Conversion Chart .................Z-154 ..... thermocouples is yellow for Type K, black
for Type J, purple for .... Electromotive Force (emf): The potential difference
between the two ... Fahrenheit: A temperature scale defined by 32° at the ice
point and.
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Temperature Handbook Contents A - Z
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Technical Reference Section
Table of Contents
Z-3
Temperature Measurement
Z-4
Thermocouples
Z-16
Probe Response Times
Z-51
Resistance Temperature Measurement
Z-53
Infrared Temperature Measurement
Z-57
Cryogenic Temperature Measurement
Z-94
Humidity & Dewpoint
Z-100
Electrical Noise Reduction
Z-104
Temperature Control
Z-110
Safety
Z-128
Data Storage and Transmission
Z-149
ITS-90
Z-158
Standards
Z-194
Non-Electric Temperature Measurement
Z-197
Thermocouple Reference Data
Z-198
RTD & Thermistor Reference Data
Z-250
Conversion Charts
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Z Section Table of Contents Technical Reference Section......Z-2 Z Section Table of Contents .......Z-3 Frequently Asked Temperature Questions............Z-4 Temperature Measurement and Control Glossary................Z-5 Practical Guidelines for Temperature Measurement....Z-13 Physical Properties of Thermoelement Materials.......Z-16 OMEGACLAD® Sheath Selection Guide ......................Z-17 Introduction to Practical Temperature Measurements ...Z-19 Using Thermocouples.............Z-21 Using RTD’s............................Z-33 Using Thermistors ..................Z-36 Nicrosil/Nisil Type N Thermocouple ........................Z-41 The Choice of Sheathing for Mineral Insulated Thermocouples.......................Z-45 Temperature Properties of Some Metals, Elements and Compounds .....................Z-48 Thermocouple Properties .........Z-49 Metal Sheathed and Exposed Thermocouple Response Times in Air ............................Z-51 Metal Sheathed and Exposed Thermocouple Response Times in Water .......................Z-52 OMEGA® Interchangeable Thermistor Applications..........Z-53 Resistance Elements and RTD’s ..............................Z-54 Introduction to Infrared Pyrometers .............................Z-57 Principles of Infrared Thermometry ..........................Z-59 Infrared Temperature Measurement: Theory and Application.......................Z-63 Noncontact Temperature Measurement: Theory and Application.......................Z-67 Fiber Optics ..............................Z-70 Handheld Infrared Thermometers for All Applications .......................Z-74 Principles of Infrared Thermocouples.......................Z-76 Microcomputer-Based Infrared Temperature Transducers......Z-81 Infrared Thermocouples Extended Temperature Ranges ...................................Z-84
Infrared Window Data...............Z-86 IR Quick Help ...........................Z-87 Table of Total Infrared Emissivity ...............................Z-88 Cryogenic Temperature Sensors: CY7 Series Silicon Diodes .....Z-90 Resolution and Accuracy of Cryogenic Temperature Measurements........................Z-94 Heat Wave: A National Problem ..............Z-100 Dewpoint.................................Z-102 Equilibrium Relative Humidity: Saturated Salt Solutions.......Z-103 Two-Wire Transmitters For Temperature Applications ..........................Z-104 How to Use Ferrite Cores With Instrumentation ............Z-105 “Electromagnetic Compatibility” and CE Conformity ...............Z-106 Low Noise Thermocouple System .................................Z-108 Introduction to Temperature Controllers and Selection Considerations .....................Z-110 Temperature Control: Tuning a PID Controller........Z-115 Controller Operation ...............Z-118 SSR Thermal Considerations..Z-119 OMEGA PT41 Precision Clock/Timer/Controller Functions..............................Z-122 Solid State Relays ..................Z-124 Intrinsic Safety ........................Z-128 Intrinsic Safety Circuit Design ..................................Z-131 Selecting a Recorder ..............Z-149 Overview of IEEE-488 ............Z-151 ASCII Code Values and Hexadecimal Conversion Chart .................Z-154 The RS-232 Standard.............Z-157 Guidelines for Realizing the ITS-90...................................Z-158 The International Temperature Scale of 1990 .......................Z-186 International Standard Codes ...................................Z-194 Application Notes: Low-Cost Non-Electric Temperature Gauges ...........Z-197 ITS-90 Thermocouple Direct and Inverse Polynomials ......Z-198 Tungsten-Rhenium Thermocouples: Calibration Equivalents.........Z-202 Z-3
Thermocouple Reference Tables Revised to ITS-90 Type J, Deg. C .....................Z-203 Type K, Deg. C.....................Z-204 Type E, Deg. C.....................Z-207 Type S, Deg. C.....................Z-208 Type R, Deg. C.....................Z-210 Type B, Deg. C.....................Z-212 Type N, Deg. C.....................Z-214 Type J, Deg. F......................Z-216 Type K, Deg. F .....................Z-218 Type E, Deg. F .....................Z-221 Type T, Deg. F .....................Z-225 Type S, Deg. F .....................Z-225 Type R, Deg. F .....................Z-228 Type B, Deg. F .....................Z-231 Type N, Deg. F .....................Z-237 Type C, Deg. C.....................Z-239 Type C, Deg. F .....................Z-241 Tungsten and Tungsten/ Rhenium: Thermocouple Tables...........Z-246 CHROMEGA® vs. Gold-0.07 Atomic Percent Iron Thermocouple Table of Temp. vs.Thermoelectric Voltage .................................Z-247 Space for Transmitters in Probe Assembly Heads ....Z-249 Platinum Resistance Temp. Detector: Interchangability Tolerance Chart.....................................Z-250 ITS-90 Polynomial for RTD Temperature vs. Resistance ..Z-251 RTD Temp. vs. Resistance Table For European Curve, Alpha = .00385 .....................Z-252 RTD Temp. vs. Resistance Table For American Curve, Alpha = .00392 .....................Z-255 Thermistor Resistance vs. Temp............Z-256 Resistance vs. Temperature for Series “700” Linear Thermistor Pairs ........Z-258 Temperature Conversion Chart Between C and F........Z-259 Conversion Factors for Physical Units of Measure....Z-261 Ohm’s Law, Summary ............Z-263 Conversion Factors for Electrical Units of Measure...Z-264
Frequently Asked Questions Q. How many feet of T/C wire can I run? A. For a specific instrument, check its specifications to see if there are any limits to the input impedance. However as a rule of thumb, limit the resistance to 100 Ohms resistance maximum, and this depends on the gage of the wire; the larger the diameter, the less resistance/foot, the longer the run can be. However, if the environment is electrically noisy, then a transmitter may be required which transmits a 4-20 mA signal that can be run longer distances and is more resistant to noise. Q. Should I use a grounded or ungrounded probe? A. It depends on the instrumentation. If there is any chance that there may be a reference to ground (common in controllers with nonisolated inputs), then an ungrounded probe is required. If the instrument is a handheld meter, then a grounded probe can almost always be used. Q. What size relay do I need to control my heaters? A. This must be calculated from known parameters. Take the total wattage of heaters and divide this value in watts by the voltage rating of the heaters in volts. The answer will be in amperes, and solid state and mechanical relays are rated by “current rating” in amperes. Q. Can I send my 4-20 mA control output to a chart recorder to monitor a process input? A. No. A control output is designed to control a valve or some equivalent control device. If you need to send an analog signal to a recording device, then choose a controller that has a “retransmission or recorder output” option. Q. Can I split my one T/C signal to two separate instruments? A. No. The T/C signal is a very lowlevel millivolt signal, and should only be connected to one device. Splitting to two devices may result in bad readings or loss of signal. The solution is to use a “dual” T/C probe, or convert one T/C output to a 4-20 mA signal by using a transmitter or signal conditioner; then the new signal can be sent to more than one instrument. Q. What are the accuracies and temperature ranges of the various thermocouples?
A. They are summarized in the tables on the first few pages of Section H. It is important to know that both accuracy and range depend on such things as the thermocouple alloys, the temperature being measured, the construction of the sensor, the material of the sheath, the media being measured, the state of the media (liquid, solid, or gas) and the diameter of either the thermocouple wire (if it is exposed) or the sheath diameter (if the thermocouple wire is not exposed but is sheathed). Q. Why can't I use ANY multimeter for measuring temperature with thermocouples? What errors will result if I don't use a thermocouple temperature meter? A. The magnitude of the thermoelectric voltage depends on the closed (sensing) end as well as the open (measuring) end of the particular thermocouple alloy leads. Temperature sensing instruments that use thermocouples take into account the temperature of the measuring end to determine the temperature at the sensing end. Most millivoltmeters do not have this capability, nor do they have the ability to do non-linear scaling to convert a millivoltage measurement to a temperature value. It is possible to use lookup tables to correct a particular millivoltage reading and calculate the temperature being sensed. However, the correction value needs to be continuously recalculated, as it is generally not constant over time. Small changes in temperature at the measuring instrument and the sensing end will change the correction value. Q. How can I choose between thermocouples, resistance temperature detectors (RTD’s), thermistors and infrared devices when measuring temperature? A. You have to consider the characteristics and costs of the various sensors as well as the available instrumentation. In addition: THERMOCOUPLES generally can measure temperatures over wide temperature ranges, inexpensively, and are very rugged, but they are not as accurate or stable as RTD’s and thermistors. RTD’s are stable and have a fairly wide temperature range, but are not as rugged and inexpensive as thermocouples. Since they require the use of
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electric current to make measurements, RTD’s are subject to inaccuracies from self-heating. THERMISTORS tend to be more accurate than RTD’s or thermocouples, but they have a much more limited temperature range. They are also subject to selfheating. INFRARED SENSORS can be used to measure temperatures higher than any of the other devices and do so without direct contact with the surfaces being measured. However, they are generally not as accurate and are sensitive to surface radiation efficiency (or more precisely, surface emissivity). Using fiber optic cables, they can measure surfaces that are not within a direct line of sight. Q. What are the two most often overlooked considerations in selecting an infrared temperature measuring device? A. The surface being measured must fill the field of view, and the surface emissivity must be taken into account. Q. What are the best ways of overcoming electrical noise problems? A. 1) Use low noise, shielded leads, connectors and probes. 2) Use instruments and connectors that suppress EMI and RF radiation. 3) Consider using analog signal transmitters, especially current transmitters. 4) Evaluate the possibility of using digitized signals. Q. If a part is moving, can I still measure temperature? A. Yes. Use infrared devices or direct contacting sensors plus a slip ring assembly. Q. Can a two-color infrared system be used to measure low emissivity surfaces? A. Only if at high temperature, say, above 700°C (1300°F). Q. What error will result if the spot size of the infrared pyrometer is larger than the target size? A. It would be indeterminate. The value would be a weighted average that wouldn’t necessarily be repeatable. Q. What readout should be used with the OS36, OS37 and OS38 units? A. Using the DP5000, BS6000, or the HH-200 would be best.
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Presenting . . . OMEGA’s Temperature Measurement and Control Glossary A comprehensive glossary of terms used in the field of temperature measurement and control. A helpful reference tool for scientists, engineers, and technicians!
Absolute Zero: Temperature at which thermal energy is at a minimum. Defined as 0 Kelvin, calculated to be –273.15°C or –459.67°F. AC: Alternating current; an electric current that reverses its direction at regularly recurring intervals. Accuracy: The closeness of an indication or reading of a measurement device to the actual value of the quantity being measured. Usually expressed as ± percent of full scale output or reading. Adaptor: A mechanism or device for attaching non-mating parts. ADC: Analog-to-Digital Converter: an electronic device which converts analog signals to an equivalent digital form, in either a binary code or a binary-coded decimal code. When used for dynamic waveforms, the sampling rate must be high to prevent aliasing errors from occurring. Address: The label or number identifying the memory location where a unit of information is stored. Aliasing: If the sample rate of a function (fs) is less than two times the highest frequency value of the function, the frequency is ambiguously presented. The frequencies above (fs/2) will be folded back into the lower frequencies producing erroneous data. Alloy 11: A compensating alloy used in conjunction with pure copper as the negative leg to form extension wire for platinum—platinumrhodium thermocouples Types R and S. Alloy 200/226: The combination of compensating alloys used with tungsten vs. tungsten/26%-rhenium thermocouples as extension cable for applications under 200°C. Alloy 203/225: The combination of compensating alloys used with tungsten/3%-rhenium vs. tungsten/25%-rhenium thermocouples as extension cable for applications under 200°C. Alloy 405/426: The combination of compensating alloys used with tungsten/5%-rhenium vs. tungsten/26%-rhenium thermocouples as extension cable for applications under 870°C. ALOMEGA®: An aluminum nickel alloy used in the negative leg of a type K thermocouple (registered trademark of OMEGA ENGINEERING, INC.). Alphanumeric: A character set that contains both letters and digits. Alumel: An aluminum nickel alloy used in the negative leg of a Type K thermocouple (Trade name of Hoskins Manufacturing Company). Ambient Compensation: The design of an instrument such that changes in ambient temperature do not affect the readings of the instrument. Ambient Conditions: The conditions around the transducer (pressure, temperature, etc.). Ambient Temperature: The average or mean temperature of the surrounding air which comes in contact with the equipment and instruments under test. Ammeter: An instrument used to measure current. Ampere (amp): A unit used to define the rate of flow of electricity (current) in a circuit; units are one coulomb (6.25 x 108 electrons) per second. Amplifier: A device which draws power from a source other than the input signal and which produces as an output an enlarged reproduction of the essential features of its input. Amplitude: A measurement of the distance from the highest to the lowest excursion of motion, as in the case of mechanical body in oscillation or the peak-to-peak swing of an electrical waveform. Analog Output: A voltage or current signal that is a continuous function of the measured parameter. Analog-to-Digital Converter (A/D or ADC): A device or circuit that outputs a binary number corresponding to an analog signal level at the input. Angstrom: Ten to the minus tenth (10–10) meters or one millimicron, a unit used to define the wavelength of light. Designated by the symbol Å. ANSI: American National Standards Institute. Anti-Reset Windup: This is a feature in a three-mode PID controller which prevents the integral (auto reset) circuit from functioning when the temperature is outside the proportional band. Application Program: A computer program that accomplishes specific tasks, such as word processing.
ASCII: American Standard Code for Information Interchange. A seven or eight bit code used to represent alphanumeric characters. It is the standard code used for communications between data processing systems and associated equipment. ASME: American Society of Mechanical Engineers. Assembler: A program that translates assembly language instructions into machine language instructions. ASTM: American Society for Testing and Materials. Asynchronous: A communication method where data is sent when it is ready without being referenced to a timing clock, rather than waiting until the receiver signals that it is ready to receive. ATC: Automatic temperature compensation. Auto-Zero: An automatic internal correction for offsets and/or drift at zero voltage input. Automatic Reset: 1. A feature on a limit controller that automatically resets the controller when the controlled temperature returns to within the limit bandwidth set. 2. The integral function on a PID controller which adjusts the proportional bandwidth with respect to the set point to compensate for droop in the circuit, i.e., adjusts the controlled temperature to a set point after the system stabilizes. AWG: American Wire Gage. Background Noise: The total noise floor from all sources of interference in a measurement system, independent of the presence of a data signal. Backup: A system, device, file or facility that can be used as an alternative in case of a malfunction or loss of data. Bandwidth: A symmetrical region around the set point in which proportional control occurs. Basic: A high-level programming language designed at Dartmouth College as a learning tool. Acronym for Beginner’s All-purpose Symbolic Instruction Code. Baud: A unit of data transmission speed equal to the number of bits (or signal events) per second; 300 baud = 300 bits per second. BCD, Buffered: Binary-coded decimal output with output drivers, to increase line-drive capability. BCD, Parallel: A digital data output format where every decimal digit is represented by binary signals on four lines and all digits are presented in parallel. The total number of lines is 4 times the number of decimal digits. BCD, Serial: A digital data output format where every decimal digit is represented by binary signals on four lines and up to five decimal digits are presented sequentially. The total number of lines is four data lines plus one strobe line per digit. BCD, Three-State: An implementation of parallel BCD, which has 0, 1 and high-impedance output states. The high-impedance state is used when the BCD output is not addressed in parallel connect applications. Beryllia: BeO (Beryllium Oxide), a high-temperature mineral insulation material; toxic when in powder form. BIAS Current: A very low-level DC current generated by a panel meter and superimposed on a signal. This current may introduce a measurable offset across a very high source impedance. Binary Coded Decimal (BCD): The representation of a decimal number (base 10, 0 through 9) by means of a 4-bit binary nibble. Binary: Refers to the base 2 numbering system, in which the only allowable digits are 0 and 1. Pertaining to a condition that has only two possible values or states. Bipolar: The ability of a panel meter to display both positive and negative readings. Bit: Acronym for binary digit. The smallest unit of computer information, it is either 0 or 1. Blackbody: A theoretical object that radiates the maximum amount of energy at a given temperature, and absorbs all the energy incident upon it. A blackbody is not necessarily black. (The name blackbody was chosen because the color black is defined as the total absorption of light energy.) BNC: A quick disconnect electrical connector used to interconnect and/or terminate coaxial cables.
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Temperature Measurement and Control Glossary Boiling Point: The temperature at which a substance in the liquid phase transforms to the gaseous phase; commonly refers to the boiling point of water which is 100°C (212°F) at sea level. BPS: Bits per second. Breakdown Voltage Rating: The dc or ac voltage which can be applied across insulation portions of a transducer without arcing or conduction above a specific current value. BTU: British thermal unit. The quantity of thermal energy required to raise one pound of water at its maximum density, 1 degree F. One BTU is equivalent to .293 watt hours, or 252 calories. One kilowatt hour is equivalent to 3412 BTU. Bulb (Liquid-in-Glass Thermometer): The area at the tip of a liquid-inglass thermometer containing the liquid reservoir. Burn-In: A long term screening test (either vibration, temperature or combined test) that is effective in weeding out premature failures because it simulates actual or worst case operation of the device, accelerated through a time, power, and temperature relationship. Burst Proportioning: A fast-cycling output form on a time proportioning controller (typically adjustable from 2 to 4 seconds) used in conjunction with a solid state relay to prolong the life of heaters by minimizing thermal stress. Bus: Parallel lines used to transfer signals between devices or components. Computers are often described by their bus structure (i.e., S-100, IBM PC). Byte: The representation of a character in binary. Eight bits.
CMV (Common-Mode Voltage): The AC or DC voltage which is tolerable between signal and ground. One type of CMV is specified between SIG LO and PWR GND. In differential meters, a second type of CMV is specified between SIG HI or LO and ANA GND (METER GND). Color Code: The ANSI established color code for thermocouple wires in the negative lead is always red. Color Code for base metal thermocouples is yellow for Type K, black for Type J, purple for Type E and blue for Type T. Common Mode: The output form or type of control action used by a temperature controller to control temperature, i.e. on/off, time proportioning, PID. Common-Mode Rejection Ratio: The ability of an instrument to reject interference from a common voltage at it’s input terminals with relation to ground, usually expressed in dB (decibels). Communication: Transmission and reception of data among data processing equipment and related peripherals. Compensated Connector: A connector made of thermocouple alloys used to connect thermocouple probes and wires. Compensating Alloys: Alloys used to connect thermocouples to instrumentation. These alloys are selected to have similar thermal electric properties as the thermocouple alloys (however, only over a very limited temperature range). Compensating Loop: Lead wire resistance compensation for RTD elements where an extra length of wire is run from the instrument to the RTD and back to the instrument, with no connection to the RTD. Compensation: An addition of specific materials or devices to counteract a known error. Compiler: A program that translates a high-level language, such as Basic, into machine language. Conductance: The measure of the ability of a solution to carry an electrical current. (See Equivalent Conductance) Conduction: The conveying of electrical energy or heat through or by means of a conductor. Confidence Level: The range (with a specified value of uncertainty, usually expressed in percent) within which the true value of a measured quantity exists. Conformity Error: For thermocouples and RTD’s, the difference between the actual reading and the temperature shown in published tables for a specific voltage input. Connection Head: An enclosure attached to the end of a thermocouple which can be cast iron, aluminum or plastic within which the electrical connections are made. Constantan: A copper-nickel alloy used as the negative lead in Type E, Type J, and Type T thermocouples. Continuous Spectrum: A frequency spectrum that is characterized by non-periodic data. The spectrum is continuous in the frequency domain and is characterized by an infinite number of frequency components. Control Character: A character whose occurrence in a particular context starts, modifies or stops an operation that affects the recording, processing, transmission or interpretation of data. Control Mode: The output form or type of control action used by a temperature controller to control temperature, i.e., on/off, time proportioning, PID. Control Point: The temperature at which a system is to be maintained. Convection: 1. The circulatory motion that occurs in a fluid at a nonuniform temperature owing to the variation of its density and the action of gravity. 2. The transfer of heat by this automatic circulation of fluid. Counts: The number of time intervals counted by the dual-slope A/D converter and displayed as the reading of the panel meter, before addition of the decimal point. CPS: Cycles per second; the rate or number of periodic events in one second, expressed in Hertz (Hz). CPU: Central processing unit. The part of the computer that contains the circuits that control and perform the execution of computer instructions. Critical Damping: Critical damping is the smallest amount of damping at which a given system is able to respond to a step function without overshoot. Cryogenics: Measurement of temperature at extremely low values, i.e., below –200°C. CSA: Canadian Standards Administration. Current Proportioning: An output form of a temperature controller which provides a current proportional to the amount of control required. Normally, a 4 to 20 milliamp current proportioning band.
Calender-van Dusen Equation: An equation that defines the resistance-temperature value of any pure metal that takes the form of (RT = RO) (1 + AT + BT2) for values between the ice point (0°C) and the freezing point of antimony (630.7°C) and the form RT = RO [1 + AT + BT2 + C (T–100)T2] between the oxygen point (–183.0°C) and the ice point (0°C). Calibration: The process of adjusting an instrument or compiling a deviation chart so that its reading can be correlated to the actual value being measured. Calorie: The quantity of thermal energy required to raise one gram of water 1°C at 15°C. Cavitation: The boiling of a liquid caused by a decrease in pressure rather than an increase in temperature. Celsius (Centigrade): A temperature scale defined by 0°C at the ice point and 100°C at the boiling point of water at sea level. Ceramic Insulation: High-temperature compositions of metal oxides used to insulate a pair of thermocouple wires. The most common are Alumina (Al2O3), Beryllia (BeO), and Magnesia (MgO). Their application depends upon temperature and type of thermocouple. High-purity alumina is required for platinum alloy thermocouples. Ceramic insulators are available as single and multihole tubes or as beads. Ceramic: Polycrystalline ferroelectric materials which are used as the sensing units in piezoelectric accelerometers. There are many different grades, all of which can be made in various configurations to satisfy different design requirements. Character: A letter, digit or other symbol that is used as the representation of data. A connected sequence of characters is called a character string. Chatter: The rapid cycling on and off of a relay in a control process due to insufficient bandwidth in the controller. CHROMEGA®: A chromium-nickel alloy which makes up the positive leg of type K and type E thermocouples (registered trademark of OMEGA ENGINEERING, INC.). Clear: To restore a device to a prescribed initial state, usually the zero state. Clipping: The term applied to the phenomenon which occurs when an output signal is limited in some way by the full range of an amplifier, ADC or other device. When this occurs, the signal is flattened at the peak values, the signal approaches the shape of a square wave, and high frequency components are introduced. Clipping may be hard, as is the case when the signal is strictly limited at some level, or it may be soft, in which case the clipping signal continues to follow the input at some reduced gain. Clock: The device that generates periodic signals for synchronization. Closeness of Control: Total temperature variation from a desired set point of system. Expressed as “closeness of control” is ±2°C or a system bandwidth with 4°C, also referred to as “amplitude of deviation.” CMR (Common-Mode Rejection): The ability of a panel meter to eliminate the effect of AC or DC noise between signal and ground. Normally expressed in dB at dc to 60 Hz. One type of CMR is specified between SIG LO and PWR GND. In differential meters, a second type of CMR is specified between SIG LO and ANA GND (METER GND).
