Flexible Inkjet Printed Sensor for Liquid Level Monitoring - IEEE Xplore

Flexible Inkjet Printed Sensor for Liquid Level Monitoring Milica Kisic, Nelu Blaz, Cedo Zlebic, Ljiljana Zivanov, Mirjana Damnjanovic Department of Power, Electronic and Telecommunication, Faculty of Technical Sciences, University of Novi Sad, Serbia [email protected]

Abstract: In this work a method for liquid level measurement is presented, with a support of experimental results. The presented liquid-level measurement method uses two sensors: level and reference interdigital capacitors. Proposed sensors are fabricated in low-cost inkjet printing technology with nanoparticle silver ink on a NoveleTM coated polyethylene terephthalate (PET) substrate. The sensors comprise a two interpenetrating comb electrodes with 1.5 mm width of comb electrode, 1.5 mm spacing between each comb and 15 fingers with length of 135 mm and 8 fingers with length of 19 mm for level and reference sensor, respectively. Capacitance of the level sensor fixed on a test tube increases as the liquid level increases (up to 43.5 mm). The sensor characteristics for three liquids with different permittivity are presented. The reference sensor at the bottom of the tube is completely covered by liquid and its capacitance changes as the permittivity of the liquid changes. Experimental results for three different liquids confirmed simple liquid level measurement method which enables the measurement regardless of the liquid type. 1. INTRODUCTION Liquid level measurements have been investigated by many researchers, since they have been required and very important in many industrial processes, and different kind of applications (like pharmaceutical, food, chemical, bioanalytical), for recharged areas, floods, leachate in landfill, etc. There are various types of level-sensing probes with invasive or non-invasive methods employed to monitor a liquid level [1-3]. Life period of the contacttype level sensors may be limited, because their characteristics can be changed due to the physical or chemical reaction between the liquid and the probe material. On the other hand, the noncontact type levelsensing probes are costly and require various environmental and experimental precautions during the usage. Some techniques for detection and monitoring of the liquid level are: acoustic [4], ultrasonic [5] or optical [6]. Fiber-optical sensors require good acoustic reflection properties, suffer from scattering effect for different container shapes, the waves scattered through the bubbles, thin-film residue on the sensor in the case of fluids [7-9]. Composite resonant millimeter-sized piezoelectric

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cantilevers were fabricated for detection of changes in liquid level at micron level using a lead zirconate titanate (PZT) actuated millimeter-sized cantilever [10]. In [11-13] are presented some magnetic field sensors that are based on powering effect via oscillating magnetic fields. Time-domain reflectometry (TDR) can be successfully used for a simultaneous real-time qualitative and quantitative monitoring levels of liquids [14, 15]. In [16] it is presented innovative approach for low cost continuous liquid level monitoring based on virtual instrumentation using differential pressure sensors. Besides the specified liquid level measurements, capacitive sensors are one of the most used ways for detection of liquid level [17-20]. The various types of sensors applying polymeric foils were realized for different applications, purposes and unique possibilities, since they are lightweight, mechanically flexible, wearable, stretchable, easy to install, adjustable to the aspect of application, and suitable for use in different conditions. Sensors realized on the flexible substrates can be formed into desired shapes, can withstand bending to very small radii of curvature and can be placed on any type of surface.

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38th Int. Spring Seminar on Electronics Technology

Fig. 1. Design and cross–section of the sensor on the tube with electric field lines created by comb electrodes above the tube’s wall and liquid: a) level and b) reference sensor.

In this work, capacitive sensors were manufactured in inkjet printing technology on flexible substrate. Flexible level sensor was bended in the form of a circular cylinder and fixed on a glass tube (Fig. 1a). Liquid level measurement was based on the fact that the permittivity of liquids inside the tube is higher than permittivity of the air, so the capacitance increases with higher level of the liquid. The other, reference sensor is planar and fixed at the bottom of the tube (Fig. 1b). Its capacitance changes depending on the permittivity of the liquid. Three liquids with different permittivity were tested and experimental results are presented.

2. DESIGN OF INTERDIGITAL CAPACITIVE SENSORS Design of the level and reference sensors, and cross–sections of the sensors placed on the tube with electric field lines created by comb electrodes. The level sensor comprises two interpenetrating comb electrodes with sl = 1.5 mm width of comb electrode, wl = 1.5 mm spacing between each comb, and 15 fingers of Ll = 135 mm length. The fringing electric field between electrodes penetrates above the interdigital electrodes and it is proportional to the spacing between the center-lines of the sensing and the driven fingers [21]. Liquid level sensor was placed around the outer side of the test tube (outer radius of 24 mm, 2 mm thickness of the wall), in which liquid is added. The level sensor is designed in that way to fringing electric field penetrates inside the test tube (Fig. 1a). The capacitance of the interdigital level 978-1-4799-8860-0/15/$31.00 ©2015 IEEE

