determined with a limited number of parameters (temperature, pressure, salinity, and .... Helical coil springs, Burdon tubes, bellows, or elastic membranes are used in rheostatic transducers with a large relative ...... M. K. Zhokhovskii, Tr. Metrol.
PHYSICOCHEMICAL MEASUREMENTS
PRECISION METHODS
FOR MEASURING OCEAN PHYSICO-
CHEMICAL CHARACTERISTICS
Yu. S. II'inykh and Yu. D. Chashechkin
UDC 551.46
This survey analyzes the current state of measuring equipment for basic ocean physicochemical characteristics, its metrological capabilities, and standards support.
Various aspects of practical hydrometeorology, environmental ecological status monitoring, weather forecasting, and evaluation require improved distribution accuracy and objectivity for world ocean status and variation. Further progress in physical oceanography, based on identification, classification, and prediction of fundamental dynamical processes (currents, gyres, coherent structures, inland waters, and turbulence), is impossible without highly accurate measurement of such ocean parameters as temperature (6T = 0.002~ salinity (6S = 0.001%o), and density (6p = 10 - 7 g/cm3). It is necessary that spatial resolution be 0.01 m and time averaging be over no more than 0.03 sec. Study of the exact structure and microstructure of the ocean therrnoclinic entails resolution of vertical (pressure) gradient layers a few centimeters thick [1-4]. It is thought that the equilibrium state of sea water can be unambiguously determined with a limited number of parameters (temperature, pressure, salinity, and density); the first three are generally used as independent parameters [5]. However, study of world ocean structure has shown that its hydrochemical fields vary significantly and that its chemical indices are related to water mass physical and biological fields. The existence of f'me structure depends on redistribution of physicochemical impurities and gas bubble concentration and of biological and chemical impurities in the gradient layers. Some environmental physicochemical characteristics are measured in situ with high accuracy and can be used as tracers in studying hydrodynamic processes in the ocean [6]. Figure 1 shows typical graphs representing the main hydrophysical processes and their fine structure. Study of climate-forming factors, i.e., the formation dynamics of water masses, halo- and thermoclines, ice cover, etc., requires simultaneous determination of these parameters on the scale of the world ocean and over time intervals measured in millennia. Especially stringent requirements are imposed on climatic process indicators (T and S) and factors having a material influence on climate, some of which are subject to strong anthropogenic effects (CO1 and O2), together with closely associated hydrogen ion activity (pH) and redox potential (Eh). The distinctive feature of the latter is the presence of a significant gradient in the upper ocean layer. Figure 2 shows the characteristic curves for these parameters. Temperature Measuring Devices. One of the most important characteristics of sea water is temperature. A detailed analysis of the temperature transducers is given in [7, 8]. Temperature transducers can be divided into point-type and distributed or integral on the basis of the spatiotemporal characteristic of the process to be measured. The actual construction of an integral transducer can be that of an ordinary marine cable, two strands of which are connected at the bottom. The resistance of such a cable characterizes the integral layer temperature and does not react to small-scale fluctuations. Such fluctuations are investigated with precision transducers, among which we must distinguish wire and semiconductor thermistors. They are highly sensitive, have good metrological characteristics, and are simple and reliable in construction. The periodical literature most often describes use of wire thermistors, which have less technological variation and high stability, and special-purpose equipment with semiconductor thermistors, which are more sensitive and faster but have greater technological variation. Wire thermistors are specifically used in water samplers [9] and ACITT [10], while semiconductor thermistors are employed in a number of turbulimeters [11], toward thermal gradient devices [11, 12], and other devices for which highly accurate absolute temperature measurements are not required. Translated from Izmeritel'naya Tekhnika, No. 9, pp. 56-64, September, 1993.