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Temperature Measurement and Control Glossary Current: The rate of flow of electricity. The unit is the ampere (a) defined as 1 ampere = 1 coulomb per second. Curve Fitting: Curve fitting is the process of computing the coefficients of a function to approximate the values of a given data set within that function. The approximation is called a “fit”. A mathematical function, such as a least squares regression, is used to judge the accuracy of the fit. Cycle Time: The time, usually expressed in seconds, for a controller to complete one on/off cycle.
Drift: A change of a reading or a set point value over long periods due to several factors including change in ambient temperature, time, and line voltage. Droop: A common occurrence in time-proportional controllers. It refers to the difference in temperature between the set point and where the system temperature actually stabilizes due to the timeproportioning action of the controller. Dual Element Sensor: A sensor assembly with two independent sensing elements. Dual-Slope A/D Converter: An analog-to-digital converter which integrates the signal for a specific time, then counts time intervals for a reference voltage to bring the integrated signal back to zero. Such converters provide high resolution at low cost, excellent normal-mode noise rejection, and minimal dependence on circuit elements. Duplex: Pertaining to simultaneous two-way independent data communication transmission in both directions. Same as “full duplex”. Duplex Wire: A pair of wires insulated from each other and with an outer jacket of insulation around the inner insulated pair. Duty Cycle: The total time to one on/off cycle. Usually refers to the on/off cycle time of a temperature controller. Dynamic Calibration: Calibration in which the input varies over a specific length of time and the output is recorded vs. time.
Damping: The reduction of vibratory movement through dissipation of energy. Types include viscous, coulomb, and solid. Data Base: A large amount of data stored in a well-organized manner. A data base management system (DBMS) is a program that allows access to the information. dB (Decibel): 20 times the log to the base 10 of the ratio of two voltages. Every 20 dB’s correspond to a voltage ratio of 10, every 10 dB’s to a voltage ratio of 3.162. For instance, a CMR of 120 dB provides voltage noise rejection of 1,000,000/1. An NMR of 70 dB provides voltage noise rejection of 3,162/1. DC: Direct current; an electric current flowing in one direction only and substantially constant in value. Deadband: 1. For chart records: the minimum change of input signal required to cause a deflection in the pen position. 2. For temperature controllers: the temperature band where heat is turned off upon rising temperature and turned on upon falling temperature expressed in degrees. The area where no heating (or cooling) takes place. Debug: To find and correct mistakes in a program. Decimal: Refers to a base ten number system using the characters 0 through 9 to represent values. Default: The value(s) or option(s) that are assumed during operation when not specified. Degree: An incremental value in the temperature scale, i.e., there are 100 degrees between the ice point and the boiling point of water in the Celsius scale and 180°F between the same two points in the Fahrenheit scale. Density: Mass per unit of volume of a substance, i.e.: grams/cu.cm. or pounds/cu.ft. Deviation: The difference between the value of the controlled variable and the value at which it is being controlled. Differential Input: A signal-input circuit where SIG LO and SIG HI are electrically floating with respect to ANALOG GND (METER GND, which is normally tied to DIG GND). This allows the measurement of the voltage difference between two signals tied to the same ground and provides superior common-mode noise rejection. Differential: For an on/off controller, it refers to the temperature difference between the temperature at which the controller turns heat off and the temperature at which the heat is turned back on. It is expressed in degrees. Digit: A measure of the display span of a panel meter. By convention, a full digit can assume any value from 0 through 9, a 1⁄2-digit will display a 1 and overload at 2, a 3⁄4-digit will display digits up to 3 and overload at 4, etc. For example, a meter with a display span of ±3999 counts is said to be a 3 3⁄4 digit meter. Digital Output: An output signal which represents the size of an input in the form of a series of discrete quantities. Digital-to-Analog Converter (D/A or DAC): A device or circuit to convert a digital value to an analog signal level. DIN (Deutsche Industrial Norm): A set of German standards recognized throughout the world. The 1⁄8 DIN standard for panel meters specifies an outer bezel dimension of 96 x 48 mm and a panel cutout of 92 x 45 mm. DIN 43760: The standard that defines the characteristics of a 100 ohm platinum RTD having a resistance vs. temperature curve specified by a = 0.00385 ohms per degree. Discharge Time Constant: The time required for the output-voltage from a sensor or system to discharge 37% of its original value in response to a zero rise time step function input. This parameter determines a low frequency response. Disk Operating System (DOS): Program used to control the transfer of information to and from a disk, such as MS DOS. Displacement: The measured distance traveled by a point from its position at rest. Peak to peak displacement is the total measured movement of a vibrating point between its positive and negative extremes. Measurement units expressed as inches or milli-inches. Dissipation Constant: The ratio for a thermistor which relates a change in internal power dissipation to a resultant change of body temperature.
Echo: To reflect received data to the sender. For example, keys depressed on a keyboard are usually echoed as characters displayed on the screen. Electrical Interference: Electrical noise induced upon the signal wires that obscures the wanted information signal. Electromotive Force (emf): The potential difference between the two electrodes in a cell. The cell emf is the cell voltage measured when no current is flowing through the cell. It can be measured by means of a pH meter with high input impedance. Electronic Industries Association (EIA): A standards organization specializing in the electrical and functional characteristics of interface equipment. EMF: Electromotive force. A rise in (electrical) potential energy. The principal unit is the volt. EMI: Electromagnetic interference. Emissivity: The ratio of energy emitted by an object to the energy emitted by a blackbody at the same temperature. The emissivity of an object depends upon its material and surface texture; a polished metal surface can have an emissivity around 0.2 and a piece of wood can have an emissivity around 0.95. Endothermic: A process is said to be endothermic when it absorbs heat. End Point (Potentiometric): The apparent equivalence point of a titration at which a relatively large potential change is observed. Enthalpy: The sum of the internal energy of a body and the product of its volume multiplied by the pressure. Environmental Conditions: All conditions to which a transducer may be exposed during shipping, storage, handling, and operation. Eprom: Erasable Programmable Read-Only Memory. The PROM can be erased by ultraviolet light or electricity. Error: The difference between the value indicated by the transducer and the true value of the measured value being sensed. Usually expressed in percent of full scale output. Error Band: The allowable deviations to output from a specific reference norm. Usually expressed as a percentage of full scale. Eutectic Temperature: The lowest possible melting point of a mixture of alloys. Excitation: The external application of electrical voltage current applied to a transducer for normal operation. Exothermic: A process is said to be exothermic when it releases heat. Expansion Factor: Correction factor for the change in density between two pressure measurement areas in a constricted flow. Explosion-Proof Enclosure: An enclosure that can withstand an explosion of gases within it and prevent the explosion of gases surrounding it due to sparks, flashes or the explosion of the container itself, and maintain an external temperature which will not ignite the surrounding gases. Exposed Junction: A form of construction of a thermocouple probe where the hot or measuring junction protrudes beyond the sheath material so as to be fully exposed to the medium being measured. This form of construction usually gives the fastest response time.
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Temperature Measurement and Control Glossary Host: The primary or controlling computer in a multiple part system. Hysteresis: The difference in output when the measurand value is first approached with increasing and then with decreasing values. Expressed in percent of full scale during any one calibration cycle. See Deadband
Fahrenheit: A temperature scale defined by 32° at the ice point and 212° at the boiling point of water at sea level. Ferrule: A compressible tubular fitting that is compressed onto a probe inside a compression fitting to form a gas-tight seal. Field of View: A volume in space defined by an angular cone extending from the focal plane of an instrument. File: A set of related records or data treated as a unit. Firmware: Programs stored in PROM’s. Flag: Any of various types of indicators used for identification of a condition or event, for example, a character that signals the termination of a transmission. Floppy Disk: A small, flexible disk carrying a magnetic medium in which digital data is stored for later retrieval and use. FM: Factory Mutual Research Corporation. An organization which sets industrial safety standards. FM Approved: An instrument that meets a specific set of specifications established by Factory Mutual Research Corporation. FORTRAN: Formula Translation language. A widely used high-level programming language well suited to problems that can be expressed in terms of algebraic formulas. It is generally used in scientific applications. Freezing Point: The temperature at which a substance goes from the liquid phase to the solid phase. Frequency: The number of cycles over a specified time period over which an event occurs. The reciprocal is called the period. Frequency Modulated Output: A transducer output which is obtained in the form of a deviation from a center frequency, where the deviation is proportional to the applied stimulus. Frequency, Natural: The frequency of free (not forced) oscillations of the sensing element of a fully assembled transducer. Frequency Output: An output in the form of frequency which varies as a function of the applied input. Full Scale Output: The algebraic difference between the minimum output and maximum output.
Impedance: The total opposition to electrical flow (resistive plus reactive). Infrared: an area in the electromagnetic spectrum extending beyond red light from 760 nanometers to 1000 microns (106 nm). It is the form of radiation used for making non-contact temperature measurements. Input Impedance: The resistance of a panel meter as seen from the source. In the case of a voltmeter, this resistance has to be taken into account when the source impedance is high; in the case of an ammeter, when the source impedance is low. Insulated Junction: See Ungrounded Junction Insulation Resistance: The resistance measured between two insulated points on a transducer when a specific dc voltage is applied at room temperature. Integral: A form of temperature control. See Automatic Reset (2) Interchangeability Error: A measurement error that can occur if two or more probes are used to make the same measurement. It is caused by a slight variation in characteristics of different probes. Interface: The means by which two systems or devices are connected and interact with each other. Interrupt: To stop a process in such a way that it can be resumed. Intrinsically Safe: An instrument which will not produce any spark or thermal effects under normal or abnormal conditions that will ignite a specified gas mixture. IPTS-48: International Practical Temperature Scale of 1948. Fixed points in thermometry as specified by the Ninth General Conference of Weights and Measures which was held in 1948. IPTS-68: International Practical Temperature Scale of 1968. Fixed points in thermometry set by the 1968 General Conference of Weights and Measures. ISA: Instrument Society of America. Isolation: The reduction of the capacity of a system to respond to an external force by use of resilient isolating materials. Isothermal: A process or area that is a constant temperature.
Gain: The amount of amplification used in an electrical circuit. Galvanometer: An instrument that measures small electrical currents by means of deflecting magnetic coils. Ground: 1. The electrical neutral line having the same potential as the surrounding earth. 2. The negative side of DC power supply. 3. Reference point for an electrical system. Grounded Junction: A form of construction of a thermocouple probe where the hot or measuring junction is in electrical contact with the sheath material so that the sheath and thermocouple will have the same electrical potential.
Joule: The basic unit of thermal energy. Junction: The point in a thermocouple where the two dissimilar metals are joined. K: When referring to memory capacity, two to the tenth power (1024 in decimal notation). Kelvin: Symbol K. The unit of absolute or thermodynamic temperature scale based upon the Celsius scale with 100 units between the ice point and boiling point of water. 0°C = 273.15K (there is no degree (°) symbol used with the Kelvin scale). Kilowatt (kw): Equivalent to 1000 watts. Kilowatt Hour (kwh): 1000 watthours. Kilovolt amperes (kva): 1000 volt amps. KVA: Kilovolt amperes (1000 volt amps).
Half-Duplex: One way at a time data communication; both devices can transmit and receive data, but only one at a time. Handshake: An interface procedure that is based on status/data signals that assure orderly data transfer as opposed to asynchronous exchange. Hardcopy: Output in a permanent form (usually a printout) rather than in temporary form, as on disk or terminal display. Hardware: The electrical, mechanical and electromechanical equipment and parts associated with a computing system, as opposed to its firmware or software. Heat: Thermal energy. Heat is expressed in units of calories or BTU’s. Heat Sink: 1. Thermodynamic. A body which can absorb thermal energy. 2. Practical. A finned piece of metal used to dissipate the heat of solid state components mounted on it. Heat Transfer: The process of thermal energy flowing from a body of high energy to a body of low energy. Means of transfer are: conduction; the two bodies contact. Convection; a form of conduction where the two bodies in contact are of different phases, i.e. solid and gas. Radiation: all bodies emit infrared radiation. Heat Treating: A process for treating metals where heating to a specific temperature and cooling at a specific rate changes the properties of the metal. Hertz (Hz): Units in which frequency is expressed. Synonymous with cycles per second. Hexadecimal: Refers to a base sixteen number system using the characters 0 through 9 and A through F to represent the values. Machine language programs are often written in hexadecimal notation. Hold: Meter HOLD is an external input which is used to stop the A/D process and freeze the display. BCD HOLD is an external input used to freeze the BCD output while allowing the A/D process to continue operation.
Lag: 1. A time delay between the output of a signal and the response of the instrument to which the signal is sent. 2. A time relationship between two waveforms where a fixed reference point on one wave occurs after the same point of the reference wave. Latent Heat: Expressed in BTU per pound. The amount of heat needed (absorbed) to convert a pound of boiling water to a pound of steam. Leakage Rate: The maximum rate at which a fluid is permitted or determined to leak through a seal. Limits of Error: A tolerance band for the thermal electric response of thermocouple wire expressed in degrees or percentage defined by ANSI specification MC-96.1 (1975). Linearity: The closeness of a calibration curve to a specified straight line. Linearity is expressed as the maximum deviation of any calibration point on a specified straight line during any one calibration cycle. Load: The electrical demand of a process expressed as power (watts), current (amps) or resistance (ohms). Load Impedance: The impedance presented to the output terminals of a transducer by the associated external circuitry. Logarithmic Scale: A method of displaying data (in powers of ten) to yield maximum range while keeping resolution at the low end of the scale.
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Temperature Measurement and Control Glossary Loop Resistance: The total resistance of a thermocouple circuit caused by the resistance of the thermocouple wire. Usually used in reference to analog pyrometers which have typical loop resistance requirements of 10 ohms. LSD (Least-Significant Digit): The rightmost active (non-dummy) digit of the display. LS-TTL Compatible: For digital input circuits, a logic 1 is obtained for inputs of 2.0 to 5.5 V which can source 20 µA, and a logic 0 is obtained for inputs of 0 to 0.8 V which can sink 400 µA. For digital output signals, a logic 1 is represented by 2.4 to 5.5 V with a current source capability of at least 400 µA, and a logic 0 is represented by 0 to 0.6 V with a current sink capability of at least 16 MA. “LS” stands for Low-power Schottky. LS-TTL Unit Load: A load with LS-TTL voltage levels, which will draw 20 µA for a logic 1 and –400 µA for a logic 0.
NEMA-4: A standard from the National Electrical Manufacturers Association, which defines enclosures intended for indoor or outdoor use primarily to provide a degree of protection against windblown dust and rain, splashing water, and hose-directed water. NEMA-7: A standard from the National Electrical Manufacturers Association, which defines explosion-proof enclosures for use in locations classified as Class I, Groups A, B, C or D, as specified in the National Electrical Code. NEMA-12: A standard from the National Electrical Manufacturers Association, which defines enclosures with protection against dirt, dust, splashes by non-corrosive liquids, and salt spray. NEMA-Size Case: An older US case standard for panel meters, which requires a panel cutout of 3.93 x 1.69 inches. Network: A group of computers that are connected to each other by communications lines to share information and resources. Nibble: One half of a byte. Nicrosil/Nisil: A nickel-chrome/nickel-silicone thermal alloy used to measure high temperatures. Inconsistencies in thermoelectric voltages exist in these alloys with respect to the wire gage. NMR (Normal-Mode Rejection): The ability of a panel meter to filter out noise superimposed on the signal and applied across the SIG HI to SIG LO input terminals. Normally expressed in dB at 50/60 Hz. Noise: An unwanted electrical interference on the signal wires. Normal-Mode Rejection Ratio: The ability of an instrument to reject interference usually of line frequency (50–60 Hz) across its input terminals. NPT: National Pipe Thread. Null: A condition, such as balance, which results in a minimum absolute value of output.
M: Mega; one million. When referring to memory capacity, two to the twentieth power (1,048,576 in decimal notation). Manual Reset (Adjustment): The adjustment on a proportioning controller which shifts the proportioning band in relationship to the set point to eliminate droop or offset errors. Manual Reset (Switch): The switch in a limit controller that manually resets the controller after the limit has been exceeded. Maximum Operating Temperature: The maximum temperature at which an instrument or sensor can be safely operated. Maximum Power Rating: The maximum power in watts that a device can safely handle. Mean Temperature: The average of the maximum and minimum temperature of a process equilibrium. Measurand: A physical quantity, property, or condition which is measured. Measuring Junction: The thermocouple junction referred to as the hot junction that is used to measure an unknown temperature. Melting Point: The temperature at which a substance transforms from a solid phase to a liquid phase. Mica: A transparent mineral used as window material in hightemperature ovens. Microamp: One millionth of an ampere, 10–6 amps. Microcomputer: A computer which is physically small. It can fit on top of or under a desk; based on LSI circuitry, computers of this type are now available with much of the power currently associated with minicomputer systems. Micron: One millionth of a meter, 10–6 meters. Microvolt: One millionth of a volt, 10–6 volts. Mil: One thousandth of an inch (.001≤). Milliamp: One thousandth of an amp, 10–3 amps, symbol mA. Millimeter: One thousandth of a meter, symbol mm. Millivolt: Unit of electromotive force. It is the difference in potential required to make a current of 1 millampere flow through a resistance of 1 ohm; one thousandth of a volt, symbol mV. Mineral-insulated Thermocouple: A type of thermocouple cable which has an outer metal sheath and mineral (magnesium oxide) insulation inside separating a pair of thermocouple wires from themselves and from the outer sheath. This cable is usually drawn down to compact the mineral insulation and is available in diameters from .375 to .010 inches. It is ideally suited for hightemperature and severe-duty applications. Minor Scale Division: On an analog scale, the smallest indicated division of units on the scale. Modem: Modulator/Demodulator. A device that transforms digital signals into audio tones for transmission over telephone lines, and does the reverse for reception. MSD (Most-Significant Digit): The leftmost digit of the display. Mueller Bridge: A high-accuracy bridge configuration used to measure three-wire RTD thermometers. Multiplex: A technique which allows different input (or output) signals to use the same lines at different times, controlled by an external signal. Multiplexing is used to save on wiring and I/O ports.
Octal: Pertaining to a base 8 number system. O.D.: Outside diameter. Offset: The difference in temperature between the set point and the actual process temperature. Also referred to as droop. Ohmmeter: An instrument used to measure electrical resistance. On/off Controller: A controller whose action is fully on or fully off. Open Circuit: The lack of electrical contact in any part of the measuring circuit. An open circuit is usually characterized by rapid large jumps in displayed potential, followed by an off-scale reading. Operating System: A collection of programs that controls the overall operation of a computer and performs such tasks as assigning places in memory to programs and data, processing interrupts, scheduling jobs and controlling the overall input/output of the system. Optical Isolation: Two networks which are connected only through an LED transmitter and photoelectric receiver with no electrical continuity between the two networks. Output: The electrical signal which is produced by an applied input to the transducer. Output Impedance: The resistance as measured on the output terminals of a pressure transducer. Output Noise: The RMS, peak-to-peak (as specified) ac component of a transducer’s dc output in the absence of a measurand variation. Overshoot: The number of degrees by which a process exceeds the set point temperature when coming up to the set point temperature. Parallax: An optical illusion which occurs in analog meters and causes reading errors. It occurs when the viewing eye is not in the same plane, perpendicular to the meter face, as the indicating needle. Parallel Transmission: Sending all data bits simultaneously. Commonly used for communications between computers and printer devices. Parity: A technique for testing transmitting data. Typically, a binary digit is added to the data to make the sum of all the digits of the binary data either always even (even parity) or always odd (odd parity). Peltier Effect: When a current flows through a thermocouple junction, heat will either be absorbed or evolved depending on the direction of current flow. This effect is independent of joule I2 R heating. Peripheral: A device that is external to the CPU and main memory, i.e., printer, modem or terminal, but is connected by the appropriate electrical connections. Phase: A time-based relationship between a periodic function and a reference. In electricity, it is expressed in angular degrees to describe the voltage or current relationship of two alternating waveforms. Phase Difference: The time expressed in degrees between the same reference point on two periodic waveforms.
N/C (No Connection): A connector point for which there is no internal connection. NBS: National Bureau of Standards. NEC: National Electric Codes. Negative Temperature Coefficient: A decrease in resistance with an increase in temperature.
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Temperature Measurement and Control Glossary Proportioning Control with Integral and Derivative Functions: Three mode PID controller. A time-proportioning controller with integral and derivative functions. The integral function automatically adjusts the system temperature to the set point temperature to eliminate droop due to the time proportioning function. The derivative function senses the rate of rise or fall of the system temperature and automatically adjusts the cycle time of the controller to minimize overshoot or undershoot. Protection Head: An enclosure usually made out of metal at the end of a heater or probe where connections are made. Protection Tube: A metal or ceramic tube, closed at one end, into which a temperature sensor is inserted. The tube protects the sensor from the medium into which it is inserted. Protocol: A formal definition that describes how data is to be exchanged. PSIA: Pounds per square inch absolute. Pressure referenced to a vacuum. PSID: Pounds per square inch differential. Pressure difference between two points. PSIG: Pound per square inch gage. Pressure referenced to ambient air pressure. PSIS: Pounds per square inch standard. Pressure referenced to a standard atmosphere. Pulse Width Modulation: An output in the form of duty cycle which varies as a function of the applied measurand.
Phase Proportioning: A form of temperature control where the power supplied to the process is controlled by limiting the phase angle of the line voltage. PID: Proportional, integral, derivative. A three-mode control action where the controller has time proportioning, integral (auto reset) and derivative rate action. Piezoresistance: Resistance that changes with stress. Pixel: Picture element. Definable locations on a display screen that are used to form images on the screen. For graphic displays, screens with more pixels provide higher resolution. Platinel: A non-standard, high temperature platinum thermocouple alloy whose thermoelectric voltage nearly matches a Type K thermocouple (Trademark of Englehard Industries). Platinum: A noble metal which in its pure form is the negative wire of Type R and Type S thermocouples. Platinum 6% Rhodium: The platinum-rhodium alloy used as the negative wire in conjunction with platinum-30% rhodium to form a Type B thermocouple. Platinum 10% Rhodium: The platinum-rhodium alloy used as the positive wire in conjunction with pure platinum to form a Type S thermocouple. Platinum 13% Rhodium: The platinum-rhodium alloy used as the positive wire in conjunction with pure platinum to form a Type R thermocouple. Platinum 30% Rhodium: The platinum-rhodium alloy used as the positive wire in conjunction with platinum 6% rhodium to form a Type B thermocouple. Platinum 67: To develop thermal emf tables for thermocouples, the National Bureau of Standards paired each thermocouple alloy against a pure platinum wire (designated Platinum 2 prior to 1973, and currently Platinum 67). The thermal emf’s of any alloy combination can be determined by summing the “vs. Pt-67” emf’s of the alloys, i.e., the emf table for a Type K thermocouple is derived from the Chromel vs. Pt-67 and the Alumel vs .Pt-67 values. Polarity: In electricity, the quality of having two oppositely charged poles, one positive, one negative. Port: A signal input (access) or output point on a computer. Positive Temperature Coefficient: An increase in resistance due to an increase in temperature. Potential Energy: Energy related to the position or height above a place to which fluid could possibly flow. Potentiometer: 1. A variable resistor often used to control a circuit. 2. A balancing bridge used to measure voltage. Power Supply: A separate unit or part of a circuit that supplies power to the rest of the circuit or to a system. PPM: Abbreviation for “parts per million,” sometimes used to express temperature coefficients. For instance, 100 ppm is identical to 0.01%. Primary Standard (NBS): The standard reference units and physical constants maintained by the National Bureau of Standards upon which all measurement units in the United States are based. Probe: A generic term that is used to describe many types of temperature sensor. Process Meter: A panel meter with sizeable zero and span adjustment capabilities, which can be scaled for readout in engineering units for signals such as 4–20 mA, 10–50 mA and 1–5 V. Program: A list of instructions that a computer follows to perform a task. Prom: Programmable read-only memory. A semiconductor memory whose contents cannot be changed by the computer after it has been programmed. Proportioning Band: A temperature band expressed in degrees within which a temperature controller’s time proportioning function is active. Proportioning Control Mode: A time proportioning controller where the amount of time that the relay is energized is dependent upon the system’s temperature. Proportioning Control plus Derivative Function: A time proportioning controller with a derivative function. The derivative function senses the rate at which a system’s temperature is either increasing or decreasing and adjusts the cycle time of the controller to minimize overshoot or undershoot. Proportioning Control plus Integral: A two-mode controller with time proportioning and integral (auto reset) action. The integral function automatically adjusts the temperature at which a system has stabilized back to the set point temperature, thereby eliminating droop in the system.