sensor changes with the liquid level inside the tube, since the levels of the empty part of the tube (i.e. filled with air) and full part of the tube (filled with liquid) are changed. The capacitance between the two electrodes in the filled part of the tube depends also on the liquid’s permittivity. In order to provide that the liquid level measurement depends on the permittivity of the liquid, reference sensor is added on the outer side of the tube’s bottom. The reference sensor comprises a two interpenetrating comb electrodes with 1.5 mm width of comb electrode, 1.5 mm spacing between each comb and 8 fingers with length of each finger of 19 mm. The electric field lines pass inside the tube due to the design of the sensor. For the same level of the liquid inside the tube, the capacitance changes of the reference sensor depends only on the permittivity of the liquid.

3. LIQUID LEVEL MEASUREMENT AND RESULTS Interdigital capacitive sensors are inkjet printed as planar structures on NoveleTM coated polyethylene terephthalate (PET) substrate with nanoparticle silver ink Metalon JS-B25HV. Hence, fabricated sensors can be flexed around curved structures. Level sensor is curved and fixed with glue on the outer side of the glass tube, while reference flat sensor is glued at the bottom of the tube. Photographs of the fabricated sensors and sensors fixed on a test tube are shown in Fig. 2. The testing of the sensors is conducted using the measurement setup shown in Fig. 3.

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a)

b)

c) Fig. 2. Photograph of the fabricated: a) curved level sensor, b) flat reference sensor, and c) sensors fixed on a test tube.

Fig. 3. Photograph of the measurement setup: Impedance Analyzer HP4194A and sensors fixed on the tube filled with the liquid.

The sensors are connected to Impedance Analyzer HP4194A and changes of the capacitance value are measured in the frequency range from 1 MHz up to 9 MHz. The level of the liquid inside the test tube can be read from the scale on the test tube and liquid is precisely added using the syringe.

(~80) compared to the acetone (~20) and especially the benzene (~2), the capacitive-type liquid level sensor has the best performance for deionized water and shows higher permittivity (sensitivity of 0.105 pF/mm compared to acetone (0.083 pF/mm) and benzene (0.013 pF/mm).

In Figs. 4a, b, c are presented the capacitance values of the level sensor for level measurements from 0 mm up to 43.5 mm for deionized water, acetone and benzene with permittivity of 78, 20.7 and 2.27, respectively. Measurements are presented for first step of 1.5 mm, in order to liquid level cover first finger of the electrodes, and after that in steps of 7.5 mm. As can be seen, capacitance value increases as the liquid level increases in all three cases. Experimentally obtained capacitance versus liquid level characteristics for tree different liquids are presented together in Fig. 4d. The sensitivity of the sensor is better if the permittivity of the liquid is higher. Since the deionized water has a higher value of permittivity

For the liquid level measurement, one sensor cannot be used due to the effect of permittivity on capacitance changes. In addition to the capacitive level sensor, the reference sensor is added at the bottom of the tube in order to detect the liquid depending on its permittivity.

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Capacitance of the reference sensor is measured for the first step (change) of level (1.5 mm). Reference sensor has surface completely covered by fixed level of liquid, and hence, its capacitance changes depend only on permittivity of the liquid. In Fig. 5 can be seen that capacitance of the reference sensor varies with the liquid type (increases as the permittivity of the liquids increases).

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a)

b)

c)

d)

Fig. 4. The capacitance changes of the level sensor for different levels (in mm) of: a) deionized water, b) acetone, c) benzene and d) the measured capacitance characteristics as a function of liquid level.

4. CONCLUSION In this work, the liquid level measurement method for different liquid type is proposed and realized. Experimental results confirm that the fabricated sensor can be effectively used to detect and measure level of various types of the liquids. Presented flexible sensors can be applied to any type of liquid, in any liquid resource, tank, vessel or container. Fabricated sensors do not require additional coating layers, does not have any physical or chemical reaction with the liquid and the measurement method enables detection of the any non-conducting, inflammable or aggressive liquids. Fig. 5. The measured capacitance changes of the reference sensor for liquids with different permittivity values.

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It is shown that the proposed measurement method gives the correct result, i.e. dependent on the liquid type in the tube. The measurements are very sensitive

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to the liquid type, respectively to its permittivity values. Realized sensors have better sensitivities for liquids with higher permittivity. The tube geometry, its thickness as well as permittivity, affects the sensitivity of the sensors and could be investigated in the future work, in order to achieve better performance of the measurement method.

ACKNOWLEDGEMENT This work was supported in part by the Ministry of Education, Science and Technological Development, Republic of Serbia, on project number TR-32016 and III45021.

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