0543-1972/93/3609-1047512.50 9 1994 Plenum Publishing Corporation
1047
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Fig. 2 One of the world's best oceanographic probes, the 196-RM manufactured by the American Interocean Systems firm, utilizes two temperature channels: a platinum resistance thermometer and a thermistor. The measurement error + 0.02 ~ with a time constant of 6 msec [13]. The errors for hydrological probes are generally greater than this. The electrical circuits of almost all resistance thermometers are designed in such fashion that the sensor is in one arm of a bridge. Since the bridge gain is a nonlinear function of the resistance R t in such a measurement system, the static conversion response is also nonlinear. Several ways to linearize such circuits have been proposed [14]. Among temperature transducers based on other principles, special mention must be made of piezoquartz temperature transducers and thermocouple transducers. The former have record sensitivity (as little as 10-6~ and accuracy but also exhibit considerable measurement inertia, with a time constant of more than 5 sec [15, 16]. The latter are promising for measurement of turbulent temperature fluctuations and temperature gradients, since their standard curves are highly stable and the sensitive area is small in size, which assures the requisite dynamic properties. However, absolute temperature measurement with thermocouples is difficult under field conditions [17]. Temperature measurements are supported by an extensive standards base resting on the International Practical Temperature Scale of 1968 (IPTS-68), which utilizes several reference points provided by a system of standards and standard devices. The calibration error provided for oceanographic measuring systems by the current standards base under steady-state temperature conditions 10-3~ while the dynamic calibration error is more than 1% [18, 19]. The significant errors displayed by working measurement devices in comparison with the capabilities of the standards base are due to a number of uncontrollable factors for which little allowance is made in situ; prime among these are, e.g. [20]: the thermal inertia of the accessories that protect the sensors from the action of the aggressive medium, high hydrostatic pressure, and incident current; the presence of thermal contact between the sensor and massive measuring device housing; the variation of sensor and medium heat transfer parameters; the finite dimensions of the heat sensor, which lead to spatial distortion of the medium temperature field; the temperature sensitivity of the connecting leads and electrical parts. 104.8
There is still another mechanism that acts to limit ambient temperature measurement accuracy: the thermal effect on the medium of the current-carrying measuring device. The conditions for maintenance of thermodynamic equilibrium between the temperature detector and water were used in [21] to estimate the theoretically attainable measuring device sensitivity limit: ATmin~TDl/2(2h To/9k )l/2 ,
where T D is the measuring system inertial index (sec), h = 6.62.10 -34 J/sec is Planck's constant, TO is the water temperature (K), and k = 1.38 -10 -23 J/K is Boltzmalm's constant. The sensitivity limit for an actual measuring device with T D = 1 sec amounts to 0.5.10-4~ Similar estimates can also be made for transducers intended to measure other parameters if they are characterized by an active effect on the ambient medium. It can be assumed on this basis that it is best in some measuring systems to supply the sensor not with direct current but with alternating current or even pulsed current having a short relative pulse duration. Methods are now being developed for noncontact ocean temperature measurement from aircraft or artificial Earth satellites (AESs) [22-24]. The measurement accuracy achieved (+0.5~ does not as yet permit them to compete with contact methods, but the information obtained from AESs is irreplaceable for global oceanic processes. Pressure Measuring Devices. There is an enormous diversity of methods and tools for measuring overpressures, which have been quite fully described in [19]. The most precise instruments for measuring hydrostatic pressures in the range (101.6)- 109 Pa (up to 16,000 kg/cm 3) are piston-type gages with nonsealed pistons, which consist of an exactly size-matched piston and cylinder separated by a thin layer of oil. Dead-weight pressure gages have been used to create a national pressure standards and providing a pressure measurement error of no more than 0.05% [25]. The most common hydrostatic pressure transducers in oceanography are of the rheostatic and jet (or vibrating-rod) types. Helical coil springs, Burdon tubes, bellows, or elastic membranes are used in rheostatic transducers with a large relative resistance variation range. Their mechanical deformation is transmitted to the potentiometer wiper and provides a change of up to 100% and a measurement error of 0.5-1.5 %. Rheostatic transducers with a small resistance variation range are based on the tensometric effect produced in a wire of strip carried on the pressure sensor. The relative resistance variation range in such transducers (strain sensors) is usually 5-10%, with a pressure measurement error of 0.2-0.5 % [20]. The advantage of jet and vibrating-rod pressure transducers lies in the fact that the information-carrying parameter is the electric current oscillation frequency, which is convenient for remote telemetry and remote measurement [26, 27]. The sensor for such transducers is generally a cavity, which has both high Q and a large time constant. The advantage of this type of transducer is therefore manifest only in static measurements. Modern hydrological equipment most often utilizes strain gages, which are thin (20-50/zm) conductive films deposited on a nonconductive substrate, which is cemented to a diaphragm-type elastic element. The most promising transducer