Radiation: See Infrared Random Access Memory (RAM): Memory that can be both read and changed during computer operation. Unlike other semi-conductor memories, RAM is volatile—if power to the RAM is disrupted or lost, all the data stored is lost. Range: Those values which a transducer is intended to measure, specified by upper and lower limits. Rangeability: The ratio of the maximum flowrate to the minimum flowrate of a meter. Rankine (°R): An absolute temperature scale based upon the Fahrenheit scale with 180° between the ice point and boiling point of water. 459.67°R = 0°F. Rate Action: The derivative function of a temperature controller. Rate Time: The time interval over which the system temperature is sampled for the derivative function. Ratiometric Measurement: A measurement technique where an external signal is used to provide the voltage reference for the dualslope A/D converter. The external signal can be derived from the voltage excitation applied to a bridge circuit or pick-off supply, thereby eliminating errors due to power supply fluctuations. Read Only Memory (ROM): Memory that contains fixed data. The computer can read the data, but cannot change it in any way. Real Time: The time interval over which the system temperature is sampled for the derivative function. Record: A collection of unrelated information that is treated as a single unit. Recovery Time: The length of time which it takes a transducer to return to normal after applying a proof pressure. Reference Junction: The cold junction in a thermocouple circuit which is held at a stable, known temperature. The standard reference temperature is 0°C (32°F). However, other temperatures can be used. Refractory Metal Thermocouple: A class of thermocouples with melting points above 3600°F. The most common are made from tungsten and tungsten/rhenium alloys, Types G and C. They can be used for measuring high temperatures up to 4000°F (2200°C) in non-oxidizing, inert, or vacuum environments. Relay (Mechanical): An electromechanical device that completes or interrupts a circuit by physically moving electrical contacts into contact with each other. Relay (Solid State): A solid state switching device which completes or interrupts a circuit electrically with no moving parts. Remote: Not hard-wired; communicating via switched lines, such as telephone lines. Usually refers to peripheral devices that are located at a site away from the CPU. Repeatability: The ability of a transducer to reproduce output readings when the same measurand value is applied to it consecutively, under the same conditions, and in the same direction. Repeatability is expressed as the maximum difference between output readings. Resistance: The resistance to the flow of electric current measured in ohms (Ω). For a conductor, resistance is a function of diameter, resistivity (an intrinsic property of the material) and length.
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Temperature Measurement and Control Glossary Resistance Ratio Characteristic: For thermistors, the ratio of the resistance of the thermistor at 25°C to the resistance at 125°C. Resistance Temperature Characteristic: A relationship between a thermistor’s resistance and the temperature. Resolution: The smallest detectable increment of measurement. Resolution is usually limited by the number of bits used to quantize the input signal. For example, a 12-bit A/D can resolve to one part in 4096 (2 to the 12 power equals 4096). Resonant Frequency: The measurand frequency at which a transducer responds with maximum amplitude. Response Time: The length of time required for the output of a transducer to rise to a specified percentage of its final value as a result of a step change of input. Response Time (time constant): The time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to stabilize at 100% of the step change value. RFI: Radio frequency interference. Rheostat: A variable resistor. Rise Time: The time required for a sensor or system to respond to an instantaneous step function, measured from the 10% to 90% points on the response waveforms. Room Conditions: Ambient environmental conditions under which transducers must commonly operate. Root Mean Square (RMS): Square root of the mean of the square of the signal taken during one full cycle. RTD: Resistance temperature detector.
Single Precision: The degree of numeric accuracy that requires the use of one computer word. In single precision, seven digits are stored, and up to seven digits are printed. Contrast with Double Precision. Software: Generally, programs loaded into a computer from external mass storage but also extended to include operating systems and documentation. Source Code: A non-executable program written in a high-level language. A compiler or assembler must translate the source code into object code (machine language) that the computer can understand and process. Span: The difference between the upper and lower limits of a range expressed in the same units as the range. Span Adjustment: The ability to adjust the gain of a process or strain meter so that a specified display span in engineering units corresponds to a specified signal span. For instance, a display span of 200°F may correspond to the 16 mA span of a 4–20 mA transmitter signal. Spare: A connector point reserved for options, specials, or other configurations. The point is identified by an (E#) for location on the electrical schematic. Specific Gravity: The ratio of mass of any material to the mass of the same volume of pure water at 4°C. Specific Heat: The ratio of thermal energy required to raise the temperature of a body 1° to the thermal energy required to raise an equal mass of water 1°. Spectral Filter: A filter which allows only a specific band width of the electromagnetic spectrum to pass, i.e., 4 to 8 micron infrared radiation. Spectrum: The resolving of overall vibration into amplitude components as a function of frequency. Spectrum Analysis: Utilizing frequency components of a vibration signal to determine the source and cause of vibration. Spot Size: The diameter of the circle formed by the cross section of the field of view of an optical instrument at a given distance. Spurious Error: Random or erratic malfunction. SSR: Solid state relay. See Relay, Solid State Stability: The ability of an instrument or sensor to maintain a consistent output when a constant input is applied. Stop Bit: A signal following a character or block that prepares the receiving device to receive the next character or block. String: A sequence of characters. Super Cooling: The cooling of a liquid below its freezing temperature without the formation of the solid phase. Super Heating: 1. The heating of a liquid above its boiling temperature without the formation of the gaseous phase. 2. The heating of the gaseous phase considerably above the boiling-point temperature to improve the thermodynamic efficiency of a system. Surge Current: A current of short duration that occurs when power is first applied to capacitive loads or temperature dependent resistive loads such as tungsten or molybdenum heaters—usually lasting not more than several cycles. Syntax: The rules governing the structure of a language.
SAMA: Scientific Apparatus Makers Association. An association that has issued standards covering platinum, nickel, and copper resistance elements (RTD’s). SCR: Silicon controlled rectifier. Scroll: To move all or part of the screen material up or down, left or right, to allow new information to appear. Seebeck Coefficient: The derivative (rate of change) of thermal EMF with respect to temperature, normally expressed as millivolts per degree. Seebeck Effect: When a circuit is formed by a junction of two dissimilar metals and the junctions are held at different temperatures, a current will flow in the circuit caused by the difference in temperature between the two junctions. Seebeck EMF: The open circuit voltage caused by the difference in temperature between the hot and cold junctions of a circuit made from two dissimilar metals. Self-Heating: Internal heating of a transducer as a result of power dissipation. Sensing Element: That part of a transducer which reacts directly in response to input. Sensitivity: The minimum change in input signal to which an instrument can respond. Sensitivity Shift: A change in slope of the calibration curve due to a change in sensitivity. Sequential Access: An access mode in which records are retrieved in the same order in which they were written. Each successive access to the file refers to the next record in the file. Serial Transmission: Sending one bit at a time on a single transmission line. Compare with Parallel Transmission. Set Point: The temperature at which a controller is set to control a system. Settling Time: The time taken for the display to settle within one digit final value when a step is applied to the meter input. SI: System Internationale. The name given to the standard metric system of units. Signal: An electrical transmittance (either input or output) that conveys information. Signal Conditioner: A circuit module which offsets, attenuates, amplifies, linearizes and/or filters the signal for input to the A/D converter. The typical output signal conditioner is +2 V dc. Signal Conditioning: To process the form or mode of a signal so as to make it intelligible to, or compatible with, a given device, including such manipulation as pulse shaping, pulse clipping, compensating, digitizing, and linearizing. Single-Ended Input: A signal-input circuit where SIG LO (or sometimes SIG HI) is tied to METER GND. Ground loops are normally not a problem in AC-powered meters, since METER GND is transformer-isolated from AC GND.
Tape: A recording medium for data or computer programs. Tape can be in permanent form, such as perforated paper tape, or erasable, such as magnetic tape. Generally, tape is used as a mass storage medium, in magnetic form, and has a much higher storage capacity than disk storage, but it takes much longer to write or recover data from tape than from a disk. Teflon: A fluorocarbon polymer used for insulation of electrical wires (trademark of DuPont). Telecommunication: Synonym for data communication. The transmission of information from one point to another. TEMPCO: Abbreviation for “temperature coefficient”: the error introduced by a change in temperature. Normally expressed in %/°C or ppm/°C. Temperature Error: The maximum change in output, at any measurand value within a specified range, when the transducer temperature is changed from room temperature to specified temperature extremes. Temperature Range, Compensated: The range of ambient temperatures within which all tolerances specified for Thermal Zero Shift and Thermal Sensitivity Shift are applicable (temperature error). Temperature Range, Operable: The range of ambient temperatures, given by their extremes, within which a transducer may be operated. Exceeding compensated range may require recalibration.
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Temperature Measurement and Control Glossary Terminal: An input/output device used to enter data into a computer and record the output. Thermal Coefficient of Resistance: The change in resistance of a semiconductor per unit change in temperature over a specific range of temperature. Thermal Conductivity: The ability of a material to conduct heat in the form of thermal energy. Thermal emf: See Seebeck emf Thermal Expansion: An increase in size due to an increase in temperature expressed in units of an increase in length or increase in size per degree, i.e. inches/inch/degree C. Thermal Gradient: The distribution of a differential temperature through a body or across a surface. Thermal Sensitivity Shift: The sensitivity shift due to changes of the ambient temperature from room temperature to the specified limits of the compensated temperature range. Thermal Zero Shift: An error due to changes in ambient temperature in which the zero pressure output shifts. Thus, the entire calibration curve moves in a parallel displacement. Thermistor: A temperature-sensing element composed of sintered semiconductor material which exhibits a large change in resistance proportional to a small change in temperature. Thermistors usually have negative temperature coefficients. Thermocouple: The junction of two dissimilar metals which has a voltage output proportional to the difference in temperature between the hot junction and the lead wires (cold junction) (refer to Seebeck emf). Thermocouple Type Material (ANSI Symbol) J Iron/Constantan K CHROMEGA®/ALOMEGA® T Copper/Constantan E CHROMEGA/Constantan R Platinum/Platinum 13% Rhodium S Platinum/Platinum 10% Rhodium B Platinum 6% Rhodium/Platinum 30% Rhodium G* Tungsten/Tungsten 26% Rhenium C* Tungsten 5% Rhenium/Tungsten 26% Rhenium D* Tungsten 3% Rhenium/Tungsten 25% Rhenium *Not ANSI symbols Thermopile: An arrangement of thermocouples in series such that alternate junctions are at the measuring temperature and the reference temperature. This arrangement amplifies the thermoelectric voltage. Thermopiles are usually used as infrared detectors in radiation pyrometry. Thermowell: A closed-end tube designed to protect temperature sensors from harsh environments, high pressure, and flows. They can be installed into a system by pipe thread or welded flange and are usually made of corrosion-resistant metal or ceramic material, depending upon the application. Thomson Effect: When current flows through a conductor within a thermal gradient, a reversible absorption or evolution of heat will occur in the conductor at the gradient boundaries. Transducer: A device (or medium) that converts energy from one form to another. The term is generally applied to devices that take physical phenomena (pressure, temperature, humidity, flow, etc.) and convert them to electrical signals. Transmitter (Two-Wire): A device which is used to transmit temperature data from either a thermocouple or RTD via a two-wire current loop. The loop has an external power supply and the transmitter acts as a variable resistor with respect to its input signal. Triac: A solid state switching device used to switch alternating current wave forms. Triple Point: The temperature and pressure at which solid, liquid, and gas phases of a given substance are all present simultaneously in varying amounts. Triple Point (Water): The thermodynamic state where all three phases, solid, liquid, and gas, may all be present in equilibrium. The triple point of water is .01°C. True RMS: The true root-mean-square value of an AC or AC-plus-DC signal, often used to determine power of a signal. For a perfect sine wave, the RMS value is 1.11072 times the rectified average value, which is utilized for low-cost metering. For significantly nonsinusoidal signals, a true RMS converter is required. TTL: Transistor-to-transistor logic. A form of solid state logic which uses only transistors to form the logic gates.
TTL-Compatible: For digital input circuits, a logic 1 is obtained for inputs of 2.0 to 5.5 V which can source 40 µA, and a logic 0 is obtained for inputs of 0 to 0.8 V which can sink 1.6 mA. For digital output signals, a logic 1 is represented by 2.4 to 5.5 V with a current source capability of at least 400 µA, and a logic 0 is represented by 0 to 0.6 V with a current sink capability of at least 16 mA. TTL Unit Load: A load with TTL voltage levels, which will draw 40 µA for a logic 1 and –1.6 mA for a logic 0. Typical: Error within plus or minus one standard deviation (±1%) of the nominal specified value, as computed from the total population. UL: Underwriters Laboratories, Inc. An independent laboratory that establishes standards for commercial and industrial products. Ultraviolet: That portion of the electromagnetic spectrum below blue light (380 nanometers). Undershoot: The difference in temperature between the temperature a process goes to, below the set point, after the cooling cycle is turned off and the set point temperature. Ungrounded Junction: A form of construction of a thermocouple probe where the hot or measuring junction is fully enclosed by and insulated from the sheath material. Union: A form of pipe fitting where two extension pipes are joined at a separable coupling. Vacuum: A pressure less than atmospheric pressure. Velocity: The time rate of change of displacement; dx/dt. Vibration Transducer: Generally, any device which converts movement, either shock or steady state vibration, into an electrical signal proportional to the movement; a sensor. Volt: The (electrical) potential difference between two points in a circuit. The fundamental unit is derived as work per unit charge— (V = W/Q). One volt is the potential difference required to move one coulomb of charge between two points in a circuit using one joule of energy. Voltage: An electrical potential which can be measured in volts. Voltmeter: An instrument used to measure voltage. Watt Density: The watts emanating from each square inch of heated surface area of a heater. Expressed in units of watts per square inch. Wheatstone Bridge: A network of four resistances, an emf source, and a galvanometer connected such that when the four resistances are matched, the galvanometer will show a zero deflection or “null” reading. Window: In computer graphics, a defined area in a system not bounded by any limits; unlimited “space” in graphics. Word: Number of bits treated as a single unit by the CPU. In an 8-bit machine, the word length is 8 bits; in a sixteen-bit machine, it is 16 bits. Working Standard: A standard of unit measurement calibrated from either a primary or secondary standard which is used to calibrate other devices or make comparison measurements. Zero Adjustment: The ability to adjust the display of a process or strain meter so that zero on the display corresponds to a non-zero signal, such as 4 mA, 10 mA, or 1 V dc. The adjustment range is normally expressed in counts. Zero Offset: 1. The difference expressed in degrees between true zero and an indication given by a measuring instrument. 2. See Zero Suppression Zero Power Resistance: The resistance of a thermistor or RTD element with no power being dissipated. Zero Suppression: The span of an indicator or chart recorder may be offset from zero (zero suppressed) such that neither limit of the span will be zero. For example, a temperature recorder which records a 100° span from 400° to 500° is said to have 400° zero suppression. Zero Voltage Switching: The making or breaking of circuit timed such that the transition occurs when the voltage wave form crosses zero voltage; typically only found in solid state switching devices.
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Practical Guidelines for Temperature Measurement Temperature can be measured via a diverse array of sensors. All of them infer temperature by sensing some change in a physical characteristic. Six types with which the engineer is likely to come into contact are: thermocouples, resistance temperature devices (RTD’s and thermistors), infrared radiators, bimetallic devices, liquid expansion devices, and change-of-state devices. It is well to begin with a brief review of each. Thermocouples consist essentially of two strips or wires made of different metals and joined at one end. As discussed later, changes in the temperature at that juncture induce a change in electromotive force (emf) between the other ends. As temperature goes up, this output emf of the thermocouple rises, though not necessarily linearly. Resistance temperature devices capitalize on the fact that the electrical resistance of a material changes as its temperature changes. Two key types are the metallic devices (commonly referred to as RTD’s), and thermistors. As their name indicates, RTD’s rely on resistance change in a metal, with the resistance rising more or less linearly with temperature. Thermistors are based on resistance change in a ceramic semiconductor; the resistance drops nonlinearly with temperature rise. Infrared sensors are noncontacting devices. As discussed later, they infer temperature by measuring the thermal radiation emitted by a material. Bimetallic devices take advantage of the difference in rate of thermal expansion between different metals. Strips of two metals are bonded together. When heated, one side will expand more than the other, and the resulting bending is translated into a temperature reading by mechanical linkage to a pointer. These devices are portable and they do not require a power supply, but they are usually not as accurate as thermocouples or RTD’s and they do not readily lend themselves to temperature recording. Fluid-expansion devices, typified by the household thermometer, generally come in two main
classifications: the mercury type and the organic-liquid type. Versions employing gas instead of liquid are also available. Mercury is considered an environmental hazard, so there are regulations governing the shipment of devices that contain it. Fluid-expansion sensors do not require electric power, do not pose explosion hazards, and are stable even after repeated cycling. On the other hand, they do not generate data that are easily recorded or transmitted, and they cannot make spot or point measurements. Change-of-state temperature sensors consist of labels, pellets, crayons, lacquers or liquid crystals whose appearance changes when a certain temperature is reached. They are used, for instance, with steam traps – when a trap exceeds a certain temperature, a white dot on a sensor label attached to the trap will turn black. Response time typically takes minutes, so these devices often do not respond to transient temperature changes, and accuracy is lower than with other types of sensors. Furthermore, the change in state is irreversible, except in the case of liquid-crystal displays. Even so, change-of-state sensors can be handy when one needs confirmation that the temperature of a piece of equipment or a material has not exceeded a certain level, for instance for technical or legal reasons, during product shipment.
The workhorses In the chemical process industries, the most commonly used temperature sensors are thermocouples, resistive devices and infrared devices. There is widespread misunderstanding as to how these devices work and how they should be used. Thermocouples: Consider first the thermocouple, probably the mostoften-used and least-understood of the three. Essentially, a thermocouple consists of two alloys joined together at one end and open at the other. The emf at the output end (the open end; V1 in Figure 1a) is a function of the temperature T1 at the closed end. As the temperature rises, the emf goes up. Z-13
Often the thermocouple is located inside a metal or ceramic shield that protects it from a variety of environments. Metal-sheathed thermocouples are also available with many types of outer coatings, such as polytetrafluoroethylene, for trouble-free use in corrosive solutions. The open-end emf is a function of not only the closed-end temperature (i.e., the temperature at the point of measurement) but also the temperature at the open end (T2 in Figure 1a). Only by holding T2 at a standard temperature can the measured emf be considered a direct function of the change in T1. The industrially accepted standard for T2 is 0°C; therefore, most tables and charts make the assumption that T2 is at that level. In industrial instrumentation, the difference between the actual temperature at T2 and 0°C is usually corrected for electronically, within the instrumentation. This emf adjustment is referred to as the cold-junction, or CJ, correction. Temperature changes in the wiring between the input and output ends do not affect the output voltage, provided that the wiring is of thermocouple alloy or a thermoelectric equivalent (Figure 1a). For example, if a thermocouple is measuring temperature in a furnace and the instrument that shows the reading is some distance away, the wiring between the two could pass near another furnace and not be affected by its temperature, unless it becomes hot enough to melt the wire or permanently change its electrothermal behavior. The composition of the junction itself does not affect the thermocouple action in any way, so long as the temperature, T1, is kept constant throughout the junction and the junction material is electrically conductive (Figure 1b). Similarly, the reading is not affected by insertion of non-thermocouple alloys in either or both leads, provided that the temperature at the ends of the “spurious” material is the same (Figure 1c).
A V1 B
T1
T2 T3
(a)
Figure 1a
T1
D
F V1
T1
G T1
E T2
(b)
Figure 1b
A V1 B B
T1 T3
C
T2
C T3
(c)
This ability of the thermocouple to work with a spurious metal in the transmission path enables the use of a number of specialized devices, such as thermocouple switches. Whereas the transmission wiring itself is normally the thermoelectrical equivalent of the thermocouple alloy, properly operating thermocouple switches must be made of goldplated or silver-plated copper alloy elements with appropriate steel springs to ensure good contact. So long as the temperatures at the input and output junctions of the switch are equal, this change in composition makes no difference. It is important to be aware of what might be called the Law of Successive Thermocouples. Of the two elements that are shown in the upper portion of Figure 1d, one thermocouple has T1 at the hot end and T2 at the open end. The second thermocouple has its hot end at T2 and its open end at T3. The emf level for the thermocouple that is measuring T1 is V1; that for the other thermocouple is V2. The sum of the two emfs, V1 plus V2, equals the emf V3 that would be generated by the combined thermocouple operating between T1 and T3. By virtue of this law, a thermocouple designated for one open-end reference temperature can be used with a different open-end temperature. RTD’s: A typical RTD consists of a fine platinum wire wrapped around a mandrel and covered with a
Figure 1c
A
D V1
V2
B
T1
E
T2 T2
T3 D
A V1
Figure 1. Assuming that certain conditions are met (text), thermocouple performance is not affected by temperature changes in wiring (a), the composition of the junction (b), nor the insertion of non-thermocouple alloys in the leads (c). As also discussed in text, thermocouple readings can be additive (d).
T3 = V1+ V2
B T1 (d)
T2
E T3
Figure 1d
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protective coating. Usually, the mandrel and coating are glass or ceramic. The mean slope of the resistance vs. temperature plot for the RTD is often referred to as the alpha value (Figure 2), alpha standing for the temperature coefficient. The slope of the curve for a given sensor depends somewhat on the purity of the platinum in it. The most commonly used standard slope, pertaining to platinum of a particular purity and composition, has a value of 0.00385 (assuming that the resistance is measured in ohms and the temperature in degrees Celsius). A resistance vs. temperature curve drawn with this slope is a so-called European curve, because RTD’s of this composition were first used extensively on that continent. Complicating the picture, there is also another standard slope, pertaining to a slightly different platinum composition. Having a slightly higher alpha value of 0.00392, it follows what is known as the American curve. If the alpha value for a given RTD is not specified, it is usually 0.00385. However, it is prudent to make sure of this, especially if the temperatures to be measured are high. This point is brought out in Figure 2, which shows both the European and American curves for the most widely used RTD, namely one that exhibits 100 ohms resistance at 0°C. Thermistors: The resistancetemperature relationship of a thermistor is negative and highly nonlinear. This poses a serious problem for engineers who must design their own circuitry. However, the difficulty can be eased by using thermistors in matched pairs, in such a way that the nonlinearities offset each other. Furthermore, vendors offer panel meters and controllers that compensate internally for thermistors’ lack of linearity. Thermistors are usually designated in accordance with their resistance at 25°C. The most common of these ratings is 2252 ohms; among the others are 5,000 and 10,000 ohms. If not specified to the contrary, most instruments will accept the 2252 type of thermistor.
