Elsevier

ISA Transactions

Volume 115, September 2021, Pages 250-258
ISA Transactions

Practice article
A constant phase impedance sensor for measuring conducting liquid level

https://doi.org/10.1016/j.isatra.2021.01.024Get rights and content

Highlights

  • Investigation of the constant phase impedance sensors for level measurement for the first time.

  • Determination of the response characteristics of the sensor.

  • Phenomena for the constant phase behavior.

  • A phase detection circuit for interfacing the sensor.

  • Fabrication is simple, inexpensive, sensitive and free from frequency error.

Abstract

In the contact type capacitive liquid level sensors, when an electrode with the insulating film is immersed in polar/ionic medium, it shows constant phase behavior at metal–insulator interface due to the formation of the double layer. This double layer effect is frequently modeled by a pure capacitor but its capacitance value depends on signal frequency. Therefore, when such a sensor is excited by sinusoidal ac, the conducting liquid level measurement suffers from the error due to the fluctuation of input signal frequency. This is because the excitation frequency applied from the source meter may fluctuate. This important issue is rarely discussed for the capacitive level sensors. In addition, the design and realization of the capacitive level sensor require special arrangements for the minimization of parasitic earth capacitance, offset capacitance, and the capacitances due to leads and contact electrodes. In this paper, we propose a novel constant phase impedance sensor for the measurement of conducting liquid levels in the range of 0–4 cm for the first time. The phase angle of the device changes due to a change in the liquid level. Two important characteristics parameters of the sensor are the constant phase angle for a certain frequency range and the fractional order in the range of 0–1. Some important features of the sensor are significant sensitivity (2.1/cm, probe 1) due to small change in conducting liquid, stable output due to fluctuation of input frequency, and the fabrication of the sensor is very simple and inexpensive. The device is finally interfaced to a phase detection circuit to convert the phase angle into a voltage signal.

Introduction

Liquid level measurement and control in a storage tank is an important task in the process industry for the efficient running of a plant. Some commonly used techniques for the measurement of the liquid level are mechanical float, capacitive, resistive, optical and ultrasound and electromagnetic, etc. [1], [2], [3]. Such methods use various parameters such as buoyancy, dielectric constant, the reflection of light, sound waves, and magnetic field from liquid surfaces. Methods of level measurement can be broadly classified into two categories namely contact-type and non-contact type. In contact-type, the sensing probe directly comes in contact with the liquid for example mechanical float, capacitive, resistive, and pressure sensing types. Contactless methods include those in which the level is measured without any contact of the sensor with the liquid such as capacitive, optical, and ultrasound sensors [4], [5], [6]. In all these methods, some parameters of the sensor change with the change in the liquid level. These changes are converted by a transducer into an electrical or mechanical signal. The output of the transducer is interfaced to an electronic circuit, signal conditioned, and fed to a calibrated output indicator. Several research works have been reported to use fiber optic or fiber Bragg grating techniques for level measurement [7], [8], [9], [10]. Optical methods offer high sensitivity, immunity to electromagnetic interference, resistance to corrosion, but suffer from limited measurement range, error due to external light source, and complex structure [11]. The time of flight of an incident wave and the wave reflected from the liquid surface is an important parameter in ultrasound sensors and multisensory arrays for level measurement [11], [12]. This method of measurement is accurate and effective but it requires complex signal processing of the receiver signal.