of this kind has a "silicon on sapphire" structure [28]. The best Russian-produced pressure transducers based on such structures are equivalent to foreign parts and make it possible to obtain a relative error of no more than 0.05% and a time constant of about 10-2 sec in measuring overpressures of up to 600 kg/crn2. When working with the upper layers of the ocean, such transducers measure depth to within a few centimeters. The good metrological characteristics and high speed of such transducers permit their use for measuring the average and fluctuation pressures in turbulent currents [30]. The main error source for tensiometric pressure transducers is the dependence of the tensiometric sensitivity coefficient on temperature. As in the case of temperature transducers, the measurement circuit for pressure transducers is usually a bridge, one of whose arms includes the sensor. The output signal of such a bridge can be represented in the form [8]: U(e ,T)=l/l l + k ( l+~6Tje],
where k is the bridge tensiometric sensitivity, e is the temperature coefficient of tensiometric sensitivity, V is the bridge supply voltage, and e is the relative sensing element deformation. Complete temperature compensation of tensiometric sensitivity obviously occurs when the condition O~U(e,T) aTOe
~0
is satisfied. Hence we can obtain an equation that relates the temperature coefficient of tensiometric sensitivity to supply voltage: I V(T)
dV(T) s dT ---- 1+e~3T"
1049
+
J
o Uou t
1 Fig. 3
It follows from this equation that, if the supply voltage temperature coefficient V(T) is set approximately equal in absolute value to the temperature coefficient of tensiometric sensitivity, this satisfies the temperature compensation condition (with some degree of approximation). The result is adequate. The uncompensated thermal sensitivity does not exceed +__0.004% K -1 for a real value e = -2"10 -3 K -1 over the range 0-20~ Similar circuits are used in the pressure transducers from the Velvine Electric [31] and Kulite Semiconductor [32] firms, which have temperature coefficients of tensiometric sensitivity amounting to less than 0.01% K -1 over the range 0-60~ Figure 3 shows a temperature compensation circuit using an active element (adc amplifier). Its gain depends on the temperature change. The universality of this temperature compensation principle should be noted. Similar circuits are used in transducers for other physicochemical characteristics. Salinity (Electrical Conductivity). The third independent parameter in the equation of state is the salinity. By definition, salinity characterizes the relative dry salt content per unit sea water mass (%o) and is, on the Practical Salinity Scale adopted in 1978 (PSS-78), closely related to the electrical conductivity of water by the equation: S =0,008 --0,1692 K[/2_~25,3851K1.~-{-14,0941K~2 --7,0261 K/25-1-2,7081K~2 , where K15 is the ratio of the conductivity of a specific sea water sample at 15~ and a normal atmospheric pressure of 1 atm to that of a potassium chloride (KC1) mass concentration of 32.4356/10 -3 at the same temperature and pressure [33]. A new International Equation of State for Sea Water was adopted (in place of the Knudsen-Eckman equation) on the basis of the PSS78 scale in 1980, making it possible to determine water density from in situ pressure, temperature, and density measurements with an accuracy of 10 -6 [34], which is better than for any other method of determining this parameter. The measurement unit for the salinity of sea water prior to introduction of the PSS-78 was its conductivity relative to that of "normal" water with a salinity of 35%0, which was assigned a value of 4.2902 S/m at a temperature of 15~ The new scale incorporates the concept of relative electrical conductivity, with which there is no need to know the absolute conductivity of either the sea water or KC1 solution. One can proceed from the requirements on the accuracy of sea water salinity determination and the form of the last equation to define the requirements on density measurement accuracy. The relative error in measuring this quantity should be no more than 0.5.10 -4 (2" 10 -4 S/m). The importance of this parameter explains the large number of publications on methods and hardware for measuring the conductivity of sea water under laboratory and field conditions. An analysis of the most widely used conductivity transducers is given [7, 35]. Several categories have been adopted for conductivity measuring devices. Four groups are distinguished on the basis of the electromagnetic field source: direct current, low-frequency, high-frequency, and pulsed. There are contact and noncontact modes of electromagnetic energy delivery to the test subject. The type of transducer used for the field/conductor interaction gives us balance, direct-reading, analog, and frequency-based systems. The most suitable type of transducer is chosen in each specific case as a function of requirements determined by the measurement conditions, reliability, sensitivity threshold, etc. Proper choice of the relationship between operating speed and resolution is important. Virtually all transducers are characterized by high-speed electrical circuits, but there is actually a considerable amount of spatial and
1050
Li
Fig. 4