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Resistance
Practical Guidelines for Temperature Measurement Cont'd a surface, be sure that the surface completely fills the field of view. If the target surface does not at first fill the field of view, move closer, or use an instrument with a more narrow field of view. Or, simply take the background temperature into account (i.e., adjust for it) when reading the instrument.
α = .00392 (American Curve)
α = .00385 (European Curve)
100 ohms
0°C
Temperature
Figure 2. A given RTD embodies either of two standard resistance-vs.-temperature relationships, often referred to as alpha values. The wise engineer will not use an RTD, especially for high-temperature measurements, without being aware of its alpha value
Infrared sensors: These measure the amount of radiation emitted by a surface. Electromagnetic energy radiates from all matter regardless of its temperature. In many process situations, the energy is in the infrared region. As the temperature goes up, the amount of infrared radiation and its average frequency go up. Different materials radiate at different levels of efficiency. This efficiency is quantified as emissivity, a decimal number or percentage ranging between 0 and 1 or 0% and 100%. Most organic materials, including skin, are very efficient, frequently exhibiting emissivities of 0.95. Most polished metals, on the other hand, tend to be inefficient radiators at room temperature, with emissivity or efficiency often 20% or less. To function properly, an infrared measurement device must take into account the emissivity of the surface being measured. This can often be looked up in a reference table. However, bear in mind that tables cannot account for localized conditions such as oxidation and surface roughness. A sometimes practical way to measure temperature with infrared when the emissivity level is not known is to
“force” the emissivity to a known level, by covering the surface with masking tape (emissivity of 95%) or a highly emissive paint. Some of the sensor input may well consist of energy that is not emitted by the equipment or material whose surface is being targeted, but instead is being reflected by that surface from other equipment or materials. Emissivity pertains to energy radiating from a surface, whereas “reflection” pertains to energy reflected from another source. Emissivity of an opaque material is an inverse indicator of its reflectivity – substances that are good emitters do not reflect much incident energy, and thus do not pose much of a problem to the sensor in determining surface temperatures. Conversely, when one measures a target surface with only, say, 20% emissivity, much of the energy reaching the sensor might be due to reflection from, e.g., a nearby furnace at some other temperature. In short, be wary of hot, spurious reflected targets. An infrared device is like a camera, and thus covers a certain field of view. It might, for instance, be able to “see” a 1-degree visual cone or a 100-degree cone. When measuring Z-15
Selection guides RTD’s are more stable than thermocouples. On the other hand, as a class, their temperature range is not as broad: RTD’s operate from about -250 to 850°C, whereas thermocouples range from about -270 to 2,300°C. Thermistors have a more restrictive span, being commonly used between -40 and 150°C, but offer high accuracy in that range. Thermistors and RTD’s share a very important limitation. They are resistive devices, and accordingly they function by passing a current through a sensor. Even though only a very small current is generally employed, it creates a certain amount of heat and thus can throw off the temperature reading. This selfheating in resistive sensors can be significant when dealing with a still fluid (i.e., one that is neither flowing nor agitated), because there is less carry-off of the heat generated. This problem does not arise with thermocouples, which are essentially zero-current devices. Infrared sensors, though relatively expensive, are appropriate when the temperatures are extremely high. They are available for up to 3,000°C (5,400°F), far exceeding the range of thermocouples or other contact devices. The infrared approach is also attractive when one does not wish to make contact with the surface whose temperature is to be measured. Thus, fragile or wet surfaces, such as painted surfaces coming out of a drying oven, can be monitored in this way. Substances that are chemically reactive or electrically noisy are ideal candidates for infrared measurement. The approach is likewise advantageous in measuring temperature of very large surfaces, such as walls, that would require a large array of thermocouples or RTD’s for measurement. ®
Physical Properties of Thermoelement Materials Thermoelement Material Property Melting point (solidus temp.) °C °F Resistivity µΩ·cm at 0°C at 20°C Ω cmil/ft at 0°C at 20°C Temperature coefficient of resistance, Ω/Ω· °C (0 to 100°C)
J Iron
J, C, T Constantan
T Copper
K, E Chromel
K Alumel
N Nicrosil
N Nisil
1490 2715
1220 2228
1083 1981
1427 2600
1399 2550
1420 2590
1330 2425
1860 3380
1850 3362
1769 3216
1927 3501
1826 3319
8.57 9.67 51.5 58.2
48.9 48.9 294.2 294
1.56 1.724 9.38 10.37
70 70.6 421 425
28.1 29.4 169 177
97.4 97.8
32.5 34.6
19.0 19.6 114.3 117.7
18.4 18.9 110.7 114.0
9.83 10.4 59.1 62.4
19.0
17.5
114.5
106
65 x 10-4
-0.1 x 10-4
Coefficient of thermal expansion 11.7 x 10-6 14.9 x 10-6 in./in. °C (20 to 100°C) Thermal conductivity at 100°C Cal·cm/s·cm2·°C BTU·ft/h·ft2·°F
4.3 x 10-4 4.1 x 10-4
23.9 x 10-4
13.3 x 10-4 12.1 x 10-4
16.6 x 10-6 13.1 x 10-6 12.0 x 10-6
0.162 39.2
0.0506 12.2
0.901 218
0.046 11.1
0.071 17.2
0.0358 8.67
0.0664 16.07
Specific heat at 20°C, cal/ g·°C
0.107
0.094
0.092
0.107
0.125
0.11 8.52
0.12 8.70
Density g/cm3 lb/in3
7.86 0.284
8.92 0.322
8.92 0.322
Tensile strength (annealed) MPa psi
345 50,000
552 80,000
241 35,000
Magnetic attraction
strong
none
none
8.73 0.315
8.60 0.311
0.3078
655 95,000
586 85,000
none
moderate
R J Pt13% Rh Pt10% Rh
R,E B Platinum Pt30% Rh
15.6 x 10-4 16.6 x 10-4 39.2 x 10-4 13.3 x 10-4
9.0 x 10-6
9.0 x 10-6
9.0 x 10-6
0.088 21.3
0.090 21.8
0.171 41.4
B Pt6% Rh
20.6 x 10-4
0.032
0.3143
19.61 0.708
19.97 0.721
21.45 0.775
17.60 0.636
20.55 0.743
690 100,000
621 90,000
317 46,000
310 45,000
138 20,000
483 70,000
276 40,000
none
none
none
none
none
none
none
Omegalloy® Nicrosil
Omegalloy® Nisil
Platinum 13% Rhodium
Platinum 10% Rhodium
Pure Platinum
Platinum 30% Rhodium
Platinum 6% Rhodium
N=Neg JN,TN KP, P=Pos JP ENa TP EP ElementNominal Chemical Composition, % Iron 99.5 … … … b Carbon … … … b … … … Manganese b Sulfur … … … b … … … Phosphorus b Silicon … … … b Nickel 45 … 90 b Copper 55 100 … b Chromium … … 10 Aluminum … … … … Platinum … … … … Rhodium … … … … Magnesium … … … …
ALOMEGA®
CHROMEGA®
Copper
Constantan
Iron
Nominal Chemical Composition of Thermoelements
KN
NP
NN
RP
SP
RN, SN
BP
BN
… … 2 … … 1 95 … … 2 … … …
… … … … … 1.4 84.4 … 14.2 … … … …
… … … … … 4.4 95.5 … … … … … 0.15
… … … … … … … … … … 87 13 …
… … … … … … … … … … 90 10 …
… … … … … … … … … … 100 … …
… … … … … … … … … … 70.4 29.6 …
… … … … … … … … … … 93.9 6.1 …
aTypes JN, TN and EN thermoelements usually contain small amounts of various elements for control of thermal emf, with corresponding reductions in the nickel or copper content, or both. bThemoelectric iron ((JP) contains small but varying amounts of these elements.
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OMEGACLAD® SHEATH SELECTION GUIDE APPLICATIONS U Heat Treating Metal Parts U Gas or Oil Fired Furnaces U Fuel Fired Heat Exchangers U Ceramic Materials Firing U Powder Metal Sintering U Steel Carburizing Furnaces U Vacuum/Atmosphere Melting & Annealing U Solid Waste Incinerators U Heat Process Fluidized Beds U R&D Tube or Box Furnaces
The metallic sheath on the outside of an OMEGACLAD® probe is used to protect the internal thermocouple wires from chemically active atmospheres. In some cases, even hot air can damage thermocouple wires and cause them to permanently lose calibration. Selection of the best type of metal sheath to employ is based on our customers’ intended use, the industry in which they work, and the country where they are located. For instance, the most common OMEGA® metal sheaths are 304 stainless steel and Inconel 600. These are accepted in most industries, including food processing. Stainless steel 304 is a common alloy, readily available and low in the cost of both materials and manufacture. Some industries, however, such as petroleum, medical, nuclear, aircraft, and power generation, have their own standards and may require more complicated and expensive alloys. Listed below are the sheath materials that OMEGA Engineering uses to make OMEGACLAD®. Any materials not on this list must be customized; direct inquiries will have to made to OMEGA South for pricing, availability and size limitations.
304 Stainless Steel
OMEGA SUPERCLAD™
OMEGA Engineering uses a lowcarbon version of 304 stainless, called 304L, mainly because it is easier to weld. In general, it is interchangeable with plain 304.
This alloy has excellent resistance to air at high temperatures. It has an aluminum oxide layer on the surface that prevents further oxidation. This oxidation resistance allows thermocouple probes to operate for extended periods before EMF drift “decalibrates” the thermocouple. It is also popular for its resistance to hydrogen gas and its high strength at high temperatures. Because of form limitations and difficulty in processing, it is more expensive than any of the alloys discussed above.
Applications: Food & beverage processing Chemical processing Dairy Hospital equipment Pharmaceutical equipment Nuclear reactor equipment Containers for mild corrosives Temperature limitations: up to 1,600°F for cyclic processes. Use Inconel 600 for extended use around or above 1,650°F
Inconel 600 This high nickel and chromium content alloy is more expensive than most stainless steels. It is good for extended use at high temperatures and resists corrosion by most simple acids and very pure water. Applications: Furnace components Chemical & food processing Nuclear power generation Caustic chemicals Temperature limitations: up to 2,100°F
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Applications: Furnace components Gas turbine industry Catalytic converter components Aerospace jet & rocket engines Refractory anchors Waste incinerators Temperature limitations: Approx. 2,220°F Also is acceptable in heated hydrogen, ≈ 2000°F
SUPER OMEGACLAD® SHEATH
THERMOCOUPLE WIRE
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CERAMIC INSULATION
310 Stainless Steel
321 Stainless Steel
Hastelloy-X
This is commonly used at higher temperatures because it resists scaling up to 1,900°F. It is stronger and resists air attack better than 304SS at these higher temperatures. Also good in fossil fuel gases at elevated temperatures.
This alloy is similar to 304 stainless except that it incorporates titanium. It is intended for welded components that are exposed to high temperatures, and is especially well suited to long exposure to air and combustion atmospheres of around 800°F.
This alloy is expensive due to the addition of iron, chromium and molybdenum. It has very good high temperature strength and good oxidation resistance. It is a relatively old alloy, less costly and with better performance than some newer alloys.
Applications: (Higher temperatures) Air heaters Baking equipment Chemical processing equipment Furnace parts Heat exchangers and electric power equipment (that does not come in contact with sulphur) Petroleum refining
Applications: Aircraft exhausts & manifolds Jet engine parts Stack liners Welded equipment Chemical processing equipment
Applications: Gas Turbines for power generation Aerospace applications Industrial furnaces Boiler & pressure vessels
Temperature limitations: up to 1,600°F
Temperature limitations: up to 2,150°F
Temperature limitations: up to 1,900°F
316 (& 316L) Stainless Steel Better corrosion resistance to most chemicals, salts, and acids than most stainless steels due to the addition of molybdenum. It has good resistance to sulphur- or chlorinebearing liquids. Applications: Marine trim exteriors Chemical and food processing Petroleum refining equipment Pharmaceutical equipment Paper & pulp Textile finishing Temperature limitations: up to 1,600°F continuously in air or in cyclic corrosive environments, slightly higher in air. Thermocouple Wire Stripper for OMEGACLAD® wire. See PST Series Strippers in Section H.
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Practical Temperature Measurements*
T
I. C. Sensor V or I
R RESISTANCE
RESISTANCE
VOLTAGE TEMPERATURE
Advantages
Thermistor
R
V
Disadvantages
RTD
TEMPERATURE
T
VOLTAGE or CURRENT
Thermocouple
TEMPERATURE
T TEMPERATURE
T
□ Self-powered □ Simple □ Rugged □ Inexpensive □ Wide variety □ Wide temperature range
□ Most stable □ Most accurate □ More linear than thermocouple
□ High output □ Fast □ Two-wire ohms measurement
□ Most linear □ Highest output □ Inexpensive
□ Non-linear □ Low voltage □ Reference required □ Least stable □ Least sensitive
□ Expensive □ Current source required □ Small ∆ R □ Low absolute resistance □ Self-heating
□ Non-linear □ Limited temperature range □ Fragile □ Current source required □ Self-heating
□ T0 0. 11 (typical) T < 0 The exact values for coefficients α , β , and δ are determined by testing the RTD at four temperatures and solving the resultant equations. This familiar equation was replaced in 1968 by a 20th order polynomial in order to provide a more accurate curve fit. The plot of this equation shows the RTD to be a more linear device than the thermocouple:
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THE THERMISTOR
12 .390 .344 .293
8
Equivalent Linearities Type S Thermocouple vs. Platinum RTD
4
0
200
400
600
Like the RTD, the thermistor is also a temperature sensitive resistor. While the thermocouple is the most versatile temperature transducer and the PRTD is the most stable, the word that best describes the thermistor is sensitive. Of the three major categories of sensors, the thermistor exhibits by far the largest parameter change with temperature.
Resistance Temperature Coefficient - RTD
Type S µv/°C Seebeck Coefficient
16
Thermistors are generally composed of semiconductor materials. Although positive temperature coefficient units are available, most thermistors have a negative temperature coefficient (TC); that is, their resistance decreases with increasing temperature. The negative T.C. can be as large as several percent per degree Celsius, allowing the thermistor circuit to detect minute changes in temperature which could not be observed with an RTD or thermocouple circuit.
800
Temperature, °C
Figure 46
Practical Precautions
The price we pay for this increased sensitivity is loss of linearity. The thermistor is an extremely non-linear device which is highly dependent upon process parameters. Consequently, manufacturers have not standardized thermistor curves to the extent that RTD and thermocouple curves have been standardized.
The same practical precautions that apply to thermocouples also apply to RTD’s, i.e., use shields and twisted-pair wire, use proper sheathing, avoid stress and steep gradients, use large extension wire, keep good documentation and use a guarded integrating dvm. In addition, the following precautions should be observed.
An individual thermistor curve can be very closely approximated through use of the Steinhart-Hart equation:18
Construction - Due to its construction, the RTD is somewhat more fragile than the thermocouple, and precautions must be taken to protect it.
v or R
Self-Heating - Unlike the thermocouple, the RTD is not self-powered. A current must be passed through the device to provide a voltage that can be measured. The current causes Joule (I2R) heating within the RTD, changing its temperature. This self-heating appears as a measurement error. Consequently, attention must be paid to the magnitude of the measurement current supplied by the ohmmeter. A typical value for selfheating error is 12ºC per milliwatt in free air. Obviously, an RTD immersed in a thermally conductive medium will distribute its Joule heat to the medium, and the error due to self-heating will be smaller. The same RTD that rises 1ºC per milliwatt in free air will rise only 110 ºC per milliwatt in air which is flowing at the rate of one meter per second.10
RTD Thermocouple
T Figure 47 1 T = A + BlnR + C (In R)3
To reduce self-heating errors, use the minimum ohms measurement current that will still give the resolution you require, and use the largest RTD you can that will still give good response time. Obviously, there are compromises to be considered.
where: T = Degrees Kelvin
Thermal Shunting - Thermal shunting is the act of altering the measurement temperature by inserting a measurement transducer. Thermal shunting is more a problem with RTD’s than with thermocouples, as the physical bulk of an RTD is greater than that of a thermocouple.
Small RTD
Large RTD
Fast Response Time Low Thermal Shunting High Self-Heating Error
Slow Response Time Poor Thermal Shunting Low Self-Heating Error
R = Resistance of the thermistor A,B,C = Curve-fitting constants
Thermal EMF - The platinum-to-copper connection that is made when the RTD is measured can cause a thermal offset voltage. The offset-compensated ohms technique can be used to eliminate this effect. 10 Refer
Thermistor
to Bibliography 6.
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A, B, and C are found by selecting three data points on the published data curve and solving the three simultaneous equations. When the data points are chosen to span no more than 100ºC within the nominal center of the thermistor’s temperature range, this equation approaches a rather remarkable ±.02°C curve fit. Somewhat faster computer execution time is achieved through a simpler equation: B T= ——— - C In R-A where A, B, and C are again found by selecting three (R,T) data points and solving the three resultant simultaneous equations. This equation must be applied over a narrower temperature range in order to approach the accuracy of the Steinhart-Hart equation.
Linear Thermistors
MONOLITHIC LINEAR TEMPERATURE SENSOR A recent innovation in thermometry is the integrated circuit temperature transducer. It is available in both voltage and current-output configurations. Both supply an output that is linearily proportional to absolute temperature. Typical values are 1 µA/K and 10 mV/K. Except for the fact that they offer a very linear output with temperature, these devices share all the disadvantages of thermistor devices and thus have a limited temperature range. The same problems of selfheating and fragility are evident, and they require an external power source. These devices provide a convenient way to produce an analog voltage proportional to temperature. Such a need arises in a hardware thermocouple reference junction compensation circuit (see Figure 15).
A great deal of effort has gone into the development of thermistors which approach a linear characteristic. These are typically 2- or 4-leaded devices requiring external matching resistors to linearize the characteristic curve. The modern data acquisition system with its computing controller has made this kind of hardware linearization unnecessary.
+
+ i = 1µ A/K 10mv/ K 10kΩ
Measurement The high resistivity of the thermistor affords it a distinct measurement advantage. The four-wire resistance measurement is not required as it is with RTD’s. For example, a common thermistor value is 5000 ohms at 25’C. With a typical T.C. of 4%/ºC, a measurement lead resistance of 100 produces only a .05°C error. This error is a factor of 500 times less than the equivalent RTD error.
To DVM
To DVM
B
A CURRENT SENSOR
VOLTAGE SENSOR Figure 48
Disadvantages - Because they are semiconductors, thermistors are more susceptible to permanent decalibration at high temperatures than are RTD’s or thermocouples. The use of thermistors is generally limited to a few hundred degrees Celsius and manufacturers warn that extended exposures even well below maximum operating limits will cause the thermistor to drift out of its specified tolerance.
APPENDIX A
Thermistors can be made very small which means they will respond quickly to temperature changes. It also means that their small thermal mass makes them especially susceptible to self-heating errors. Thermistors are a good deal more fragile than RTD’s or thermocouples and they must be carefully mounted to avoid crushing or bond separation.
11
The Empirical Laws of Thermocouples11 The following examples illustrate the empirically derived “laws” of thermocouples which are useful in understanding and diagnosing thermocouple circuits.
Refer to Bibliography 2.
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APPENDIX B +
Fe
Cu
C
Cu
T
v
--
Thermocouple Characteristics
Fe Tl
C
Over the years, specific pairs of thermocouple alloys have been developed to solve unique measurement problems. Idiosyncrasies of the more common thermocouples are discussed here.
Isother mal Block T1
THE LAW OF INTERMEDIATE METALS
We will use the term standard wire error to refer to the common commercial specifications published in the Annual Book of ASTM Standards. It represents the allowable deviation between the actual thermocouple output voltage and the voltage predicted by the tables in NBS Monograph 125.
Inserting the copper lead between the iron and constantan leads will not change the output voltage V, regardless of the temperature of the copper lead. The voltage V is that of an Fe-C thermocouple at temperature T1.
Fe
+
Noble Metal Thermocouples - The noble metal thermocouples, types B, R, and S, are all platinum or platinum-rhodium thermocouples and hence share many of the same characteristics.
Fe T
v C
--
T
Diffusion - Metallic vapor diffusion at high temperatures can readily change platinum wire calibration; therefore, platinum wires should only be used inside a non-metallic sheath such as high-purity alumina. The one exception to this rule is a sheath made of platinum, but this option is prohibitively expensive.
C
C Isother mal Block T1
Stability - The platinum-based couples are by far the most stable of all the common thermocouples. Type S is so stable that it is specified as the standard for temperature calibration between the antimony point (630.74°C) and the gold point (1064.43ºC).
THE LAW OF INTERIOR TEMPERATURES The output voltage V will be that of an Fe-C couple at Temperature T, regardless of the external heat source applied to either measurement lead.
Type B - The B couple is the only common thermocouple that exhibits a double-valued ambiguity. C
+
C T
v Fe
Fe
--
T Fe
Isother mal Block T1 Pt
Due to the double-valued curve and the extremely low Seebeck coefficient at low temperatures, Type B is virtually useless below 50°C. Since the output is nearly zero from 0°C to 42°C, Type B has the unique advantage that the reference junction temperature is almost immaterial, as long as it is between 0º and 40ºC. Of course, the measuring junction temperature is typically very high.
THE LAW OF INSERTED METALS
v
The voltage V will be that of an Fe-C thermocouple at temperature T, provided both ends of the platinum wire are at the same temperature. The two thermocouples created by the platinum wire (FePt and Pt -Fe) act in opposition.
Double-Value Region
0
All of the above examples assume the measurement wires are homogeneous; that is, free of defects and impurities.
12
Refer to Bibliography 3
42
T, ˚C
Base Metal Thermocouples Unlike the noble metal thermocouples, the base metal couples have no specified chemical composition. Any combination of metals can be used which results in a voltage vs. temperature curve fit that is within the standard wire errors. This leads to some rather interesting metal combinations. Constantan, for example, is not a specific metal alloy at all, but a generic name for a whole series of copper-nickel alloys. Incredibly, the Constantan used in a type T (copperConstantan) thermocouple is not the same as the Constantan used in the type J (iron -Constantan) couple.12 Z-38
Z
ASTM STANDARD WIRE ERRORS14
Type E - Although Type E standard wire errors are not specified below 0°C, the type E thermocouple is ideally suited for low temperature measurements because of its high Seebeck coefficient (58 µV/°C), low thermal conductivity and corrosion resistance. The Seebeck coefficient for Type E is greater than all other standard couples, which makes it useful for detecting small temperature changes. Type J - Iron, the positive element in a J couple, is an inexpensive metal rarely manufactured in pure form. J thermocouples are subject to poor conformance characteristics because of impurities in the iron. Even so, the J couple is popular because of its high Seebeck coefficient and low price. The J couple should never be used above 760°C due to an abrupt magnetic transformation that can cause decalibration even after the instrument cools.
170 °C
871
± 8.5 °C
± 4.4 1
/2 % Slope
TYPE B 24 AWG
0
v=
± 1.4
T1
Cu
1
/4 %
(T1 _ T2)
C
TYPE R,S 24 AWG
_ Cu Voltmeter
0
T2 (Ambient Reference)
871 °C
316
Cu
± 4.4 °C
TYPE T
Type T - This is the only couple with published standard wire errors for the temperature region below 0°C; however, type E is actually more suitable at very low temperatures because of its higher Seebeck coefficient and lower thermal conductivity. Type T has the unique distinction of having one copper lead. This can be an advantage in a specialized monitoring situation where a temperature difference is all that is desired. The advantage is that the copper thermocouple leads are the same metal as the dvm terminals, making lead compensation unnecessary. Types K & Nicrosil-Nisil - The Nicrosil-Nisil thermocouple, type N, is similar to type K, but it has been designed to minimize some of the instabilities in the conventional Chromel-Alumel combination. Changes in the alloy content have improved the order/disorder transformations occurring at 500˚C, and a higher silicon content in the positive element improves the oxidation resistance at elevated temperatures. A full description with characteristic curves is published in NBS Monograph 161.13 Tungsten - Tungsten-rhenium thermocouples are normally used at high temperature in reducing or vacuum environments, but never in an oxidizing atmosphere because of the high reaction rates. Pure tungsten becomes very brittle when heated above its recrystallization temperature (about 1200°C). To make the wire easier to handle, rhenium alloys are used in both thermocouple legs. Types G (tungsten vs. tungsten 26% rhenium), C (tungsten 5% rhenium vs. tungsten 26% rhenium) and D (tungsten 3% rhenium vs. tungsten 25% rhenium) thermocouples are available in bare wire forms as well as complete probe assemblies. All materials conform to published Limits of Error. ®
Refer to Bibliography 14. 14 Refer to Bibliography 3. 13
1482 °C
± 3.7 °C
Cu
+
538
± 1.7 1
/2 %
TYPE E 8 AWG
_
101
_
371 °C
59 93
2% ± 1.2
± 2.8 °C
± .8 3
/4 %
TYPE T 14 AWG
0
277
760 °C
± 5.7 °C ± 2.2 3
/4 %
TYPE J 8 AWG
0
At high temperatures, small thermocouple wire is affected by diffusion, impurities, and inhomogeneity more so than large wire. The standard wire errors reflect this relationship.
277
°C
1260
± 9.5 ± 2.2 3
/4 %
TYPE K 8 AWG
Note that each NBS wire error specification carries with it a wire size. The noble metal thermocouples (B, R, and S) are specified with small (24 ga.) wire for obvious cost reasons.
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1260 °C
1093
982
AWG DIA, MILS DIA, mm
±2.2°C
34 %
Wire Size AWG
24 or 28 20 14
8 10 12 14 16 18 20 22 24 26 28
Error
±9.5°C
0°C
277°C
871
TYPE K
8
TEMPERATURE RANGE vs. WIRE SIZE vs. ERROR
TYPE B E J K N (AWG 14) N (AWG 28) R S T W-Re
METAL + Platinum 6% Rhodium Nickel 10% Chromium Iron Nickel I0% Chromium Nicrosil Nicrosil Platinum13% Rhodium Platinum 10% Rhodium Copper Tungsten 5% Rheniurn
Platinum 30% Rhodium
STANDARD COLOR CODE + –
Ω/DOUBLE FOOT 20 AWG
128 102 81 64 51 40 32 25 20 16 13
SEEBECK COEFFICIENT S(µV/ºC) @ T (ºC)
°C STANDARD WIRE ERROR (SEE APPENDIX B)
0.2
6
600
4.4 to 8.6
3.3 2.6 2.1 1.6 1.3 1 0.8 0.6 0.5 0.4 0.3 NBS SPECIFIED † MATERIAL RANGE† (ºC) 0 to 1820*
Constantan Constantan
Violet White
Red Red
0.71 0.36
58.5 50.2
0 0
1.7 to 4.4 1.1 to 2.9
-270 to 1000 - 210 to 760
Nickel Nisil
Yellow Red –
0.59 –
39.4 39
0 600
1.1 to 2.9 –
-270 to 1372 0 to 1300
Nisil
–
–
26.2
0
–
-270 to 400
Platinum
–
0.19
11.5
600
1.4 to 3.8
-50 to 1768
Platinum Constantan Tungsten 26% Rhenium
–
0.19 0.30
10.3 38
600 0
1.4 to 3.8 0.8 to 2.9
-50 to 1768 -270 to 400
–
19.5
600
–
Blue
Red –
0 to 2320
* Type B double-valued below 42°C - curve fit specified only above 130°C † Material range is for 8 AWG wire; decreases with decreasing wire size
BIBLIOGRAPHY Thermocouple Well: Lower gradient, protects wire and allows user to change thermocouple without interrupting process.
1. 2. 3. 4.
5. 6. 7.
8. 9. 10. 11.
Connector: Composed of same metals as thermocouple, for minimum connection error.
12. 13. 14.
Exposed
Ungrounded
Grounded 15. 16.
Exposed Junction: Wires unprotected, faster response. Ungrounded Junction: Best protection, electronically isolated. Grounded Junction: WIres protected, faster response.
17. 18.
Thermocouple Washers: Couple built into washer; convenient mounting.
Charles Herzfeld, F.G. Brickwedde: Temperature - Its Measurement and Control in Science and Industry, Vol. 3, Part 1, Reinhold, New York, 1962. Robert P. Benedict: Fundamentals of Temperature, Pressure and Flow Measurements, John Wiley & Sons, Inc., New York, 1969. Manual on the Use of Thermocouples in Temperature Measurement, ASTM Special Publication 470A, Omega Press, Stamford, Connecticut 06907, 1974. Thermocouple Reference Tables, NBS Monograph 125, National Bureau of Standards, Washington, D.C., 1979. Also, TemperatureMillivolt Reference Tables-Section T, Omega Temperature Measurement Handbook, Omega Press, Stamford Connecticut 06907,1983. H. Dean Baker, E.A. Ryder, N.H. Baker: Temperature Measurement in Engineering, Omega Press, Stamford, Connecticut 06907, 1953. Temperature Measurement Handbook, Omega Engineering, Inc., Stamford, Connecticut. R.L. Anderson: Accuracy of Small Diameter Sheathed Thermocouples for the Core Flow Test Loop, Oak Ridge National Laboratories, ORNL-54011 (available from National Information Service), April, 1979. R. R Reed: Branched Thermocouple Circuits in Underground Coal Gasification Experiments, Proceedings of the 22nd ISA International Instrumentation Symposium, Instrument Society of America, 1976. R.J. Moffat: The Gradient Approach to Thermocouple Circuitry, from Temperature - Its Measurement and Control in Science and Industry, Reinhold, New York, 1962 R.P. Reed: A Diagnostics-Oriented System for Thermocouple Thermometry, Proceedings of 24th ISA International Instrumentation Symposium, Instrument Society of America, 1978. Harry R. Norton: Handbook of Transducers for Electronic Measuring Systems, Prentice-Hall, Englewood Cliffs, New Jersey. C.H. Meyers: Coiled Filament Resistance Thermometers, NBS Journal of Research, Vol. 9, 1932. Bulletin 9612, Rev. B: Platinum Resistance Temperature Sensors, Rosemount Engineering Co., 1962. Burley, Powell, Burns & Scroger: The Nicrosil vs. Nisil Thermocouple: Properties and Thermoelectric Reference Data, NBS Monograph 161, U.S. Dept. of Commerce, Washington, D.C., 1978 J.P Tavener: Platinum Resistance Temperature Detectors - State of the Art, Measurements & Control, Measurements & Data Corporation, Pittsburgh, PA., April, 1974. J.P. Evans and G.W. Burns: A Study of Stability of High Temperature Platinum Resistance Thermometers, in Temperature - Its Measurement and Control in Science and Industry, Reinhold, New York, 1962. D.D. Pollock: The Theory and Properties of Thermocouple Elements, ASTM STP 492, Omega Press, Stamford, Connecticut 06907, 1979. YSI Precision Thermistors, Yellow Springs Instruments, Yellow Springs, Ohio, 1977.
* Hewlett Packard Company makes no warranty as to the accuracy or completeness of the foregoing material and disclaims any responsibility therefor. (Editor’s Note: Thermocouple data which conform to ITS-90 are given in “ITS-90 Thermocouple Direct and Inverse Polynomials.”) OMEGA ENGINEERING, INC. gratefully acknowledges the HEWLETT PACKARD COMPANY for permission to reproduce Application Note 290-Practical Temperature Measurements.
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Nicrosil/Nisil Type N Thermocouples The Nicrosil/Nisil Type N thermocouple offers better stability than existent base-metal Types E, J, K and T. It is now available and in widespread use worldwide.
DR. NOEL A. BURLEY
T he
ANSI standard base-metal ther mocouples, designated E, J, K and T (Ref. 1), show inherent ther moelectric instability related to time- and/or temperature-dependent instabilities in several of their physical, chemical, nuclear, structural and electronic properties. This paper reviews the major thermoelectric properties of the new nickel-base thermocouple system Nicrosil versus Nisil (designated type N), in which very high thermoelectric stability has been achieved by a judicious choice of elemental component concentrations.
INSTABILITY OF CONVENTIONAL BASE-METAL THERMOCOUPLES There are three principal characteristic types and causes of thermoelectric instability in the standard base-metal thermoelement materials: 1. A gradual and generally cumulative drift in thermal EMF on long exposure at elevated temperatures. This is observed in all base-metal thermoelement materials and is majnly due to compositional changes caused by oxidation, in particular internal oxidation (Figures 1 and 2), and to neutron irradiation which can produce transmutation in nuclear reactor environments. 2. A short-term cyclic change in thermal EMF on heating in the temperature range about 250º to 650ºC, which occurs in types KP (or EP) and JN (or TN and EN). This kind of EMF instability is thought to be due to some form of structural change like magnetic shortrange order (Figures 3 and 4). 3. A time-independent perturbation in thermal EMF in specific temperature ranges. This is due to compositiondependent magnetic transformations which perturb the thermal EMF’s in type KN in the range of about 25º to 225ºC (Figure 5), and in type JP above about 730ºC.
ULTRA-HIGH STABILITY OF NICROSILINISIL (TYPE N) THERMOCOUPLE Nicrosil and Nisil thermocouple alloys (Ref. 2) show greatly enhanced thermoelectric stability (Ref. 3) relative to the other standard base-metal thermocouple alloys because their compositions (Table 1) are such as to virtually eliminate or substantially reduce the causes of thermoelectric instability described above. This is achieved primarily by increasing component solute concentrations (chromium and silicon) in a base of nickel above those required to cause a transition from internal to external modes of oxidation, and by selecting solutes (silicon and magnesium) which preferentially oxidize to form a diffusion-barrier, and hence oxidation inhibiting films. The thermal EMF instabilities of the short-term cyclic kind occurring in KP and JN alloys have virtually been eliminated in nicrosil (NP) by setting the chromium content at 14.2 weight-%. The increase in the silicon content of nisil (NN) to 4.4 weight-% has suppressed the magnetic transformation of this new alloy to below room temperature. Virtual freedom from nuclear transmutation effects is achieved by eliminating such elements as manganese, cobalt and iron from the specified compositions of both alloys. The ver y high ther moelectric stability of the Nicrosil/Nisil (type N) thermocouple is illustrated in Figures 1 and 2. The influence of thermoelement conductor cross-sectional area upon the thermal-EMF constancy of Nicrosil/Nisil is shown in Figure 6.
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300 #8K (KP/KN) 100
#8K
6
Z
200
(KP/KN) #14K
4
0 #14 NIC/NIS (KP/JN) (KP/JN)
–200
100
2
#8 NIC/NIS
#14 E #8E
0
0 200
#8K 4
(JP/JN) #8J
THERMAL EMF DRIFT (uv)
–600
CALIBRATION TEMPERATURE 497°C
#14 J –800 250 #14 0 NIC/NIS
2
#10 NIC/NIS
#14 E #8 E
0 200
0 #8K 4
#12 NIC/NIS 100
2 0 200
0 #8K 4
100 2
#14 NIC/NIS
–500
0 300
0
6
#8J
–1000
#8K
200
4
CALIBRATION TEMPERATURE 777°C #16 NIC/NIS
100 –1500
2
#14 J
0
0 0
300
600
900
1200
DRIFT (°C)
(JP/JN)
100
THERMAL EMF DRIFT (uv)
–400
1500
0
200
EXPOSURE TIME AT 777°C (h) AT
600
800
1000
1200
EXPOSURE TIME (h) 1077°C, 1152°C, 1202°C
FIGURE 2. Long-term thermal-EMF drifts in air, at three constant aging (and calibration) temperatures for Nicrosil/Nisil T/Cs in five wire gauges (#). Corresponding thermal-EMF drifts for 8 AWG (#8) type K T/Cs at two of these temperatures are also given. The drifts are changes from EMF output values existent after 80 hours of exposure at the constant aging temperature (Ref. 3).
FIGURE 1. Long-term thermal-EMF drifts in air, at two calibration temperatures, for 14 AWG (#14) Nicrosil/Nisil (N) and E, J and K T/Cs. ThermalEMF drifts for 8 AWG (#8) E and J T/Cs are also given. The drifts are changes from EMF output values existent after 20 hrs of exposure at constant aging temperature of 777°C (Ref. 3).
As Figure 2 shows, 8 AWG type K thermocouples appear to be markedly more unstable as temperatures progressively exceed about 1050ºC. In contrast, it is clear from Figure 6 that type N thermocouples, in a range of wire sizes finer than 8 AWG, can be used reliably for extended periods of time at temperatures up to at least 1200ºC. Indeed, it has recently been
400
demonstrated (Ref. 4) that, in oxidizing atmospheres, the ther moelectric stability of the Nicrosil/Nisil thermocouple, in wire sizes not finer than 10 AWG, is about the same as that of the noble-metal thermocouples of ANSI types R and S up to about 1200ºC.
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Type N Thermocouples PROMULGATION AS A STANDARD No new ther mocouple will sur vive for universal adoption and use unless it is formally promulgated by national standards authorities around the world. It is for tunate that the Nicrosil/Nisil thermocouple system is in vigorous process of being so promulgated.
0.2
0.6
30 Days
0.5
0.1
∆S (uV/°C)
0.05 0.4
0 –0.05
0.3
The ASTM, through its Committee E-20 on Temperature Measurement, has shown considerable interest in Nicrosil versus Nisil, and has kept matters relating to the development, availability and use of the new thermocouple under continual review.
5 min
–0.1 45 min
3 Days
0.2
–0.2 7h
0.1
45 min
ground state 0 200
400
600
3 Days 30 Days
–0.3
800
200
400
600
TEMPERATURE (°C)
FIGURE 3 (Left). Changes in the Seebeck coefficient (∆S) of a typical type KP thermoelement vs. platinum on initial heating, as a function of constant aging temperature for the indicated times (Ref. 3). FIGURE 4 (Right). Similar changes of a type JN thermoelement (Ref. 3).
TABLE.1- NOMINAL COMPOSITIONS OF ANSI STANDARD BASE-METAL THERMOELEMENT ALLOYS, AND NICROSIL AND NISIL ALLOYS ALLOY CHEMICAL COMPOSITION (WEIGHT-%) ANSI (1) Cr Si Mn Al Co Mg Cu Ni Fe DESIGNATION (+)KP, EP 9.5 0.4 bal (-)KN 1.0 3.0 2.0 0.4 0.015 bal (+)JP 0.3 bal (-)JN, EN, TN 1.0 0.5 54 44 0.5 (+)TP (+)NP (nicrosil) (-)NN (nisil)
100 14.2 1.4 4.4
0.10
bal bal
TABLE 2-VARIANTS OF TYPE KN ALLOY CHEMICAL COMPOSITION (WEIGHT-%) Mn Al Si co Ni KN1 3.02 1.90 1.19 0.41 balance KN2 1.67 1.25 1.56 0.72 balance KN3 2.50 1.00 balance KN4 0.43 2.39 0.23 balance Z-43
Recently, relevant subcommittees of ASTM E-20 have produced several publications containing information on the properties and characteristics of the Nicrosil versus Nisil thermocouple. A document quoting several of the EMF-temperature tables from NBS Monograph 161 (Ref. 2) was published (Ref. 6) for information. A formal ASTM Standard (E1223) is promulgated, while Type N data is now included in ASTM Standard E230. Again, in the recently published third edition of the ASTM Manual on the Use of Thermocouples (Ref. 8), various properties and characteristics of Nicrosil versus Nisil are summarized. Based mainly on the above information, several crucial actions now have been taken by the supreme standardizing bodies in several important countries: 1. The Instrument Society of America (ISA), in October 1983, promulgated the Nicrosil/Nisil system as a U.S. Standard Ther mocouple bearing the letter-designation “type N.” 2. The British Standards Institute (BSI) has recently promulgated a standard on the type N thermocouple identified as B.S.4937: Part 8. 3. The Japan Society for the Promotion of Science, through its Committee TC19 (Temperature), is nearing the conclusion of its deliberation on type N, leading to the issue of a Japan Industrial Standard (JIS). These actions have ensured that the type N ther mocouple and its allied pyrometric instrumentation and ancillary circuitry elements are now commercially available in a number of major countries around the world.
DISCUSSION The various types of ther moelectric instability described in this paper can cause substantial changes in thermoelectromotive force and, hence, calibration in ANSI-standard letterdesignated base-metal thermocouples types E, J, K and T. In the case of Nicrosil/Nisil, however, thermoelectric instability due to these causes is
REFERENCES 1. American National Standards Institute (ANSI) Standard MC96.1-1975, Instrument Society of America (1976), pp. vi and 1. 2. N.A. Burley, et al., U.S. National Bureau of Standards Monograph 161, NBS* Washington (1978). 3. N.A. Burley, et al., Temperature, Its Measurement and Control in Science and Industry, vol. 5, part 2, Instrument Society of America (1982), p. 1159. 4. N.A. Burley, Proc. 11th IMEKO Conference (Sensors), Houston, TX, 1988, p. 155. 5. R.L. Powell, et al., U.S. National Bureau of Standards Monograph 125, NBS* Washington (1974). 6. American Society for Testing and Materials (ASTM), Annual Book of Standards, vol. 14.01 (1983), p. 859. 7. ASTM Standard E 1223-87. 8. Manual on the Use of Thermocouples in Temperature Measurement, ASTM Special Technical Publication 470 B (1981). 9. N.A. Burley, et al., “The Nicrosil versus Nisil Type N Thermocouple: A Commercial Reality,” Australian Department of Defence Report MRL-R-903 (1983).
360
400
440
480
DEVIATION (uV)
40
520
1
KN2
0
0 KN1 KN3
–40
–1
–2
–80
KN4
–120
0
40
120
80
160
TEMPERATUREDEVIATION (K)
60
320
–3
200
240
TEMPERATURE (°C)
FIGURE 5. Deviations of the measured values of the thermal EMFs of several type KN thermoelements vs. platinum from reference table values (Ref. 5). Variants of type KN are given in Table 2. WIRE GAUGE (A W G) 16
250
14
12
10
8 6
200
5 1202°C 4
150 1152°C
3 100
DRIFT (°C)
Use of type N thermocouples in several countries has already demonstrated a number of advantages: enhanced pyrometric accuracy, improved product quality and performance, lower reject rates, enhanced energy utilization, lower pyrometric maintenance costs, and improved productivity.
TEMPERATURE (K) 280
THERMAL EMF DRIFT (uv)
virtually eliminated or substantially attenuated over the entire temperature range up to 1230˚C. ANSI-standard base-metal thermocouples types E, J, K and T can, therefore, be regarded as obsolescent. Their replacement by Nicrosil/Nisil thermocouples would lead, in most cases, to demonstrable technological and economic advantages for science and industry at large. Indeed, the enhanced calibration stability and longevity of the type N thermocouple, taken into account with its ability to operate at considerably higher upper operating temperatures than conventional base-metal thermocouples, make it ideally suited to scientific, technological and industrial applications where temperature measurements are critical.
2
1077°C 50
1
0 0
0.2
0.4
0.6
0.8
0 1.0
LOG CROSS-SECTIONAL AREA
FIGURE 6. Relationship between total thermalEMF drift (after 1000 hrs of exposure in air at each of three test temperatures) and crosssectional area of Nicrosil/Nisil T/C wires. The drifts are changes from EMF output values existent after 80 hours of exposure (Ref. 3).
THE AUTHOR
*The NBS is now NIST (National Institute of Standards and Technology).
Reproduced with permission of H.L. Daneman, Box 31056, Sante Fe, NM 87594
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DR. NOEL A. BURLEY, D.App.Sc., C. Eng., F.I.M., F.A.I.M., is General Manager, Research and Development, for BellIRH Pty., Ltd., an Australian company specializing in the manufacture of electrical and electronic components, instruments and sensors. It has considerable expertise and established reputation in temperature control. Contact Dr. Burley at Bell-IRH Pty., Ltd., 32 Paramatta Rd., Lidcombe NSW 2141, Sydney, Australia, phone: 02 648 5455.
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The Choice Of Sheathing For MineraI Insulated Thermocouples H.L. Daneman, P.E.
• • • •
Chemical isolation of wires from the surrounding atmosphere. Shielding of thermoelements from sources of electrical interference. Protection of the wires and insulation from damage due to shock. Flexibility of the final assembly allowing bending.
For two decades, people have credited MIMS construction with a greater capability than deserved. Quite frequently, this form has shown less stability, less durability and lower temperature limits than corresponding unsheathed elements. The nickel bearing MIMS thermocouples used above 400ºC (750ºF) are especially vulnerable to calibration instability and shortened lifetime - factors which bear heavily on thermocouple use and selection. HYSTERESIS Thermoelectric hysteresis is one contributor toward calibration instability. Hysteresis is a form of short-range order/disorder phenomenon occurring between 200 and 600ºC (peaking at ≈ 400ºC) for Ni-Cr alloys such as Type K. It is evidenced by a calibration change of several degrees as the thermocouple temperature is cycled within this temperature band. Type N thermocouples exhibit hysteresis of up to 5ºC when heated and cooled between 200 and 1000ºC (peaking around 750ºC). At 900ºC hysteresis is 2 to 3ºC. If the type K thermocouple, for example, will be used below 500ºC, hysteresis can be reduced by annealing overnight at 450ºC. OXIDATION Another phenomenon affecting calibration is oxidation. Ni-Cr-AI alloys (e.g., Chromel*) have limited life in air above 500ºC because of oxidation. A special form of oxidation is so-called “green rot” which is preferential oxidation of Cr in atmospheres with low oxygen content (e.g., sheaths in which the volume of air is limited and stagnant). Nicrosil resists oxidation up to about 1,250ºC (2,300ºF) and does not exhibit green rot. Several new sheath materials called “Nicrobell” (**) consist of Nicrosil with 1.5% or 3.0% niobium. Nicrobell “A” is particularly formulated to be resistant to oxidation. Another new oxidation resistant sheath material called Nicrosil + (***)
consists of Nicrosil plus 0.15% magnesium. It is reported (ref. 4) to exhibit less spalling and probably have a longer life than some Nicrobell version(s) tested.
due to metal fatigue. On heating to 900ºC, the thermal expansion of Nisil differs from SS 304 by 0.4% of length. Nicrosil has only 0.05% difference in thermal expansion compared to Nisil (the leg most likely to fracture). A sheath of Nicrosil, Nicrosil + or Niobell would therefore induce less metal fatigue in either leg of the Type N thermocouple than would stainless steel.
Nicrosil, itself, does not have satisfactory resistance to reducing atmospheres, such as encountered in most combustion or many heat treating processes. Other adaptations of Nicrosil for use as sheath material (such as Nicrobells B, C and D) can be expected to deal with typical nonoxidizing atmospheres.