temporal averaging, a consequence of the fact that the medium under investigation belongs to the class of ionic conductors, in which the charge carriers are ions, and passage of current is accompanied by mass transfer. Many researchers have been concerned with dynamic transducer calibration, but this problem still cannot be considered solved, because of the complex interaction of the transducer with the environment and the process being investigated. Specifically, transducer dynamic properties determined at the interface in a two-layer liquid [36] do not remain the same in measuring internal waves in continuously stratified media, where a specific method is needed for estimating the dynamic errors [37, 38]. Among the multiplicity of known conductivity transducers, we must single out the two types that have become the most common: contact microelectrode and noncontact transformer transducers. Development of the former was associated with the requirements on the short-range averaging scale needed to investigate small-scale inhomogeneities. The averaging scale for such transducers depends on the size of one electrode (microelectrode), which can have the form of a sphere or the end face of a fine wire [39, 40]. The second electrode is the sensor housing. The condition currents in such a transducer are densest in a region with a characteristic size of about ten microelectrode radii. The transducers have good dynamic characteristics, but they are sensitive to polarization phenomena and the temperature field produced by the measuring currents when utilized in a traditional measurement circuit. These drawbacks can be ameliorated to a substantial extent by using special methods for transducer power supply and signal processing in the electrical circuitry (pulsed probing and use of a high-frequency input signal spectrum) [41]. Figure 4 is a block diagram of one such transducer. The circuitry includes pulse generator 1 with regulable output signal amplitude, null detector 2, peak detectors 3 and 4, differential amplifier 5, and stabilized voltage source 6. One feature of the transducer is a combination of a pulsed probe signal having short relative pulse duration and analog signal processing in the circuit, which makes it possible to avoid a number of interfering factors and improve measurement accuracy and reliability. The best foreign contact-type transducers, used in the mark III CTD from Neil Brown Instrument System [42] and the model 8705-8707 CTDs from Guildline Instruments [43], have errors of not more than 10 -4. Russian CTD probes, which also have high speed, a time constant of 30 msec, and a spatial averaging volume of 1 cm3, make it possible to obtain similar accuracy [44]. Transducer working volumes are more closely grouped than for other types of measuring devices, which reduces dynamic error in investigating medium fine structure. Noncontact conductivity transducers do not make direct electrical (or, more precisely, electrochemical) contact with the medium under investigation, so that they do not have the drawbacks of polarization effects, surface contamination, etc. This creates a potential for obtaining good metrological qualities, but their averaging regions are considerably larger than for the preceding type. A transformer transducer used in a hydrophysical probe system and having a thermistor range of from 1.2 to 7.0 S/m and a measurement error of no more than 4-10 -3 S/m is described in [7]. However, such transducers cannot be used for highfrequency measurements, because of their intrinsic spatial averaging. Moreover, the substantial device size and the materials used are sources of uncontrollable errors associated with the action of pressure and temperature. The problem of metrological support of conductivity measurements for electrolytes reduces to electrolyte conductivity certification and calibration of devices for measuring the conductivity of electrolytes chosen as standards. The test procedure for resolution certification of electrolyte conductivity measuring instruments is described in [45]. However, the primary 1051
precision class 0.1 standard proposed in that paper does not meet contemporary oceanographic requirements. There is a laboratory conductivity setup that permits certification of conductivity measuring devices to within 0.02% [44]. Given the current state of the art in metrological engineering, the primary standard should have an error of no more than 0.01%. No validation procedure for such a precision class presently exists in Russia. There have been attempts to determine the salinity of sea water without measuring its conductivity. A method for measuring sea water salinity described in [46] is based on light flux attenuation in a transparent optical body (waveguide), expressed as a function of the difference in the refractive indices of the material and ambient medium. It was established in [47] that the light polarization plane rotation angle is determined by the sum of the effects of all aqueous solution components. The feasibility of determining the salinity of surface sea water from remote optical measurements was considered in [48]. Simultaneous determination of sea water density and salinity with radioactive sources was suggested in [49]. It was shown that salinity can be determined to within 0.01%o and density to within 3-10 -5 g/cm 3 with a measurement time of 10 sec. The Combined Group on Oceanographic Tables and Standards (CGOTS) [33] designed the refractive index of light in sea water and the speed of sound as the next most important parameters of sea water status (after salinity). Refractive Index, From the physical standpoint, the refractive index of light in a medium n (also known as the coefficient of refraction) is determined from the ratio of the speed of light in vacuo co and in the medium under consideration c for temperature T, impurity concentration (salinity) S, and hydrostatic pressure p. One important feature of refractive index as a state parameter is its relationship to medium density. It follows from physical theory [50] that the so-called specific refraction is def'med by the expression: n~--I n2+2
-
-
4~ 3 N'~,