COMPOSITION Composition changes in SS sheathed couples are generally greater than in Inconel (****) sheathed couples. In tests performed by Anderson, et al., the KN leg showed an increase in chromium but a decrease in aluminum. These changes in composition contributed the major portion of the resulting change in calibration of the thermocouple. Most stainless steels have from 1 to 2% of manganese. Type 304 has ≈ 2% manganese. Others have manganese concentrations varying from 1% to 10%. Inconel has up to 1% Mn. As a rule of thumb, each 1% of Mn in the sheath material contributes -10ºC calibration shift for 1,000 hours at 1,100ºC. According to Bentley, at 1,200ºC, Type N in a 3 mm diameter SS sheath drifted -24ºC in 1,000 hours.
CONTAMINATION A third influence on calibration stability is contamination. The idea behind the mineral-insulated, integrally designed, metal-sheathed thermocouple is that the uniform compression of finely divided mineral oxides (typically MgO) insulation surrounding the wires and filling the sheath would seal the internal volume, thereby eliminating contamination. The volume of the insulation compressed by swaging, rolling or drawing is on the order of 85% of solid material. This is useful, permitting the tubing to be bent and also permitting the manufacture of smaller diameter assemblies. It does, however, permit the intrusion of gas such as water vapor or air. It also permits vapor diffusion of elements composing the wires or sheath. Bentley and Morgan determined that the vapor-phase diffusion of Mn (manganese) through the MgO insulation has the greatest influence on thermocouple decalibration.
HUMIDITY There is a multiple effect of water vapor within the sheath. It is rapidly absorbed in the MgO, reducing the insulation resistance. Humidity intrusion can ruin a MIMS thermocouple assembly in as short a time as a few minutes. In lesser amounts, it destroys a protective oxide coating on Nickel-Chromium alloys, subjecting them to more rapid deterioration. The changes due to water
METAL FATIGUE Metal fatigue is another cause of shortened thermocouple life. Differing temperature coefficients of linear expansion between sheaths and wires causes strain during heating or cooling. These strains result in eventual fracture
+25
Type K (Inconel) 0
DRIFT (°C)
INTRODUCTION The mineral-insulated integrally metalsheathed (MIMS) form of thermocouple consists of matched thermocouple wires surrounded by insulating material (typically MgO) compacted by rolling, drawing or swaging until the sheath is reduced in diameter. The advantages of MIMS thermocouples are:
Type N (310 SS)
Type N (Inconel)
Type K (310 SS)
–25
–50 0
200
400
600
800
1000
1200
ElapsedTime (h) Figure 1. Drift of 3 mm diameter stainless steel sheathed and Inconel 600 sheathed type K and Nicrosil vs. Nisil thermocouples in 1200°C in vacuum. The dips in the drift curve are the result of the "in-place inhomogeneity test" where the samples were extracted from the furnace by 5 cm.
Z-45
20
1.6 mm Bare Wire 3 mm OD Mineral Insulated Metal Sheathed Thermocouple 1100°C
Insitu Drift (°C)
10
NCR 0
SS -10
-20 0
1000
2000
3000
Time (h)
Figure 2. The insitu drift in type N thermocouples with tips held at 1100°C. Curves refer to mineral insulated metal sheathed thermocouples with 3mm OD sheaths of 310 stainless steel (SS) or Nicrosil (NCR) and 1.6mm bare wire thermocouples in air. The range in drift for the latter is also indicated.
vapor can be sufficiently severe as to make affected couples useless by reducing insulation resistance. This reduced resistance can result in misleading temperature readings, premature failure or even erroneous readings after open circuiting. Water vapor can be introduced during thermocouple fabrication or repair, or even by changes in atmospheric pressure during air shipment or during long periods of storage (e.g., six months) at construction sites. Care must be taken of hermetic seals during shipment and installation. RECOMMENDATIONS Although not mentioned above , there is some relationship between the diameter of these thermocouple materials and stability and longevity at elevated temperatures. The surface of the brickwork on which electrical heaters are supported becomes conductive at elevated temperatures. This leads to flow of electrical currents through thermocouple sheaths to ground, perhaps through the measuring instrument. The temptation to use the finest sheathed thermocouples (as fine as 1 mm) should be resisted for higher temperature or corrosive industrial environments. Stainless steel is a poorer sheath for mineral-insulated, metal-sheathed thermocouples than either Inconel 600 or modified Nicrosil when used with Ni-Cr thermocouples such as Type K or Type N. The modified Nicrosil sheathed thermocouples offer improved oxidation resistance up to 1,100ºC (1,200 to 1,250ºC for Type N), reduced failures due to differential thermal expansion, improved ductility and the elimination of the drift
problems caused by the vapor diffusion of manganese from stainless steels or Inconel. Considering the current state of supply of the newer materials, one could well choose a low manganese (0.3% or less) Inconel sheathed Type K MIMS thermocouple until such time as modified Nicrosil sheathed Type K or N and appropriate supporting data become readily available. (*) CHROMEL is a trademark of the Hoskins Manufacturing Co. (**) NICROBELL is a trademark of NICROBELL Pty. Ltd. NICROBELL sheath alloys are patented in a number of countries including the USA (***) NICROSIL + is a trademark of Pyrotenax Australia Pty. Ltd. (****) INCONEL is a trademark of the International Nickel Co. Reproduced with the permission of: H.L. Daneman P.O. Box 31056 Sante Fe, NM 87594
REFERENCES 1. Anderson, R. L., Ludwig, R.L.,FAILURE OF SHEATHED THERMOCOUPLES DUE TO THERMAL CYCLING, Temperature, (1982) pp 939-951 2. Anderson, R. L., Lyons, J. D., Kollie, T G., Christie, W. H., Eby, R., DECALIBRATION OF SHEATHED THERMOCOUPLES, Temperature, (1982) pp 977-1007 3. Bentley, R. E., NEW-GENERATION TEMPERATURE PROBES, Materials Australasia, April (1987), pp. 10-13 4. Bentley, R. E., THEORY AND PRACTICE OF THERMOELECTRIC THERMOMETERY, 2nd Edition, CSIRO Div. of Applied Physics, (1990) 152 pages.
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5. Bentley, R.E., private communication, 11/22/90 6. Burley, N. A., HIGHLY STABLE NICKEL-BASE ALLOYS FOR THERMOCOUPLES, J. of the Australian Institute of Metals, May (1972), pp 101-113 7. Burley, N. A., Burns, G. W., Powell, R. L., NICROSIL AND NISIL: THEIR DEVELOPMENT AND STANDARDIZATION, Inst. Physical Conf. Ser. No. 26, (1975), pp 162-171 8. Burley, N. A., Jones, T.P., PRACTICAL PERFORMANCE OF NICROSIL-NISIL THERMOCOUPLES, Inst. Physical Conf. Ser. No. 26, (1975), pp 172-180 9. Burley, N. A., Powell, R. L., Burns, G. W., Scroger, M. G., THE NICROSIL VS NISIL THERMOCOUPLE: PROPERTIES AND THERMOELECTRIC DATA, NBS Monograph 161, April (1978), pp 1-156 10.Burley, N. A., THE NICROSIL VS NISIL THERMOCOUPLE: THE FIRST TWO DECADES, (1986) private communication 11. Burley, N. A., N-CLAD-N: A NOVEL ADVANCED TYPE N INTEGRALLY-SHEATHED THERMOCOUPLE OF ULTRA-HIGH THERMOELECTRIC STABILITY, High Temperatures-High Pressures, (1986) pp 609-616 12.Burley, N. A., NICROSIL/NISIL TYPE N THERMOCOUPLE, Measurements & Control, April (1989), pp 130-133 13.Burley, N. A., ADVANCED INTEGRALLY SHEATHED TYPE N THERMOCOUPLE OF ULTRA-HIGH THERMOELECTRIC STABILITY, Measurement, Jan-Mar 1990, pp 3641 14.Daneman, H. L., THERMOCOUPLES, Measurements & Control, June (1988), pp 242-243 15.Frank, D.E., AS TEMPERATURES INCREASE, SO DO THE PROBLEMS!, Measurements & Control, June (1988), p 245 16.Hobson, J. W., THE INTRODUCTION OF THE NICROSIL/NISIL THERMOCOUPLES IN AUSTRALIA, Australian Journal of Instrumentation and Control, October (1982), pp 102104 17.Hobson, J. W., THE K TO N TRANSITION - BUILDING ON SUCCESS, Australian Journal of Instrumentation and Control, (1985) pp 12-15 18.Northover, E. W., Hitchcock, J. A., A NEW HIGH-STABILITY NICKEL ALLOY, Instrument Practice, September (1971), pp 529-531 19.Paine, A., TYPE N AND K MIMS T/C’S, fax LNA5195, 11/23/90 20.Wang, T P., Starr, C. D., NICROSILNISIL THERMOCOUPLES IN PRODUCTION FURNACES, ISA (1978) Annual conference, pp 235-254 21.Wang, T. P., Starr, C. D., EMF STABILITY OF NICROSIL-NISIL AT 500˚C, ISA (1978) Annual conference, pp 221-233
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Material Selection Guide This chart is a guide to selection of thermocouple sheath and thermowell materials according to process fluid. It includes factors such as catalytic reaction, contamination and electrolysis. However, there are many instances where factors other than these must be considered. It is recommended that such special applications be submitted to OMEGA ENGINEERING for recommendations. These recommendations are only guides based on the most economical material selection. OMEGA ENGINEERING cannot be held responsible if these recommendations are not satisfactory for specific applications. SUBSTANCE
CONDITIONS
Acetate Solvents Acetic Acid " " " " " " " " Acetic Anhydride Acetone Acetylene Alcohol Ethyl " " Alcohol Methyl " " Aluminum Aluminum Acetate Aluminum Sulphate " " " " " " Ammonia Ammonium Chloride Ammonium Nitrate " " Ammonium Sulphate " " " " Aniline Amylacetate Asphalt
Crude or Pure 10% - 70°F 50% - 70°F 50% - 212°F 99% - 70°F 99% - 212°F 212°F 70°F 212°F 70°F 212°F Molten Saturated 10% - 70°F Saturated 70°F 10% - 212°F Saturated 212°F All concentrations 70°F All concentrations 212°F All concentrations 70°F All concentrations 212°F 5% - 70°F 10% - 212°F Saturated 212°F All concentrations 70°F
Barium Carbonate Barium Chloride " " " " Barium Hydroxide Barium Sulphite Benzaldehyde Benzene Benzine
70°F 5% - 70°F Saturated 70°F Aqueous - Hot
Benzol Boracic Acid Bromine Butadiene Butane Butylacetate Butyl Alcohol Butylenes
Hot 5% Hot or Cold 70°F
Butyric Acid " " Calcium Bisulfite Calcium Chlorate " " Calcium Hydroxide " " " " Carbolic Acid Carbon Dioxide " " Carbon Tetrachloride Chlorex Caustic Chlorine Gas " " " " Chromic Acid " " " " Citric Acid " " " " Coal Tar Coke Oven Gas Copper Nitrate Copper Sulphate Core Oils Cottonseed Oil
5% - 70°F 5% - 150°F 70°F Dilute 70°F Dilute 150°F 10% - 212°F 20% - 212°F 50% - 212°F All 212°F Dry Wet 10% - 70°F
70°F
70°F
Dry 70°F Moist 70°F Moist 212°F 5% - 70°F 10% - 212°F 50% - 212°F 15% - 70°F 15% - 212°F Concentrated 212°F Hot
Creosols Creosote Crude Cyanogen Gas Dowtherm Epsom Salt Ether
Hot and Cold 70°F
RECOMMENDED METAL Monel or Nickel 304 Stainless Steel 304 Stainless Steel 316 Stainless Steel 430 Stainless Steel 430 Stainless Steel Monel 304 Stainless Steel 304, Monel, Nickel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel Cast iron 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 316 Stainless Steel 316 Stainless Steel 304 Stainless Steel 316 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 316 Stainless Steel 316 Stainless Steel 304 Stainless Steel Monel Steel (C1018) Phosphor Bronze, Monel, Nickel 304 Stainless Steel Monel Monel 316 Stainless Steel Steel (C1018) Nichrome Steel (C1018) 304 Stainless Steel Steel (C1018), Monel, Inconel 304 Stainless Steel 304 Stainless Steel Tantalum Brass, 304 304 Stainless Steel Monel Copper Steel (C1018), Phosphor Bronze 304 Stainless Steel 304 Stainless Steel 316 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 317 Stainless Steel 316 Stainless Steel Steel (C1018), Monel Aluminum,Monel,Nickel Monel 316SS, 317SS 317 Stainless Steel Hastelloy C Hastelloy C 304 Stainless Steel 316 Stainless Steel 316 Stainless Steel 304 Stainless Steel 316 Stainless Steel 317 Stainless Steel 304 Stainless Steel Aluminum 304, 316 304, 316 316 Stainless Steel Steel (C1018), Monel, Nickel 304 Stainless Steel Steel (C 1018), Monel, Nickel 304 Stainless Steel Steel (C1018) 304 Stainless Steel 304 Stainless Steel
SUBSTANCE Ethyl Acetate Ethyl Chloride Ethylene Glycol Ethyl Sulphate Ferric Chloride " " " " Ferric Sulphate Ferrous Sulphate Formaldehyde Freon Formic Acid " " Gallic Acid " " Gasoline Glucose Glycerine Glycerol Heat Treating Hydrobromic Acid Hydrochloric Acid " " " " " " " " " " Hydrocyanic Acid Hydrofluoric Acid Hydrogen Peroxide " " Hydrogen Sulphide Iodine Kerosene Lactic Acid " " " " Lacquer Latex Lime Sulphur Linseed Oil Magnesium Chloride " " Magnesium Sulphate Malic Acid Mercury
RECOMMENDED METAL Monel 304 Stainless Steel Steel (C1018) Monel 316 Stainless Steel Tantalum Tantalum 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel Steel (C1018) 316 Stainless Steel 316 Stainless Steel Monel Monel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 446 Stainless Steel Hastelloy B Hastelloy C Hastelloy B Hastelloy C Hastelloy B Hastelloy B Hastelloy B 316 Stainless Steel Hastelloy C 316 Stainless Steel 316 Stainless Steel 316 Stainless Steel Tantalum 304 Stainless Steel 304 Stainless Steel 316 Stainless Steel Tantalum 316 Stainless Steel Steel (C1018) Steel (C1018), 304, Monel 304 Stainless Steel Monel Nickel Monel 316 Stainless Steel Steel (C1018) , 304, Monel Steel (1020) 304, Nickel Carpenter #20
CONDITIONS 70°F 70°F 1% - 70°F 5% - 70°F 5% - Boiling 5% - 70°F Dilute 70°F 5% - 70°F 5% - 150°F 5% - 70°F 5% - 150°F 70°F 70°F 70°F 48% - 212°F 1% - 70°F 1% - 212°F 5% - 70°F 5% - 212°F 25% - 70°F 25% - 212°F 70°F 212°F Wet and dry 70°F 70°F 5% - 70°F 5% - 150°F 10% - 212°F 70°F
70°F 5% - 70°F 5% - 212°F Cold and Hot Cold and Hot
Methane 70°F Milk Mixed Acids (Sulphuric and Nitric - all temp. and %) Molasses Muriatic Acid Nap Natural Gas Neon Nickel Chloride Nickel Sulphate Nitric Acid " " " " " " " " " " " " Nitrobenzene Nitrous Acid Oleic Acid Oleum Oxalic Acid " " Oxygen " Palmitic Acid Petroleum Ether PhenoI Pentane Phosphoric Acid " " " " " " " "
70°F 70°F 70°F 70°F 70°F Hot and Cold 5% - 70°F 20% - 70°F 50% - 70°F 50% - 212°F 65% - 212°F Concentrated - 70°F Concentrated - 212°F 70°F 70°F 70°F 5% - Hot and Cold 10% - 212°F 70°F Liquid
Steel (C1018), 304, Monel, Nickel Tantalum 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 316 Stainless Steel 304 Stainless Steel Tantalum 304 Stainless Steel 304 Stainless Steel 316 Stainless Steel 316 Stainless Steel 304 Stainless Steel Monel Steel (C1018) 304 Stainless Steel 316 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 316 Stainless Steel Hastelloy C Hastelloy B
1% - 70°F 5% - 70°F 10% - 70°F 10% - 212°F 30% - 70°F
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SUBSTANCE
CONDITIONS
Picric Acid Potassium Bromide Potassium Carbonate Potassium Chlorate Potassium Chloride " " Potassium Hydroxide " " " " Potassium Nitrate " " Potassium Permanganate Potassium Sulphate " " Potassium Sulphide Propane Pyrogallic Acid Quinine Bisulphate Quinine Sulphate Resin Rosin Sea Water Salommoniac Salicylic Acid Shellac Soap Sodium Bicarbonate " " Sodium Bisulphate Sodium Carbonate " " Sodium Chloride " " " " " " Sodium Fluoride Sodium Hydroxide Sodium Hypochlorite Sodium Nitrate Sodium Peroxide Sodium Phosphate Sodium Silicate Sodium Sulphate Sodium Sulphide Sodium Sulphite Steam Stearic Acid Sulphur Dioxide " " Sulphur " Sulphuric Acid " " " " " " " " " " " " " " Tannic Acid Tar
70°F 70°F 1% - 70°F 70°F 5% - 70°*F 5% - 212°F 5% - 70°F 25% - 212°F 50% - 212°F 5% - 70°F 5% - 212°F
Tartaric Acid " " Tin Tolvene Trichloroethylene Turpentine Varnish Vegetable Oils Vinegar Water " Whiskey, Wine Xylene Zinc Zinc Chloride Zinc Sulphate " " " "
5% - 70°F 5% - 70°F 5% - 212°F 70°F
RECOMMENDED METAL 304 Stainless Steel 316 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 316 Stainless Steel 304 Stainless Steel 304 Stainless Steel
304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel Dry 316 Stainless Steel Dry 304 Stainless Steel 304 Stainless Steel Molten 304 Stainless Steel Monel Monel Nickel 304 Stainless Steel 70°F 304 Stainless Steel All concentrations 70°F 304 Stainless Steel 5% - 150°F 304 Stainless Steel Monel 5% - 70°F 304 Stainless Steel 5% - 150°F 304 Stainless Steel 5% - 70°F 316 Stainless Steel 5% - 150°F 316 Stainless Steel Saturated - 70°F 316 Stainless Steel Saturated - 212°F 316 Stainless Steel 5% - 70°F Monel 304 Stainless Steel 5% still 316 Stainless Steel Fused 317 Stainless Steel 304 Stainless Steel Steel (C1018) Steel (C1018) 70°F 304 Stainless Steel 70°F 316 Stainless Steel 150°F 304 Stainless Steel 304 Stainless Steel 304 Stainless Steel Moist Gas - 70°F 316 Stainless Steel Gas - 575°F 304 Stainless Steel Dry - Molten 304 Stainless Steel Wet 316 Stainless Steel 5% - 70°F Carp. 20, Hastelloy B 5% - 212°F Carp. 20, Hastelloy B 10% - 70°F Carp. 20, Hastelloy B 10% - 212°F Carp. 20, Hastelloy B 50% - 70°F Carp. 20, Hastelloy B 50% - 212°F Carp. 20, Hastelloy B 90% - 70°F Carp. 20, Hastelloy B 90% - 212°F Hastelloy D 70˚F 304 Stainless Steel Steel (C1018), 304, Monel, Nickel 70°F 304 Stainless Steel 150°F 316 Stainless Steel Molten Cast Iron Aluminum, Phosphor Bronze, Monel Steel (C1018) 304 Stainless Steel 304 Stainless Steel Steel (C1018), 304, Monel 304 Stainless Steel Fresh Copper, Steel (C1018), Monel Salt Aluminum 304, Nickel Copper Molten Cast Iron Monel 5% - 70°F 304 Stainless Steel Saturated - 70°F 304 Stainless Steel 25% - 212°F 304 Stainless Steel
Melting Temperatures Very High Temperature of Some Important Metals Sheath Materials Approximate melting points are given only as a guide for material selection since many factors including atmosphere, type of process, mounting, etc., all affect the operating maximum.
Tungsten ............
°F
6000 ..................... Rhenium
Sheath Material
Molybdenum
Rec. Useful Temp.
Melting Point
Environmental Conditions
4000ºF
4730ºF
Not Rec.
Fair
Fair
Good
Oxidizing
Hydrogen
Inert
Vacuum
Tantalum
4500ºF
5425ºF
Not Rec.
Not Rec.
Not Rec.
Good
Platinum
3050ºF
3223ºF
Very Good
Poor
Poor
Poor
Tantalum............ Molybdenum ....... Niobium ........... . (Columbium) Chromium ....... ... Titanium ............ Zirconium ........... Iron................... Cobalt ............... Nickel ...............
5000 ..................... Osmium ..................... Iridium 4000 ..................... Rhodium ..................... Platinum ..................... Vanadium 3000 ..................... Palladium
Beryllium ........... Manganese .........
.................... Stainless ....................} Steels .................... .................... Cast Irons
Uranium ............ Copper ..............
2000 ................Gold (24 Karat)
Silver ................
{
Brasses
Magnesium......... Zinc................... Lead ................. Bismuth ............. Tin.................... Indium ..............
Thermometry Fixed Points THERMOELECTRIC FIXED POINT
}
}
18 Karat 12 Karat 10 Karat
Gold Alloys
}
.........Aluminum Silver Solders 1000 ........ Cadmium 500
Gallium .............
Common ...................... Solders
Mercury .............
0ºF
}
Boiling point of oxygen Sublimation point of carbon dioxide Freezing point of mercury Ice Point Triple point of water Boiling point of water Triple point of benzoic acid Boiling point of naphthalene Freezing point of tin Boiling point of benzophenone Freezing point of cadmium Freezing point of lead Freezing point of zinc Boiling point of sulfur Freezing point of antimony Freezing point of aluminum Freezing point of siIver Freezing point of gold Freezing point of copper Freezing point of palladium Freezing point of platinum
MELTING POINTS FROM THE PRACTICAL INTERNATIONAL TEMPERATURE SCALE IPTS-68 -183.0 ºC - 78.5 - 38.9 0 0.01 100.0 122.4 218 231.9 305.9 321.1 327.5 419.6 444.7 630.7 660.4 961.9 1064.4 1084.5 1554 1772
-297.3 ºF -109.2 - 38 32 32 212 252.3 424.4 449.4 582.6 610 621.5 787.2 832.4 1167.3 1220.7 1763.5 1948 1984.1 2829 3222
Extension Grade Wires for Platinum and Tungsten-Rhenium Alloys + Copper Compensating alloys made into extension wire for tungsten-rhenium thermocouples and platinum-rhodium thermocouples closely match the emf of the thermocouples over limited range
Pt/Rh Hot Junction
Lead Junctions – Alloy No. 11
Pt.
Z-48
• The alloy 405/426 combination is used with Tungsten 5% Re vs Tungsten 26% Re. • The alloy 200/226 combination is used with Tungsten vs Tungsten 26% Re. • The alloy 203/225 combination is used with Tungsten 3% Re vs Tungsten 25%. • The Combination copper/alloy #11 is used with platinum-rhodium alloys vs pure platinum.
Z
Thermoelectric Alloy Property Data
ALLOY or DESIGNATION Pure Metals Iron Nickel Molybdenum Aluminum (H-P) Copper Gold Silver Tungsten Rhenium Platium Ref Rhodium Platinum Pt- 6%Rh Pt-10%Rh Pt-13% Rh Pt-20% Rh Pt-30% Rh Pt-40% Rh Nickel Alloys Constantan CHROMEGA® P ALOMEGA®
Compensating Alloys Alloy #11 Alloy #200 Alloy #203 Alloy #205 Alloy #225 Alloy #226 Alloy #260
Changes in Thermocouple Resistance with Increasing Temperature
N=Neg, P=Pos
99.9+% 99.98% 99.9+% 99.99+% 99.98% 99.999% 99.99% 99.99% 99.99% 99.999+% 99.99%
66 39 42 17.4 9.44 13.4 9.3 42 61.2 33.0
60 .0062 37 .0064 31 .0036 15 .0038 9.24 .0041 13.17 .0039 8.83 .0038 33 .0036 117 59.13 .00386 25.8 .0029
.0065 .0068 .0047 .0044 .0043 .0040 .0041 .0048 .00393 .0046
90 100 250 16.3 76 46 52 285 360 60 275
34 48 120 6.8 32 19 24 80 170 24 120
2 2 2 5 1.5 1.5 1.5 2 2
40 36 16 60 46 36 46 3 10 38 16
1536 1452 2610 660 1083 1063 960.8 3410 3170 1769 1966
7.9 8.9 10.2 2.71 8.93 19.30 10.5 19.3 20.0 21.45 12.42
94%Pt- 6%Rh 90% Pt-10% Rh 87% Pt-13% Rh 80% Pt-20% Rh 70% Pt-30% Rh 60% Pt-40% Rh
101 114 119 124 116 108
95 111 114 116 112 101
.0020 .0017 .0016 .0014 .0014 .0014
85 95 105 140 160 190
37 46 48 72 74 78
1.5 1.5 1.5 1.5 1.5 1.5
34 32 32 32 26 26
1810 1830 1840 1870 1910 1920
20.51 19.95 19.55 18.65 17.52 16.54
150 80 165 95 170 85
2 32 2 27 2 32
1270 1430 1400
8.86 8.73 8.60
55% Cu-45% Ni 315 294 90% Ni-10% Cr - 425 95% Ni-2% Mn-2% Al - 177
Tungsten Alloys Tungsten-3% Re Tungsten-5% Re Tungsten-25% Re Tungsten-26% Re
1. “Percent purity or composition” column refers to matching thermocouple grade alloy.
RESISTIVITY TEMP COEEF. TENSILE Ω cmil/ft OF RESISTANCE STRENGTH ELONGATION Melting (at 0ºC) (0-100ºC) (psi x 1000) (percent) point Density 0 C (g/cm3) Hard Annld Hard Annld Hard Annld Hard Annld
PERCENT PURITY or Notes composition
(1)
.0019 .0016 .0015 .0013 .0013 .0013
.00003 .00002 .00032 .00032 .00188 .00188
97% W- 3% Re 95% W- 5% Re 75% W-25% Re 74% W-26% Re
-
55 70 165 170
-
-
320 320 300 300
180 200 210 200
-
10 10 10 10
3410 3350 3130 3120
Pt alloys Tungsten Tungsten- 3% Re Tungsten- 5% Re Tungsten-25% Re Tungsten-26% Re Tungsten-26% Re
-
30 470 470 510 180 160 750
-
.0014 .0003 .0012 -
105 -
50 -
2 -
30 -
1090 1430 1400 1410 1370 1450 1520
19.4 19.4 19.7 19.7
8.91 8.73 8.60 8.58 8.88 8.85 7.42
Ratio of Resistance at Temperature Indicated to Resistance at 0°C (32°F) N=Neg P=Pos 0°C 20°C 200°C 400°C 600°C 800°C 1000°C 1200°C 1400°C 1500°C Thermoelements (32°F) (68°F) (392°F) (752°F) (1112°F) (1472°F) (1832°F) (2192°F) (2552°F) (2732°F) JP JN, TN, EN TP KP, EP KN NP NN RP SP RN, SN BP BN
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
1.13 0.999 1.11 1.01 1.05 1.01 1.07 1.03 1.03 1.06 1.03 1.03
2.46 0.996 1.86 1.09 1.43 1.02 1.13 1.31 1.33 1.77 1.26 1.40
4.72 0.994 2.75 1.19 1.64 1.07 1.27 1.60 1.65 2.50 1.51 1.78
7.84 1.02 3.70 1.25 1.82 1.08 1.39 1.89 1.95 3.18 1.76 2.14
12.0 1.056 4.75 1.30 1.98 1.08 1.55 2.16 2.23 3.81 1.98 2.47
13.07 1.092 5.96 1.37 2.15 1.10 1.68 2.41 2.50 4.40 2.20 2.78
… … … 1.43 2.32 … … 2.66 2.76 4.94 2.41 3.08
… … … … … … … 2.90 3.01 5.42 2.62 3.37
… … … … … … … 3.01 3.13 5.66 2.73 3.51
Resistance of Thermocouples, ohms per foot at 20°C ( 68°F)
Awg. No.
Diameter in.
KN
KP,EP
TN,JN,EN
TP
JP
NP
NN
RN, SN
RP
SP
BP
BN
16 20 24 30 36
0.0508 0.0320 0.0201 0.0100 0.0050
0.0683 0.173 0.438 1.77 7.08
0.164 0.415 1.05 4.25 17.0
0.1113 0.287 0.728 2.94 11.8
0.00402 0.0102 0.0257 0.1032 0.4148
0.0276 0.0699 0.1767 0.710 2.86
.2230 .5664 1.436 5.800 23.20
.08458 .2148 .5445 2.20 8.800
0.0247 0.0624 0.1578 0.6344 2.550
0.0456 0.1149 0.4656 2.965 12.25
0.0445 0.1125 0.2847 1.144 4.600
0.0447 0.1130 0.2859 1.149 4.620
0.0414 0.1046 0.2647 1.064 4.277
Z-49
Thermocouple Types
Trade Names of Alloys ANSI DESIGNATION
Iron-Constantan (ANSI Symbol J) The Iron-Constantan “J” curve thermocouple with a positive iron wire and a negative Constantan wire is recommended for reducing atmospheres. The operating range for this alloy combination is 1600°F for the largest wire sizes. Smaller size wires should operate in correspondingly lower temperatures. Copper-Constantan (ANSI Symbol T) The CopperConstantan “T” curve thermocouple, with a positive copper wire and a negative Constantan wire, is recommended for use in mildly oxidizing and reducing atmospheres up to 750°F. They are suitable for applications where moisture is present. This alloy is recommended for low temperature work since the homogeneity of the component wires can be maintained better than with other base metal wires. Therefore, errors due to inhomogeneity of wires in zones of temperature gradients are greatly reduced. CHROMEGA -ALOMEGA (ANSI Symbol K) The CHROMEGA®-ALOMEGA® “K” curve thermocouple with a positive CHROMEGA® wire and a negative ALOMEGA® wire is recommended for use in clean oxidizing atmospheres, The operating range for this alloy is 2300°F for the largest wire sizes. Smaller wire sizes should operate in correspondingly lower temperatures. ®
ALLOY (Generic or Trade Names)
JP JN, EN, or TN KP or EP KN TP RN or SN RP SP
Iron Constantan, Cupron, Advance CHROMEGA®, Tophel, T1, Thermokanthal KP ALOMEGA®, Nial, T2, Thermokanthal KN Copper Pure Platinum Platinum 13% Rhodium Platinum 10% Rhodium
Trade Names: Advance T - Driver Harris Co., CHROMEGA® and ALOMEGA® - OMEGA Engineering, Inc., Cupron, Nial and Trophel -Wilbur B. Driver Co., Thermokanthal KP and Thermokanthal KN -The Kanthal Corporation.
®
ANSI LETTER DESIGNATIONS -Currently thermocouple and extension wire is ordered and specified by an ANSI letter designation. Popular generic and trade name examples are CHROMEGA®/ALOMEGA® -ANSI Type K: Iron/Constantan ANSI Type J: Copper/Constantan - ANSI Type T CHROMEGA®/Constantan -ANSI Type E: Platinum/Platinum 10% Rhodium - ANSI Type S: Platinum/Platinum 13% Rhodium -ANSI Type R. The positive and negative legs are identified by letter suffixes P and N, respectively, as listed in the tables.
CHROMEGA®-Constantan (ANSI Symbol E) The CHROMEGA®-Constantan thermocouple may be used for temperatures up to 1600°F in a vacuum or inert, mildly oxidizing or reducing atmosphere. At sub-zero temperatures, the thermocouple is not subject to corrosion. This thermocouple has the highest emf output of any standard metallic thermocouple.
ANSI Symbol
E
70
T E J K N*
60 K Millivolts
Platinum-Rhodium Alloys (ANSI Symbols S, R and B) Three types of “noble-metal” thermocouples are in common use; they are: 1) a positive wire of 90% platinum and 10% rhodium used with a negative wire of pure platinum, 2) a positive wire of 87% platinum and 13% rhodium used with a negative wire of pure platinum, and 3) a positive wire of 70% platinum and 30% rhodium used with a negative wire of 94% platinum and 6% rhodium. They have a high resistance to oxidation and corrosion. However, hydrogen, carbon and many metal vapors can contaminate a platinum-rhodium thermocouple. The recommended operating range for the platinum-rhodium alloys is 2800°F, although temperatures as high as 3270°F can be measured with the Pt-30% Rh vs. Pt-6% Rh alloy combination.
80
50
J
N*
40
G* C*
30 T
20
R S B
10 0 1000
2000
3000
4000
5000
Temperature (Fahrenheit)
Tungsten-Rhenlum Alloys Three types of tungstenrhenium thermocouples are in common use for measuring temperatures up to 5000°F. These alloys have inherently poor oxidation resistance and should be used in vacuum, hydrogen or inert atmospheres.
Copper vs. Constantan CHROMEGA® vs. Constantan Iron vs. Constantan CHROMEGA® vs. ALOMEGA® OMEGALLOY® Nicrosil-Nisil G* Tungsten vs. Tungsten 26% Rhenium C* Tungsten 5% Rhenium vs. Tungsten 26% Rhenium D* Tungsten 3% Rhenium vs. Tungsten 25% Rhenium R Platinum 13% Rhodium vs. Platinum S Platinum 10% Rhodium vs. Platinum B Platinum 30% Rhodium vs. Platinum 6% Rhodium *Not an ANSI Symbol
Resistance Vs. Wire Diameter AWG No. 6 8 10 12 14 16 18 20 24 26 30 32 34 36 38 40 44 50 56
Type K Type J Type T Type E Type S Iron/ Copper/ CHROMEGA® Pt/ Diameter CHROMEGA® inches ALOMEGA® Constantan Constantan Constantan PT110%Rh 0.162 0.023 0.014 0.012 0.027 0.007 0.128 0.037 0.022 0.019 0.044 0.011 0.102 0.058 0.034 0.029 0.069 0.018 0.081 0.091 0.054 0.046 0.109 0.028 0.064 0.146 0.087 0.074 0.175 0.045 0.051 0.230 0.137 0.117 0.276 0.071 0.040 0.374 0.222 0.190 0.448 0.116 0.032 0.586 0.357 0.298 0.707 0.185 0.0201 1.490 0.878 0.7526 1.78 0.464 0.0159 2.381 1.405 1.204 2.836 0.740 0.0100 5.984 3.551 3.043 7.169 1.85 0.0080 9.524 5.599 4.758 11.31 1.96 0.0063 15.17 8.946 7.66 18.09 4.66 0.0050 24.08 14.20 12.17 28.76 7.40 0.0039 38.20 23.35 19.99 45.41 11.6 0.00315 60.88 37.01 31.64 73.57 18.6 0.0020 149.6 88.78 76.09 179.20 74.0 0.0010 598.4 355.1 304.3 716.9 185 0.00049 2408 1420 1217 2816 740
*Increase the resistance by 19% for nickel plated, type RTD wire
Type R Type RX/SX Type C† Pt/ Copper W5%Re/ PT113%Rh Alloy11** W26%Re 0.007 0.003 0.009 0.011 0.004 0.015 0.018 0.007 0.023 0.029 0.011 0.037 0.047 0.018 0.058 0.073 0.028 0.092 0.119 0.045 0.148 0.190 0.071 0.235 0.478 0.180 0.594 0.760 0.288 0.945 1.91 0.727 2.38 3.04 1.136 3.8 4.82 1.832 6.04 7.64 2.908 9.6 11.95 4.780 15.3 19.3 7.327 24.4 76.5 18.18 60.2 191 72.7 240 764 302.8 1000
**Maximum Resistance of reviewed wire
Z-50
Type CX Alloy 405 Alloy 426 0.014 0.023 0.037 0.058 0.093 0.146 0.238 0.371 0.941 1.503 3.800 5.94 9.57 15.20 24.98 38.30 95.00 380.0 1583
Type G† W/ W26%Re 0.008 0.012 0.020 0.031 0.049 0.078 0.126 0.200 0.560 0.803 2.03 3.22 5.10 8.16 12.9 20.6 51.1 204 850
†Not ANSI symbol
Type D† W3%Re/ W25%Re 0.009 0.015 0.022 0.035 0.055 0.088 0.138 0.220 0.560 0.890 2.26 3.60 5.70 9.10 15.3 23.0 56.9 227 945
Type BX Copper/ Copper* 0.000790 0.001256 0.001998 0.00318 0.00505 0.00803 0.01277 0.02030 0.05134 0.08162 0.2064 0.3282 0.5218 0.8296 1.3192 2.098 5.134 20.64 86.38
Z
Comparison of Time Constant* vs. Overall Outside Diameter of Bare Thermocouple Wires or Grounded Junction Thermocouples In Air Time constants calculated for air at room temperature and atmospheric pressure moving with velocity of 65 feet per second for thermocouples shown in Figures #1 and #2.
* The “Time Constant” or “Response TIme” is defined as the time required to reach 63.2% of an instantaneous temperature change.
1.1
2.1
1.0
2.0
.9
1.9
.8
1.8
.7
1.7
.6
1.6
Time constant of thermocouple made with exposed, butt welded 0.001 in. dia. wire = .003 sec.
.5 .4 .3
1.5 1.4 1.3 1.2
.2
1.1
.1
1/64 in.
1/32 in. 1.0
0.0 .002 .004 .006 .008 .010 .012 .014 .016 .018 .020 .022 .024 .026 .028 .030 .032 .034 .001
WIRE OR SHEATH DIAMETER - INCHES “D”
D
D
D GROUNDED Junction Fig. #2
BARE WIRE Butt Welded Fig. #1
D
BEADED-TYPE UNGROUNDEDThermocouple TYPE Fig. #3 Thermocouple Fig. #4
Because of space limitations, time constant curve is divided into 4 separate curves.
11.0
110
10.0
100
9.0
90
8.0
80
7.0
70
6.0
60
5.0
50
4.0
40
3.0
30
20
2.0
1.8 sec. 10
1.0
.03125
.0625
.09375
.125
.15625
.1875
.21875
.250
.28125
.3125
.375
1/32
1/16
3/32
1/8
5/32
3/16
7/32
1/4
9/32
5/16
3/8
WIRE OR SHEATH DIAMETER - INCHES “D”
Note: These comparisons apply to either bare “butt-welded” or “grounded” junction thermocouples. If the thermocouples are the “beaded” type or “ungrounded,” the time constant is longer. These times are only approximate and are provided for comparison purposes only. Multiply values from Time Constants by 1.5 for junctions shown in Fig. #3 and Fig. #4.
Z-51
TIME CONSTANT - SECONDS
TIME CONSTANT - SECONDS
Figure M Sheath Diameter 1⁄32" to 3⁄8 "
TIME CONSTANT - SECONDS
Time constant of thermocouple made with exposed butt welded 0.001 inch dia. wire = .003 sec.
TIME CONSTANT - SECONDS
For beaded-type and ungrounded junctions (Figures #3 or #4), multiply time constants by 1.5.
1.2
Metal Sheathed Thermocouple Probe Time Response Study in Water 2.25 .250
Ungrounded
2.00
1.75
Time in Seconds
1.50
1.25 .188
1.00
Grounded
.75
.125 .50
.25
.040
.05
.062
Exposed
.15
.10
Probe Diameter in Inches
Z-52
.20
.25
Z
OMEGA® Interchangeable Thermistor Applications
variable resistor for battery control
thermistor
variable resistor for battery control
thermistor
variable resistor for setting desired temperature
relay high gain amplifier
thermistor thermistor
Fig. 1
Fig. 6
Fig. 4
room temperature
oven #1
variable resistor for battery control
variable resistor for battery control
reference thermistor
slave thermistor
selector switch
oven #2
variable resistor for adjusting slave temperature slightly above or below temperature
refrigerator chamber
difference #2
difference #1
relay high gain amplifier
master thermistor
pressure chamber
Fig. 5
Fig. 2
Fig. 7
thermistor 1000 ohms at 25°C
meter coil 2150 ohms at 25°C
resistor 1000 ohms at 25°C
Thermistors can be used in a variety of ways. Here are a few typical applications. If you have questions concerning these or other thermistor uses, we will be happy to discuss them. TEMPERATURE MEASUREMENT-A thermistor in one leg of a Wheatstone bridge circuit will provide precise temperature information. Accuracy is limited in most applications only by the readout device. See Figure 1. Since lead length between thermistor and bridge is not normally a limiting factor, this basic system can be expanded to measure temperature at several locations from a central point. Thermistor interchangeability and large resistance change eliminate any significant error from switches or lead length. See Figure 2. METER COMPENSATION - The coil resistance of a meter movement changes with temperature, making the meter temperature dependent. Using the thermistor’s property of a high negative temperature coefficient, the coil can be compensated so total resistance due to temperature rise is essentially constant, allowing the meter to be used over a wide temperature range with minimal error. See Figure 3. DIFFERENTIAL THERMOMETERS-For accurate indication of temperature differential, two thermistors can be used in a Wheatstone bridge circuit. Thermistor interchangeability simplifies circuit design and reduces the number of components. See Figure 4.
can be placed at various points and the difference between these temperatures and the original temperature monitored at a convenient location. Measuring air temperature at different elevations with reference to ground temperature is useful for temperature inversion data and geological studies. See Figure 5. TEMPERATURE CONTROL-A system can be designed using a thermistor with a known temperature/ resistance curve to form one leg of an AC bridge and a variable resistor calibrated in temperature to form another leg. When the resistor is set to a desired temperature, bridge unbalance occurs. This unbalance is fed into an amplifier which actuates a relay to provide a source of heat or cold. When the thermistor senses the desired temperature, the bridge is balanced, opening the relay and turning off the heat or cold. See Figure 6. MASTER-SLAVE CONTROL-Occasionally there is a need to control one temperature with respect to another, such as a product going through a series of baths. The first bath acts as a master and uses a thermistor to sense temperature. Succeeding baths, also using thermistors, are slaves. When these thermistors are placed in the controller bridge, the slave baths can be kept at a temperature relative to the master bath. The master bath can be controlled with the system described earlier. The master-slave controller can be used for as many baths as necessary. See Figure 7.
To measure heat loss in a piping network, thermistors Z-53
®
Resistance Elements and RTD’s David J. King INTRODUCTION Resistance elements come in many types conforming to different standards, capable of different temperature ranges, with various sizes and accuracies available. But they all function in the same manner: each has a pre-specified resistance value at a known temperature which changes in a predictable fashion. In this way, by measuring the resistance of the element, the temperature of the element can be determined from tables, calculations or instrumentation. These resistance elements are the heart of the RTD (Resistance Temperature Detector). Generally, a bare resistance element is too fragile and sensitive to be used in its raw form, so it must be protected by incorporating it into an RTD. A Resistance Temperature Detector is a general term for any device that senses temperature by measuring the change in resistance of a material. RTD’s come in many forms, but usually appear in sheathed form. An RTD probe is an assembly composed of a resistance element, a sheath, lead wire and a termination or connection. The sheath, a closed end tube, immobilizes the element, protecting it against moisture and the environment to be measured. The sheath also provides protection and stability to the transition lead wires from the fragile element wires.
Some RTD probes can be combined with thermowells for additional protection. In this type of application, the thermowell may not only add protection to the RTD, but will also seal whatever system the RTD is to measure (a tank or boiler for instance) from actual contact with the RTD. This becomes a great aid in replacing the RTD without draining the vessel or system. Thermocouples are the old tried and true method of electrical temperature measurement. They function very differently from RTD’s but generally appear in the same configuration: often sheathed and possibly in a thermowell.
Basically, they operate on the Seebeck effect, which results in a change in thermoelectric emf induced by a change in temperature. Many applications lend themselves to either RTD’s or thermocouples. Thermocouples tend to be more rugged, free of self-heating errors and they command a large assortment of instrumentation. However, RTD’s, especially platinum RTD’s, are more stable and accurate. RESISTANCE ELEMENT CHARACTERISTICS There are several very important details that must be specified in order to properly identify the characteristics of the RTD: 1. 2. 3. 4. 5. 6.
Material of Resistance Element (Platinum, Nickel, etc.) Temperature Coefficient Nominal Resistance Temperature Range of Application Physical Dimensions or Size Restrictions Accuracy
1. Material of Resistance Element Several metals are quite common for use in resistance elements and the purity of the metal affects its characteristics. Platinum is by far the most popular due to its linearity with temperature. Other common materials are nickel and copper, although most of these are being replaced by platinum elements. Other metals used, though rarely, are Balco (an iron-nickel alloy), tungsten and iridium. 2. Temperature Coefficient The temperature coefficient of an element is a physical and electrical property of the material. This is a term that describes the average resistance change per unit of temperature from ice point to the boiling point of water. Different organizations have adopted different temperature coefficients as their standard. In 1983, the IEC (International Electrotechnical Commission) adopted the DIN (Deutsche Institute for Normung) standard of Platinum 100 ohm at 0ºC with a temperature coefficient of 0.00385 ohms per ohm degree centigrade. This is now the accepted standard of the industry in most countries, although other units are widely used. A quick explanation of how the coefficient is derived is as follows: Resistance at the boiling point (100ºC) =138.50 ohms. Resistance at ice point (0ºC) = 100.00 ohms. Divide the difference (38.5) by 100 degrees and then divide by the 100 ohm Z-54
nominal value of the element. The result is the mean temperature coefficient (alpha) of 0.00385 ohms per ohm per ºC. Some of the less common materials and temperature coefficients are: Pt TC
=
Pt TC
=
Pt TC Pt TC Copper TC Nickel TC Nickel TC
= = = = =
Balco TC = Tungsten TC =
.003902 (U.S. Industrial Standard) .003920 (Old U.S. Standard) .003923 (SAMA) .003916 (JIS) .0042 0.00617 (DIN) .00672 (Growing Less Common in U.S.) .0052 0.0045
Please note that the temperature coefficients are the average values between 0 and 100ºC. This is not to say that the resistance vs. temperature curves are truly linear over the specified temperature range. 3. Nominal Resistance
Nominal Resistance is the prespecified resistance value at a given temperature. Most standards, including IEC-751, use 0ºC as their reference point. The IEC standard is 100 ohms at 0ºC, but other nominal resistances, such as 50, 200, 400, 500, 1000 and 2000 ohm, are available. 4. Temperature Range of Application Depending on the mechanical configuration and manufacturing methods, RTD’s may be used from -270ºC to 850ºC. Specifications for temperature range will be different, for thin film, wire wound and glass encapsulated types, for example. 5. Physical Dimensions or Size Restrictions The most critical dimension of the element is outside diameter (O.D.), because the element must often fit within a protective sheath. The film type elements have no O.D. dimension. To calculate an equivalent dimension, we need to find the diagonal of an end cross section (this will be the widest distance across the element as it is inserted into a sheath).