where N is the particle concentration, and 3' is the polarizability, which should be proportional to the density p, since o is proportional to N. An illustrative comparative analysis of the complex distribution of hydrophysical state parameters in the Arctic Ocean was given in [51], where it was concluded that the vertical distributions of Ao and An are similar, as well as that these distributions resemble that of the salinity S. Among published efforts to use refractive index measurements to devise a salinity determination method, we must note the work of Rusby [52]. This researcher employed a modified Jamin interferometer as the refractometric measuring instrument; it permits attainment of a measurement accuracy of about 4-10 - 7 rel. unit in the laboratory. He investigated sea water with a salinity 33 < S < 37%0 over the temperature range 17 < T < 30~ and obtained the formula: S =35+5,3302 -10~Anq-2,274.105An2-1-3,9. 108An~+ -}--10,59An(T--20) +2,5.102An 2(T--20). This formula was employed to tabulate the relationship in the form of the function S(An20) and the correction z~S(T - 20) valid over the range 15 < T < 30~ Although the resultant tables did not cover the temperature range at depth, they came into wide use and were published together with UNESCO's International Oceanographic Tables, which relate salinity to electrical conductivity. The error in calculations made with this formula does not exceed 5.10 -3 %0. Refractive index measurement methods are divided by type into interference, goniometric, photometric, and schlieren. All of them are employed in submerged refractometric equipment. An automated hardware system consisting of a probe submersible to 500 m, a laser interferometer, and temperature and pressure sensors was described in [53]. It was noted that the accuracy of sea water density field fme structure measurement can in principle be improved over that for CTD probes and that the density determination error can be brought down to 5" 10 -6. The best-known submerged refractometers [54, 55] have a sensitivity of 5"10 -7, a refractive index measurement accuracy of about 10 - 6 , and an inertia of less than 0.01 see. The spatial resolution in vertical probing is limited only by the light beam diameter (about 5 rnm). A probe of this type, the Okean, was used to obtain refractive index prot~es down to a depth of 400 m [54]. Turbulent fluctuations in the refractive index of sea water were investigated with a submerged T6pler instrument in [56]. The high speed of refractometers makes it possible to reduce dynamic measurement errors when they are used to replace transducers with the greatest inertia in hydrological probes, i.e., temperature transducers [51]. Metrological support of refractometric instruments requires further development. It was noted in [57] that this problem has been solved for instruments that measure refractive index to within 10 -5 in the case of transparent media. There are
1052
collections of standard media (glasses and liquids) that permit monitoring of determination accuracy n down to the fifth decimal place. However, there are no generally accepted calibration methods for more accurate refractometers. Speed of Sound. The speed of sound in sea water depends in a complex manner on temperature, salinity, and pressure. There are a number of empirical analytical functions that relate the speed of sound to these parameters with def'mite accuracy. The Leroy, Wilson, and Del Grecco formulas are the most widely known. Thus, the first few terms of Del Grecco's formula have the form: c=1449,063..}-4 , 575T--5,271.10 - 2 T 2 ~ (S--35) ( 1,344---1,329.10 - 2 T--}-1,04.10 -4 T2)-]-...,
where c is the speed of sound (m/sec), T is the temperature (K), and S is the salinity (%o). This formula makes it possible to determine the speed of sound to within 0.013 m/sec over the temperature range 273-303 K and the salinity range 31-39%o at atmospheric pressure. A UNESCO working group used these formulas and empirical data to obtain an equation connecting the speed of sound and state parameters with an accuracy of 0.2 m/sec over their entire variation ranges under field conditions [4]. A variety of methods exist for measuring the speed of sound, and a quite detailed survey is given in [58]. A generalized design for a system to measure the speed of sound is described in [18]. All such measuring devices contain an ultrasound receiver, a source signal converter, a receiver signal converter, a comparator, and a recording device. The many transducers for speed of sound can be classified as having pulsed or continuous operation, in accordance with the nature of the acoustic signal produced. Pulsed transducers are most frequently used under field conditions, since they are simple and reliable. Their general shortcomings include limited measurement sensitivity and accuracy, a consequence of the characteristics of pulsed signal processing, and the need for a large measurement base (about 10 cm), which produces a substantial averaging region [58]. Among continuous-operation transducers, we should note those of the phase type, which are the simplest, and those of the interferometric type, which have the greatest sensitivity with a minimal measurement Base (less than 1 cm) and high speed, exhibiting a time constant of about 10 -2 sec [59-61]. A comparative analysis of the limiting resolution of the most common types of measuring systems for speed of sound is given in [62]. The minimum sensitivity threshold for pulsed-cyclic systems is determined by the expression ~=~(g'~12n2) 1/3 ,