Z
Resistance Elements and RTD’s Cont’d FIGURE 1. LOCATION OF THIN FILM ELEMENT IN CYLINDRICAL SHEATH WALL THICKNESS
DIAGONAL OF ELEMENT
Vibration Resistance: 50 g @ 500ºC; 200 g @ 20ºC; at frequencies from 20 to 1000 cps. THICKNESS OF ELEMENT
W OD
Permissible deviations from basic values Temperature ºC -200 -100 0 100 200 300 400 500 600 650
Class A Deviation ohms ºC ±0.24 ±0.55 ±0.14 ±0.35 ±0.06 ±0.15 ±0.13 ±0.35 ±0.20 ±0.55 ±0.27 ±0.75 ±0.33 ±0.95 ±0.38 ±1.15 ±0.43 ±1.35 ±0.46 ±1.45
Class B Temperature Deviation ºC ohms ºC -200 ±056 ±1.3 -100 ±0.32 ±0.8 0 ±0.12 ±0.3 100 ±0.30 ±0.8 200 ±0.48 ±1.3 300 ±0.64 ±1.8 400 ±0.79 ±2.3 500 ±0.93 ±2.8 600 ±1.06 ±3.3 650 ±1.13 ±3.6 700 ±1.17 ±3.8 800 ±1.28 ±4.3 850 ±1.34 ±4.6 For example, using an element that is 10 x 2 x 1.5 mm, the diagonal can be found by taking the square root of (22 + 1.52). Thus, the element will fit into a 2.5 mm (0.98") inside diameter hole. For practical purposes, remember that any element 2 mm wide or less will fit into a
10,000 hours at maximum temperature (1 year, 51 days, 16 hours continuous).
W = WIDTH OF ELEMENT
1/ " O.D. sheath with 0.010" walls, 8 generally speaking. Elements which are 1.5 mm wide will typically fit into a sheath with 0.084" bore. Refer to Figure 1.
6. Accuracy IEC 751 specifications for Platinum Resistance Thermometers have adopted DIN 43760 requirements for accuracy. DIN-IEC Class A and Class B elements are shown in the chart on this page. 7. Response Time 50% Response is the time the thermometer element needs in order to reach 50% of its steady state value. 90% Response is defined in a similar manner. These response times of elements are given for water flowing with 0. 2 m/s velocity and air flowing at 1 m/s. They can be calculated for any other medium with known values of thermal conductivity. In a 1/4" diameter sheath immersed in water flowing at 3 feet per second, response time to 63% of a step change in temperature is less than 5.0 seconds. 8. Measurement Current and Self Heating Temperature measurement is carried out almost exclusively with direct current. Unavoidably, the measuring current generates heat in the RTD. The permissible measurement currents are determined by the location of the element, the medium which is to be measured, and the velocity of moving media. A self-heating factor, “S”, gives the measurement error for the element in ºC per milliwatt (mW). With a given value of measuring current, I, the milliwatt value P can be calculated from P = I2R, where R is the RTD’s resistance value. The temperature measurement error ∆T (ºC) can then be calculated from ∆T = P x S.
RESISTANCE ELEMENT SPECIFICATIONS Stability: Better than 0.2ºC after Z-55
Temperature Shock Resistance: In forced air: over entire temperature range. In a water quench: from 200 to 20ºC. Pressure Sensitivity: Less than 1.5 x 10-4 C/PSI, reversible. Self Heating Errors & Response Times: Refer to specific Temperature Handbook pages for the type of element selected. Self Inductance From Sensing Current: Can be considered negligible for thin film elements; typically less than 0.02 microhenry for wire wound elements. Capacitance: For wire wound elements: calculated to be less than 6 PicoFarads; for film-type elements: capacitance is too small to be measured and is affected by lead wire connection. Lead connections with element may indicate about 300 pF capacitance.
LEAD WIRE CONFIGURATIONS As stated previously, a Resistance Temperature Detector (RTD) element generally appears in a sheathed form. Obviously, all of the criteria applicable to resistance elements also apply here, but rather than element size, the construction and dimensions of the entire RTD assembly must be considered. Since the lead wire used between the resistance element and the measuring instrument has a resistance itself, we must also supply a means of compensating for this inaccuracy. Refer to Figure 2 for the 2-wire configuration. BLACK
R2
RE RED ELEMENT
R1
FIGURE 2. 2-WIRE CONFIGURATION (STYLE 1) The circle represents the resistance element boundaries to the point of calibration. 3- or 4-wire configuration must be extended from the point of calibration so that all uncalibrated resistances are compensated.
The resistance RE is taken from the resistance element and is the value that will supply us with an accurate temperature measurement. Unfortunately, when we take our resistance measurement, the instrument will indicate RTOTAL: Where RT = R1 + R2 + RE This will produce a temperature readout higher than that actually being measured. Many systems can be calibrated to eliminate this. Most RTD’s incorporate a third wire with resistance R3. This wire will be connected to one side of the resistance element along with lead 2 as shown in Figure 3. This configuration provides one connection to one end and two to the other end of the sensor. Connected to an instrument designed to accept 3-wire input, compensation is achieved for lead resistance and temperature change in lead resistance. This is the most commonly used configuration.
BLACK R 3 BLACK R2 RE RED ELEMENT
R1
provided to each end of the sensor. This construction is used for measurements of the highest precision.
BLACK R 4 BLACK R 3
case. Still another configuration, now rare, is a standard 2-wire configuration with a closed loop of wire alongside (Figure 5). This functions the same as the 3-wire configuration, but uses an extra wire to do so. A separate pair of wires is provided as a loop to provide compensation for lead resistance and ambient changes in lead resistance.
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RE RED R
BLACK R 4
2
RED R 1
ELEMENT
RE RED ELEMENT
FIGURE 4. 4-WIRE CONFIGURATION (STYLE 3) With the 4-wire configuration, the instrument will pass a constant current (I) through the outer leads, 1 and 4. The voltage drop is measured across the inner leads, 2 and 3. So from V = IR we learn the resistance of the element alone, with no effect from the lead wire resistance. This offers an advantage over 3-wire configurations only if dissimilar lead wires are used, and this is rarely the
FIGURE 3. 3-WIRE CONFIGURATION (STYLE 2) If three identical type wires are used and their lengths are equal, then R1 = R2 = R3. By measuring the resistance through leads 1, 2 and the resistance element, a total system resistance is measured (R1 + R2 + RE ). If the resistance is also measured through leads 2 and 3 (R2 + R3), we obtain the resistance of just the lead wires, and since all lead wire resistances are equal, subtracting this value (R2 + R3) from the total system resistance (R1 + R2 + RE) leaves us with just RE, and an accurate temperature measurement has been made. A 4-wire configuration is also used. (See Figure 4.) Two connections are Z-56
BLACK LEAD RESISTANCE LOOP
FIGURE 5. 2-WIRE CONFIGURATION PLUS LOOP (STYLE 4)
®
R3 R2 R1
Introduction to Infrared Pyrometers Why should I use an infrared pyrometer to measure temperature in my application? Infrared pyrometers allow users to measure temperature in applications where conventional sensors cannot be employed. Specifically, in cases dealing with moving objects (i.e., rollers, moving machinery, or a conveyer belt), or where non-contact measurements are required because of contamination or hazardous reasons (such as high voltage), where distances are too great, or where the temperatures to be measured are too high for thermocouples or other contact sensors.
Emissivity (ε), a major but not uncontrollable factor in IR temperature measurement, cannot be ignored. Related to emissivity are reflectivity (R), a measure of an object’s ability to reflect infrared energy, and transmissivity (T), a measure of an object’s ability to pass or transmit IR energy. All radiation energy must be either emitted (E) due to the temperature of the body, transmitted (T) or reflected (R). The total energy, the sum of emissivity, transmissivity and reflectivity is equal to 1: E + T + R = 1.0
What should I consider about my application when selecting an infrared pyrometer? The critical considerations for any infrared pyrometer include field of view (target size and distance), type of surface being measured (emissivity considerations), spectral response (for atmospheric effects or transmission through surfaces), temperature range and mounting (handheld portable or fixed mount). Other considerations include response time, environment, mounting limitations, viewing port or window applications, and desired signal processing.
R E
Hot Source
T Infrared Pyrometer
FIELD OF VIEW What is meant by Field of View, and why is it important? The field of view is the angle of vision at which the instrument operates, and is determined by the optics of the unit. To obtain an accurate temperature reading, the target being measured should completely fill the field of view of the instrument. Since the infrared device determines the average temperature of all surfaces within the field of view, if the background temperature is different from the object temperature, a measurement error can occur (figure 1). Object A
Object B
Total infrared radiation reaching pyrometers
The ideal surface for infrared measurements is a perfect radiator, or a blackbody with an emissivity of 1.0. Most objects, however, are not perfect radiators, but will reflect and/or transmit a portion of the energy. Most instruments have the ability to compensate for different emissivity values, for different materials. In general, the higher the emissivity of an object, the easier it is to obtain an accurate temperature measurement using infrared. Objects with very low emissivities (below 0.2) can be difficult applications. Some polished, shiny metallic surfaces, such as aluminum, are so reflective in the infrared that accurate temperature measurements are not always possible.
Wall
Figure 1: Field of view
Most general purpose indicators have a focal distance between 20 and 60". The focal distance is the point at which the minimum measurement spot occurs. For example, a unit with a distance-to-spot size ratio of 120:1, and a focal length of 60" will have a minimum spot size of 0.5" at 60" distance. Close-focus instruments have a typical 0.1 to 12" focal length, while long-range units can use focal distances on the order of 50'. Many instruments used for long distances or small spot sizes also include sighting scopes for improved focusing. Field of view diagrams are available for most instruments to help estimate spot size at specific distances.
EMISSIVITY What is emissivity, and how is it related to infrared temperature measurements? Emissivity is defined as the ratio of the energy radiated by an object at a given temperature to the energy emitted by a perfect radiator, or blackbody, at the same temperature. The emissivity of a blackbody is 1.0. All values of emissivity fall between 0.0 and 1.0.
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Reflectivity is usually a more important consideration than transmission except in a few special applications, such as thin film plastics. The emissivity of most organic substances (wood, cloth, plastics, etc.) is approximately 0.95. Most rough or painted surfaces also have fairly high emissivity values.
FIVE WAYS TO DETERMINE EMISSIVITY There are five ways to determine the emissivity of the material, to ensure accurate temperature measurements: 1. Heat a sample of the material to a known temperature, using a precise sensor, and measure the temperature using the IR instrument. Then adjust the emissivity value to force the indicator to display the correct temperature.
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2. For relatively low temperatures (up to 500°F), a piece of masking tape, with an emissivity of 0.95, can be measured. Then adjust the emissivity value to force the indicator to display the correct temperature of the material. 3. For high temperature measurements, a hole (depth of which is at least 6 times the diameter) can be drilled into the object. This hole acts as a blackbody with emissivity of 1.0. Measure the temperature in the hole, then adjust the emissivity to force the indicator to display the correct temperature of the material. 4. If the material, or a portion of it, can be coated, a dull black paint will have an emissivity of approx. 1.0. Measure the temperature of the paint, then adjust the emissivity to force the indicator to display the correct temperature. 5. Standardized emissivity values for most materials are available (see pages 114-115). These can be entered into the instrument to estimate the material’s emissivity value.
TEMPERATURE MEASUREMENT THROUGH GLASS I want to measure the temperature through a glass or quartz window; what special considerations are there? Transmission of the infrared energy through glass or quartz is an important factor to be considered. The pyrometer must have a wavelength where the glass is somewhat transparent, which means they can only be used for high temperature. Otherwise, the instrument will have measurement errors due to averaging of the glass temperature with the desired product temperature.
MOUNTING How can I mount the infrared pyrometer?
SPECTRAL RESPONSE What is spectral response, and how will it affect my readings? The spectral response of the unit is the width of the infrared spectrum covered. Most general purpose units (for temperatures below 1000°F) use a wideband filter in the 8 to 14 micron range. This range is preferred for most measurements, as it will allow measurements to be taken without the atmospheric interference (where the atmospheric temperature affects the readings of the instrument). Some units use wider filters such as 8 to 20 microns, which can be used for close measurements, but are ‘‘distance-sensitive’’ against longer distances. For special purposes, very narrow bands may be chosen. These can be used for higher temperatures, and for penetrations of atmosphere, flames, and gases. Typical low band filters are at 2.2 or 3.8 microns. High temperatures above 1500°F are usually measured with 2.1 to 2.3 micron filters. Other bandwidths that can be used are 0.78 to 1.06 for high temperatures, 7.9 or 3.43 for limited transmissions through thin film plastics, and 3.8 microns to penetrate through clean flames with minimum interference.
The pyrometer can be of two types, either fixed-mount or portable. Fixed mount units are generally installed in one location to continuously monitor a given process. They usually operate on line power, and are aimed at a single point. The output from this type of instrument can be a local or remote display, along with an analog output that can be used for another display or control loop. Battery powered, portable infrared ‘‘guns’’ are also available; these units have all the features of the fixed mount devices, usually without the analog output for control purposes. Generally these units are utilized in maintenance, diagnostics, quality control, and spot measurements of critical processes.
RESPONSE TIME What else should I take into account when selecting and installing my infrared measurement system? First, the instrument must respond quickly enough to process changes for accurate temperature recording or control. Typical response times for infrared thermometers are in the 0.1 to 1 second range. Next, the unit must be able to function within the environment, at the ambient temperature. Other considerations include physical mounting limitations, viewing port/window applications (measuring through glass), and the desired signal processing to produce the desired output for further analysis, display or control.
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Principles of Infrared Thermometry W. R. Barron, Williamson Corporation
Temperature measurement can be divided into two categories: contact and noncontact. Contact thermocouples, RTDs, and thermometers are the most prevalent in temperature measurement applications. They must contact the target as they measure their own temperature and they are relatively slow responding, but they are inexpensive. Noncontact temperature sensors measure IR energy emitted by the target, have fast response, and are commonly used to measure moving and intermittent targets, targets in a vacuum, and targets that are inaccessible due to hostile environments, geometry limitations, or safety hazards. The cost is relatively high, although in some cases is comparable to contact devices. Infrared radiation was discovered in 1666 by Sir Isaac Newton, when he separated the electromagnetic energy from sunlight by passing white light through a glass prism that broke up the beam into colors of the rainbow. In 1800, Sir William Herschel took the next step by measuring the relative energy of each color. He also discovered energy beyond the visible. In the early 1900s, Planck, Stefan, Boltzmann, Wien, and Kirchhoff further defined the activity of the electromagnetic spectrum and developed quantitative data and equations to identify IR energy. This research makes it possible to define IR energy using the basic blackbody emittance curves (See Figure 1). From this plot it can be seen that objects (of a temperature greater than -273°C) emit radiant energy in an amount proportional to the fourth power of their temperature. The concept of blackbody emittance is the foundation for IR thermometry. There is, however, the term “emissivity” that adds a variable to the basic laws of physics. Emissivity is a measure of the ratio of thermal radiation emitted by a
1200°F
0.8
El = 1 - tl - rl This emissivity coefficient fits into Planck’s equation as a variable describing the object surface characteristics relative to wavelength. The majority of targets measured are opaque and the emissivity coefficient can be simplified to:
0.7
0.6
0.5
El = 1 - rl
1000°F 0.4
Exceptions are materials like glass, plastics, and silicon, but through proper selective spectral filtering it is possible to measure these objects in their opaque IR region.
0.3
0.2 VISIBLE
THEORY AND FUNDAMENTALS
Therefore:
RADIATION EMITTANCE (W/cm2/mm-1)
The fundamentals of IR thermometry are an important prerequisite for specifying an accurate monitoring system. Unfortunately, many users do not take the time to understand the basic guidelines, and consequently reject the concept of noncontact temperature measurement as inaccurate.
0.1
600°F
0 0
1
2
3
4
5
6
7
8
9
10
WAVELENGTH (mm)
BLACKBODY RADIATION CHARACTERISTICS STEFAN-BOLTZMANN LAW Q = sT4 WIEN'S DISPLACEMENT LAW l M = K/T PLANCK'S LAW Ql = Cl -5 (ec2 / lT-1) -1
Figure 1: As shown in curves representing the distribution of energy emitted by blackbodies ranging in temperature from 600°F to 1200°F, the predominant radiation is in the IR region of 0.5-14 µm, well beyond the visible region.
graybody (non-blackbody) to that of a blackbody at the same temperature. (A graybody refers to an object that has the same spectral emissivity at every wavelength; a non-graybody is an object whose emissivity changes with wavelength, e.g. aluminum.) L E = GB LBB The law of conservation of energy states that the coefficient of transmission, reflection, and emission (absorption) of radiation must add up to 1: tl + rl + al = 1 and the emissivity equals absorptivity: El = al
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There is typically a lot of confusion regarding emissivity error, but the user need remember only four things: – IR sensors are inherently colorblind. – If the target is visually reflective (like a mirror), beware – you will measure not only the emitted radiation, as desired, but also reflected radiation. – If you can see through it, you need to select IR filtering (e.g., glass is opaque at 5µm). – Nine out of ten applications do not require absolute temperature measurement. Repeatability and drift-free operation yield close temperature control. If the surface is shiny, there is an emissivity adjustment that can be made either manually or automatically to correct for emissivity error. It is a simple fix for most applications. In cases where emissivity varies and creates processing problems, consider dual- or multiwavelength radiometry to eliminate the emissivity problem.
DESIGN ELEMENTS IR thermometers come in a wide variety of configurations pertaining to optics, electronics, technology, size, and protective enclosures. All, however, have a common chain of IR energy in and an electronic signal out. This basic chain consists of collecting optics, lenses, and/or fiber optics, spectral filtering, and a detector as the front end. Dynamic processing comes in many forms, but can be summarized as amplification, thermal stability, linearization, and signal conditioning. Normal window
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From an applications standpoint, the primary characteristic of the optics is the field of view (FOV), i.e., what is the target size at a prescribed distance? A very common lens system, for example, would be a 1 in. dia. target size at a 15 in. working distance. Using the inverse square law, by doubling the distance (30 in.) the target area theoretically doubles (2 in. dia.). The actual definition of target size (area measured) will vary depending upon the supplier, and it is price dependent. Other optical configurations vary from small spot (0.030 in dia.) for close-up pinpoint measurement, to distant optics (3 in. at 30 ft) for distant aiming. It is important to note that working distance should not affect the accuracy if the FOV is filled by the target. In one technique for measuring FOV, the variable is signal loss vs. diameter. A strict rule is a 1% energy reduction, although some data are presented at half power, or 63.2% Alignment (aiming) is another optical factor. Many sensors lack that capability; the lens is aligned to the surface and measures surface temperature. This works with sizable targets, e.g., paper web, where pinpoint accuracy is not required. For small targets that use small-spot optics, and for distant optics used in remote monitoring, there are options of visual aiming, aim lights, and laser alignment. Selective spectral filtering typically uses short-wavelength filters for hightemperature applications (>1000°F, and long-wavelength filters for low temperatures –50°F). This obviously fits the blackbody distribution curves, and there are some technological advantages. For example, high temperature/short wavelength uses a very thermally stable silicon detector, and the short-wavelength design minimizes temperature error due to emissivity variations. Other selective filtering is used for plastic films (3.43 µm and 7.9 µm), glass (5.1 µm), and flame insensitivity (3.8 µm).
A variety of detectors are used to maximize the sensitivity of the sensor. As shown in Figure 2, PbS has the greatest sensitivity, while the thermopile has the least sensitivity. Most detectors are either photovoltaic, putting out a voltage when energized, or photoconductive, changing resistance when excited. These fast-responding, high sensitive detectors have a tradeoff thermal drift that can be overcome in many ways, including temperature compensation (thermistors) circuitry, temperature regulation, auto null circuitry, chopping (AC vs. DC output), and isothermal protection. Drift-free operation is available in varying degrees and is price dependent. 106 PbS
105
RELATIVE SENSIVITY
glass is usable at the short wavelength, quartz for the midrange, and germanium or zinc sulfide for the 8-14 µm range. Fiber optics are available to cover the 0.5-5.0 µm region.
Ge 104
Si
InAs InSb
3
10
THERMISTOR BOLOMETER 102
(PYROELECTRIC DETECTOR)
THIN FILM THERMOPILE
101
METAL THERMOPILE
1 0.1
0.2 0.3 0.5 0.7 1.0
2
3
5 7 10
20
WAVELENGTH (mm) CHOPPED UNCHOPPED
Figure 2: To optimize the respone of IR sensing systems, the detector’s spectral response and modulation characteristics must be considered.
In the IR thermometer’s electronics package, the detector’s nonlinear output signal, on the order of 100-1000 µV, is processed. The signal is amplified 1000 x, regulated, and linearized, and the ultimate output is a linear mV or mA signal. The trend is toward 4-20 mA output to minimize environmental electrical noise interference. This signal can also be transposed to RS 232 or fed to a PID controller, remote display, or recorder. Additional signal conditioning options involve on/off alarms, adjustable peak hold for intermittent targets, adjustable response time, and/or sample-and-hold circuitry.
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On the average, IR thermometers have a response time on the order of 300 ms, although signal outputs on the order of 10 ms can be obtained with silicon detectors. In the real world, many instruments have an adjustable response capability that permits damping of noisy incoming signals and field adjustment on sensitivity. It is not always necessary to have the fastest response available. There are cases involving induction heating and other types of applications, however, where response times on the order of 10-50 ms are required, and they are attainable through IR thermometry.
SINGLE-WAVELENGTH THERMOMETRY The basic single-wavelength design measures total energy emitted from a surface at a prescribed wavelength. The configurations range from handheld probes with a simple remote meter to sophisticated portables with simultaneous viewing of target and temperature, plus memory and/or printout capabilities. On-line, fixed-mount sensors range from simple small detectors with remote electronics (OEM designs) to rugged devices with remote PID control. Fiber optics, laser aiming, water cooling, CRT display, and scanning systems are among the options for process monitoring and control applications. There are many variations in size, performance, ruggedness, adaptability, and signal conditioning. Process sensor configuration, IR spectral filtering, temperature range, optics, response time, and target emissivity are important engineering elements that affect performance and which must be given careful consideration during the selection process. The sensor configuration can be a portable, a simple two-wire transmitter, a sophisticated ruggedized sensing unit, or a scanning device. Visual aiming, laser alignment, non-aiming, fiber optics, water cooling, output signals, and remote displays represent an overview of the various options. These are somewhat subjective, but demand engineering review. In most cases, if it is a simple application, e.g., web temperature, a simple low-cost sensor would do the job; if the application is
Infrared Thermometry Principles Cont’d
complicated, e.g., vacuum chamber or small target, then a more sophisticated sensor is a better choice. The selection of IR spectral response and temperature range is related to a specific application. Short wavelengths are for high temperature and long wavelengths are for low temperature, to coincide with the blackbody distribution curves. If transparent-type targets are involved, e.g., plastics and glass, then selective narrow-band filtering is required. For example, polyethylene film has a CH absorption band of 3.43 µm, where it becomes opaque. By filtering in this region, the emissivity factor is simplified. Likewise, most glass-type materials become opaque at 4.6 µm and narrow-band filtering at 5.1 µm permits accurate measurement of glass surface temperature. On the other hand, to look through a glass window, a sensor filtered in the 1-4 µm region would allow easy access via viewing ports into vacuum and pressure chambers. Another option, in the case of chambers, is to use a fiber-optic cable with a vacuum or pressure bushing. Optics and response time are two sensor characteristics that are, in most applications, nonissues, in that the standard FOV of approximately 1 in. at 15 in. is acceptable, and response time of