while for self-excited oscillator systems a = ~(g/27r2fn2) 1/3 and for interferometric systems a = 1/2~(gf/Q4e2) 1/3, where ~ is a parameter characterizing piezotransducers and electronic circuit noise, g is a parameter characterizing the medium, r is the repetition period of the source-exciting pulses, f is the ultrasound frequency, n is the integer wave number, Q is the resonator quality factor, and e is the degree of high-frequency signal modulation. The characteristic values obtained for these parameters were 0.6 cm/sec in the case of pulsed-cyclic systems, i0.12 cm/sec for self-excited oscillator systems, and 0.02 cm/sec for interferometric systems. All these levels are attained with an optimum probing signal amplitude, which can be calculated for each specific type of measuring device. Researchers are paying increasing attention to the sound speed field distribution in the ocean, as one of its fundamental characteristics [63, 64]. The best hydrological measurement systems include a device for measuring the speed of sound. The device in the aforementioned 196-RM system has an instrumental error of +0.1 m/sec and a response of 20 msec over the working range 1400-1600 m/sec. Measurement of the speed of sound in water to within < 0.1 m/sec is quite complicated. Unfortunately, there are no standard measuring systems for speed of sound in Russia having an error of 0.1 m/sec, so that precision transducers can be developed only if they are indirectly calibrated in a reference system with a standard liquid, generally distilled deaerated water [65]. The UTSZ-M system described in [65] permits creation of media characterized by sound speeds in the range 1402.41540.3 m/sec at atmospheric pressure (to within 0.1 m/sec). Overpressures reduce calibration accuracy, since the published polynomials that relate the speed of sound to pressure and temperature in distilled water [66] and sea water [4] have an error of no less than 0.2 m/sec. Use of sound speed transducers in measuring systems in place of inertial temperature measuring devices makes it possible to reduce dynamic measurement errors. Moreover, the measurement volumes for conductivity and sound velocity transducers can be combined [67]. This type of measuring system was investigated in [68], and it was shown that use of appropriate combined transducer design parameters can extend the passband to 100 Hz. 1053
Density Determination Methods. Density is a fundamental characteristic of the ocean, determining the character of the hydrodynamic processes taking place. However, its measurement in situ is a complex matter, since various accurate direct density measurement methods [69, 70] are unsuitable for field study of hydrophysical fields. Another group of methods comprises those of the indirect type, in which parameters connected to liquid density by definite relationships are measured. They can be classified as chemical, optical, acoustic, and STD methods, depending on the set of independent parameters used in the density computations. The chemical (argentometric) method for measuring sea water density, which is widely employed in oceanography, is based on the hypothesis that ocean water salt composition is constant. The mass content of the C l - ion, which accounts for 55% of total dissolved matter, is determined by titration with silver nitrate [71]. Salinity is determined from chlorine content with the tables given in [72]. The density is then computed, and the resultant error is 10 -5 . Optical methods are based on the connection between density and light beam refractive index. They make it possible to obtain a general visual picture of hydrophysical field variation. A more detailed analysis of the capabilities of such methods was given above. Acoustic measurement methods utilize the dependence of the speed of sound on salinity, temperature, and pressure [73]. These techniques are not highly accurate, although they have some advantages, which were pointed out above. The method that employs the equation of state and the empirical dependences of the salinity S on temperature T, conductivity C, and pressure p is most widely used at present [74]. Two kinds of measuring systems are found in practice: STP (salinity, temperature, and pressure) and STD (salinity, temperature, and depth). The difference between the two systems lies in the choice of salinity measuring device. The salinity transducer in an STP system is a device in which the destabilizing influence of temperature and pressure are compensated for with analog circuitry. The influence of T and p are taken into account by numerical methods in an STP system, by means of the equation S = S(T, R15, p). Compensation by analog methods is virtually impossible in high-precision measurements, because of the strong nonlinearity and complexity of the function S = S(T, R15, p). Preference is therefore given to the STP method in hydrophysics. The relative error in density determination by this method can be brought down to 10 - 6 when the PSS-78 and the new International Equation of State are employed. Dissolved Oxygen Transducers. The amount of oxygen dissolved in sea water does not enter directly into the equation of state, but it must be taken into account in studying processes in the ocean, since the physicochemical state of the water is associated with stable stratification and formation of thermoclinic circulation, whose contribution to overall circulation is quite significant. Moreover, we know from [75] that the dissolved 02 concentration in the upper layers of the ocean have a considerable gradient and can serve as a tracking parameter in studying hydrodynamic processes. The dissolved oxygen distribution by depth in the upper layers of the Black Sea is shown in Fig. 1. Winkler's hydrochemical method is used for chemical analysis of water samples in marine research. This technique is widely known, and the error in determining the dissolved oxygen concentration in water amounts to 0.04 ml/liter. There are also calorimetric, luminescence, radiosiotope, and other methods. However, the electrochemical method has found widest use in recent years; it was developed after Geirov's discovery in the 1920's that the electrical current through a solution depends not only on the voltage applied to the electrodes but also on the substance present in the solution and its amount. The author used the term polarography to denote his solution composition determination method. Most primary oxygen transducers (PITs) for field measurements have been based on this method. The designs of Russian and foreign PITs have been described in detail in [7]. A typical transducer design consists of an insulating cell, within which is a cathode with a small surface area and a large anode. The cell is separated from the ambient medium by an oxygenpermeable membrane. This sort of design is used in the bathyoximeter of [76]. Transducer sensitivity threshold is 0.01-0.07 ml/liter. There are also electrochemical transducers, "magnetic pump" transducers, and transducers based on the nuclear magnetic resonance effect, but they are less accurate than Winkler's method. The requirements to be imposed on such measuring devices at the present state of the art were formulated in [7]: a measurable concentration range for oxygen dissolved in sea water of 0-12 ml/liter; a working temperature range of from - 2 to + 35~ a time constant (at 20~ of no more than 2 see; a measurement error of no more than 0.2 ml/liter. More stringent requirements were imposed on measurement accuracy in [1], calling for an error of no more than 1%. The best Russian and foreign PITs, e.g., the Bencauminor, Hydrolab 11 A, and Precision Scientific 68850, provide the required measurement accuracy over the temperature range from - 2 to + 40~ at depths of more than 2000 m. The calibration equation for all polarographic PITs has the form: 1054
Co,,i =k (N t--No) exp[ (m (I / T i - I Tgr) ~-#,IP4-k~(S--Sgr) ],
where k = C02 gr/(Ngr-N 0 ) is the transducer sensitivity for temperature Tgr, salinity Sgr, oxygen content CO2gr, and instrument output signal amplitude Ngr, T i is the polymer membrane temperature, N i is the instantaneous transducer output signal amplitude, N O is the residual transducer signal amplitude, and S is the sea water salinity. When a transducer is to operate in the presence of large temperature gradients, a dynamic correction has to be introduced for determination of the true membrane temperature, together with a dynamic direction that takes into account the PIT inertia with respect to oxygen. PIT' calibration is in specially designed equipment, on which stringent requirements are imposed: the specified oxygen concentration should vary by no more than 0.05 ml/liter during the measurements; the specified temperature should not vary by more than 0.05~ during the measurements; it should be possible to set up different known water flow rates; ~it should be possible to take water samples for analysis by Winkler's method. A number of such systems have been described in [7, 77]. They permit calibration of PITs in conformity with the above requirements. Other methods have recently been developed for in situ determination of oxygen dissolved in sea water. Thus, a pulsed potentiometric 02 determination method was proposed in [78]; it is based on the maximum current measured at a constant molecular oxygen electrical reduction potential. This technique permits measurement within less than 1 sec. A method for measuring the contents of 02, CO 2, and other chemical components of sea water with fiber optics was described in [79]. A new algorithm for calculating dissolved oxygen level from data yielded by an STD probe equipped with an oxygen sensor was proposed in [80]. Hydroehemical Characteristics, One of the basic hydrochemical determinations is that of hydrogen ion activity (pH). The concept of pH was introduced into electrolytic dissociation theory in order to characterize total solution acidity. The numerical pH equals the negative base 10 logarithm of the hydrogen ion concentration: pH = - l o g [H+]. The many pH determination methods can be divided into electrometric and nonelectrometric [81]. The basic nonelectrometric methods are catalytic, calorimetric, and spectrometric. They are used for precision laboratory measurements. There are two electrometric methods: conductometric and electrochemical. The conductometric method is based on determination of solution conductance and subsequent calculation of the pH. It is employed rather infrequently and only when making measurements on strong acid and alkali solutions with a substantial salt content, when solution composition is exactly known. The electrochemical method is based on measurement of the potential or emf difference for an electrochemical cell in which the solution to be analyzed is used as the electrolyte. It is the most widely employed technique, especially for investigating sea water. Primary pH transducers are quite mature and are mass-produced. The standard transducer is a combination of two electrodes: a measuring electrode, whose potential is proportional to the pH, and a reference electrode, which is connected to the solution and has a very small, almost invariant intrinsic potential. When the transducer is placed in the test medium, it produces an output signal having the form of a potential difference:
r =%H+~Ig[H+ ], where c~ = 1.98.10 -4 T, ~b0H is the normal electrode potential (arbitrarily assumed to equal zero), and T is the temperature on the Kelvin scale. The measuring electrodes can be of the hydrogen type, fabricated from chemically pure porous platinum, quinhydronebased (containing quinhydrone as the dissociative compound), or made of metal oxide or glass, using glasses containing monovalent metals (Li, Na, K, etc.) as impurities. The most common reference electrodes are of the silver chloride type, whose potential remains constant to within +2 mV over the entire pH variation range [82]. Measuring devices for pH generally operate in the range from ph - 1 to 14 with an error of about 1% and a sensitivity of no less than 0.01 pH unit. Similar parameters are exhibited by foreign and Russian-produced transducers, such as the DPg-4 and DPr-3, which are intended for working pressures of up to 600 kPa under field conditions [8]. The interest of in situ pH measurement is due to the fact that this parameter has a significant gradient in the upper ocean layers. Figure 2 is a graph representing pH as a function of depth [83]. Marine hydrochemistry utilizes another medium characteristic analogous to pH, i.e., the oxidation-reduction or redox potential Eh = t%~ + RT log a e, where #e ~ is the standard chemical potential of the electron, a e = kV is the activity of the electron, and v is the free electron concentration in the medium.
1055
The redox potential can be measured by different methods: potentiometric, colorimetric, polarographic, and polarization-curve [84]. The most widely used, however, is the potentiometric method, which is based on measurement of the emf of an appropriate reversible electrochemical cell. It is analogous to the electrochemical pH determination method and has similar measurement errors; it differs only in electrode design and the buffer solutions employed. Like the pH, the redox potential Eh has a significant gradient in the upper layers of the ocean (from +0.8 V to - 0 . 2 V) [85], which has attracted the attention of researchers. Transducers for pH and Eh are calibrated with respect to standard solutions. High-precision master facilities for calibrating such transducers have relative measurement errors not exceeding 10 -3 [86]. The results of pH measurements under field conditions are given in [87], which proposes a method for continuous sea water pH recording along a ship's track. A special system has been developed to implement it. The pH is calculated from the electrode potential, with allowance for the temperature characteristics of the electrode and sea water and the two buffer solutions used in the system. This technique provides a total measurement error of +0.003 pH unit. Continuous pH distribution profiles in both the horizontal and vertical planes have now been obtained for some oceanic regions. A method for standardizing water pH measurements with glass electrodes on the NBS scale with just one buffer (pH 7.413) was suggested in [88]; it utilizes an equation for the apparent hydrogen ion activity coefficient as a function of salinity and temperature. The results of Eh measurements made in different oceanic regions are given in [89, 90]. Ion-Selective Transducers. The variation of sea water ionic composition is studied by methods based on the properties of ion-selective membranes, which are the principal design element for selective measuring electrodes. The membranes can be fabricated from liquid or solid ion exchangers; the latter are preferable for use in metrological engineering, by virtue of their easier manufacture. Solid ion exchange membranes provided the basis for creation of a set of electrodes that permits activity measurements for single-charge cations (K +, Na +, Li +, etc.) and anions (CI-, B r - , I - , etc.) [91]. The operating principle of a selective electrode is as follows: the membrane it contains separates the internal cavity, filled with a reference electrolyte of constant composition, from the test medium. Passage of a def'mite type of ion through the membrane results in development of a diffusive potential difference between the cavity and ambient medium. An electric field is produced between the cavity and medium after diffusion has proceeded for some time. This field creates an obstacle to cation (or anion) migration and blocks it completely at some equilibrium value E. The potential difference created between the electrode's internal cavity and the medium under these conditions is called the equilibrium membrane potential. Its magnitude is defined by the expression [92]: ~p=q~'o'~"
2,303/~T Z~T lg[ai],
where R is the universal gas constant, T is the absolute temperature, Z i is the ion charge, a i = c i f i is the ion activity, ci is the concentration of ions of species i in the medium, fi is the activity constant, and ~0 is the standard electrode potential (for a i -- 1). Electrodes with membranes selective for K + and Na + cations are often employed in physicochemical studies of natural water [92]. Types ESL-51G-05 and ESL-51G-04 sodium electrodes and various modified versions of them are mass-produced in Russia. The Orion firm has set up large-scale manufacture of such electrodes abroad [93]. The wide introduction of ion-selective electrodes in marine hydrochemical research is to a considerable extent hampered by methodological difficulties. There are "interfering" ions for all known electrodes under practical conditions. Specifically, H + has a strong interfering effect for sodium electrodes, while Cs + is detrimental to potassium electrodes. This requires development of rather complicated special procedures for solutions as complex as sea water and thus limits oppornmities for electrode use in situ. It also accounts for the low accuracy of measurements made with ion-selective transducers (about 10%). Another drawback of ion-selective electrodes is their inertia. The measurement time constant is about a minute at room temperature and 10-15 min at temperatures below 10~ which is very long for field studies. Reports have recently appeared on new methods for measuring sea water ionic composition. An x-ray fluorescence hydrochemical system for investigating sea water salt composition under shipboard conditions was described in [94]. Chlorine, potassium, and calcium contents are determined with closed and flowthrough cuvettes. The measurement errors are estimated at 0.5-1.5%, with a per-sample analysis time of about 40 sec. It is suggested that the system also be used to determine the contents of other basic salt components. Luminescence and Other Physieochemical Characteristics. Luminescence methods are considered to be promising for oceanographic research but not yet adequately refined for wide use; they essentially consist in measurement of the optical 1056
luminescence of sea water when artificial luminophors are added to it [95] or when natural chemical and biological luminescence is present [96]. Such techniques are highly sensitive (down to 10-12 mole/liter) in determining various impurities. It was shown in [6] that low-frequency fluctuations in sea water luminescence characteristics are associated with surface and interior waves, as well as with thermoclinic water stratification and turbulent and frontal variability. The refraction, absorption, and scattering indices of sound waves at various impurities, gas bubbles, and organic inclusions in the ocean can be used like the refractive index of light rays. Modern ultrasound sensors permit measurement of the refraction index to within about 10 -4 [18]. The influence of oceanographic characteristics on acoustic energy loss in sea water is discussed in [97]. A new method for determining total dissolved and gaseous nitrogen with high accuracy was proposed in [98], while a technique was suggested in [80] for determining several sea water components present in the upper ocean layers, which are characterized by significant variation, and carry information on processes taking place in the depths. The scientific and practical demand for accurate measurement of oceanographic parameters is thus continually increasing. The requirements presently imposed on measurement accuracy for some parameters are at the levels of existing standards and reference systems. While the temperature standard provides the necessary measurement precision, the conductivity standard has considerably greater error, which ultimately makes it impossible to provide the requisite density measurement accuracy. Despite the fact that a great deal of experience has been amassed in Russia and throughout the world in measuring the physicochemical parameters of the marine environment, adequate work has not been done in a number of areas. This is true, for example, of procedures for estimating dynamic measurement errors and particularly for dynamic transducer calibration. Further progress requires direct and indirect methods for determining sea water density under filed conditions. A number of standard bases failed to meet contemporary requirements. It is for this reason that UNESCO has recently created a number of new working groups concerned with development of standards bases and direct incalibration of various systems for measuring marine environmental parameters [99] [sic]. The efforts of different nations must be combined in these labor-intensive and expensive undertakings, in order to save time and money. Among the achievements of Russian metrological science in physical oceanography, we must mention the successful development of methods for laboratory simulation of hydrodynamic processes. Measurements on laboratory models are one promising way to monitor the actual accuracy of instruments intended for field research.
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