Capacitive sensors are widely reported in the literature because of its cost-effectiveness, linearity, and low power consumption. In the case of a metallic storage tank, the capacitance formed between an electrode immersed in the liquid that acts as a dielectric medium and the storage tank varies due to variation of liquid level.  For nonmetallic storage tank, two identical metallic electrodes are immersed in the liquid and the capacitance between the electrodes varies due to variation of level. These metallic electrodes may suffer from self-inductance effects. To eliminate the self-inductance effect, a modified capacitance-type level sensor was proposed by Bera et al. [2]. For non-contact capacitive conducting level measurement, the air column between the conducting liquid and the sensing electrode acts as a dielectric. But this technique suffers from error due to some changes in the dielectric property of air in humid conditions. This issue is addressed by using a uniform circular insulating cylinder as the dielectric so that the error due to air dielectric does not arise [3]. B. Jin et al. [13] use a shielded cable, instead of using active shielding, for the level measurement for conducting liquid to overcome the effect of the parasitic capacitance of active shielding. Shielded cable is used as a grounded cylindrical capacitive sensor with the inner conductor of the cable connected to the ground. A three capacitive sensors liquid measurement system is proposed with minimization of air effect in [14]. A remote grounded capacitive level sensor with a detailed analysis of the parasitic components on the active-shielding circuit is proposed in [15]. In [16], the planar capacitive sensors with leakage detection are investigated for level measurement. Liquid level in non-stationary tanks using single capacitive contact type probes with artificial neural networks for improving performance is proposed in [17]. An interdigitated capacitive sensor interfaced to a microcontroller-based circuit for water level measurement with a range of 0–30 cm and a resolution of 0.2 cm is reported in [18]. In [19] another comb-electrode capacitive sensor placed on the outer wall of a container is used for contactless level measurement for biomedical applications. In the contact mode of level sensing, the metal electrode covered by insulator film is dipped in a liquid sample. It has been observed that the capacitance of such systems is not constant but depends upon the signal frequency due to diffusion of ions forming a double layer at the metal–insulator interface with the polarizing medium like water [20], [21], [22]. This is called the frequency dispersion of the capacitance. Such systems behave as constant phase elements (CPE). It is observed that the behavior of a double layer is not purely capacitive and its capacitance value depends upon the frequency [23]. It is sometimes modeled by a pure capacitance but P. Zoltowski [20] has found in his study that characterizing interfacial double layer by a capacitance instead of CPE is misleading and it causes modeling errors in system parameters. Therefore, the CPE sensor may be a better choice for contact type liquid level measurement to overcome the issues with the capacitance type liquid level sensor discussed above. The constant phase element is in general impedance whose phase angle remains constant over a wide range of input signal frequency. The constant phase behavior arises for the formation of R-C structures (distributed time constants) along the surface or normal to the electrode surface. If the insulating film is porous, a large number of distributed time constants corresponding to each pore is possible. The origins of CPE are investigated in the literature [20], [21], [22], [24], [25] but its physical explanation is still incomplete. Besides, the capacitive contact type method for conducting liquid, the electrode insulation is the dielectric below the liquid–air interface. But above the liquid–air​ interface, the electrode insulation and the air both behave as the dielectrics. Thus, error may arise due to air dielectric which varies in humid conditions. Whereas, CPE behavior depends only on the area of electrode below the liquid–air interface, so the error due to air dielectric may not arise. Moreover, the capacitive liquid level sensors use metallic electrodes which are rigid in nature. This limits the transportation and installation of the capacitive liquid level sensors at remote locations. In addition, the capacitive sensor requires a special arrangement for the minimization of parasitic earth capacitance, offset capacitance, and the capacitances due to leads and contact electrodes. But in this work, the CPE sensor is fabricated by a thin polyimide strip which is flexible. It can be rolled and transported easily to remote locations.

The CPE has been employed in different sensing applications where the change in constant phase angle depends on the ionic conductivity of the conducting medium, the thickness of the insulating layer, and the contact area of the electrode with the electrolyte [26]. If the thickness of the insulating film of the sensor is unchanged, then the CPE for a particular conducting liquid depends on the contact area of the electrodes. The contact area varies due to a change in the liquid level.

Therefore, in the present work, the CPE impedance sensor is found a novel application of liquid level sensing based upon the fact that the CPE response depends upon the contact area of the electrode and the type of conducting liquid. A CPE is fabricated using a double-side copper-clad polyimide by depositing a thin insulating film on both surfaces. The insulating film having a suitable porous structure can be made of either polymer/oxide material. When the probe is immersed in an ionic liquid, it behaves as a CPE. The behavior of the CPE probe is studied by measuring the variation of phase angle when it is immersed into the tap water at different lengths. Three different probes are constructed by varying the thickness of the insulating film deposited on their copper cladding. Initial experiments are conducted using a PC interfaced Keysight Impedance analyzer (4294A). The sensor is excited with an input voltage of 500 mV(rms) and the signal frequency is varied from 40 Hz–1 MHz. Finally, an electronic circuit is developed for converting the phase angle change into a corresponding voltage signal. The experimental results show that the constructed CPE can be used even for a small liquid level measurement. The sensor can be fabricated with in-expensive materials in bulk quantity.

Section snippets

Working of the proposed CPI sensor

The impedance Z in frequency domain of a CPE is mathematically represented by an expression [27] Z(s)=QSβwhere S=jω is the Laplace operator, Q and β are the two characteristic parameters of the CPE and ω is the signal frequency. Here, β is a fractional exponent and its value lies within −1 to +1.

Magnitude of the impedance is given by |Z(jω)|=Qωβ and the phase angle is given by θ=πβ2 radians. It is shown in the expression that the magnitude of the CPE depends on the frequency as well as the

Experiment

An experimental arrangement consisting of a glass container, the CPE sensor probe, and a precision Agilent Impedance Analyzer (4294A) was utilized to determine the electrical responses of the sensors. The terminals of the probes were directly connected to the test fixture of the impedance analyzer (16010E). Experiments were performed to measure water levels of 400 ml contained in the 500 ml glass vessel with probe 1, probe 2, and probe 3 respectively. Two types of water were considered such as

Results and discussions

All the three sensors show constant phase behavior for a certain frequency range of 100 kHz to 1 MHz. CPE behavior at interfaces is explained by the distribution of the time constants. The primary cause of this behavior is the abnormal diffusion of ions through the porous insulating film. For a smooth surface, an ordinary diffusion takes place where the mean square displacement of diffusion of ions x2, is directly proportional to time (t) i.e  x2t, (Fick’s law). For abnormal diffusion, the

Interfacing circuit for phase angle measurement

To make a prototype system for liquid level measurement using the CPE device, the schematic of the circuit to detect the change in phase angle in terms of the voltage signal is shown in Fig. 8(a). This circuit may be suitable for interfacing the sensor which shows constant phase behavior over a wide frequency range. The circuit consists of a fractional-order differentiator circuit and an inverting amplifier. All the sensors were immersed in water and interfaced to the fractional-order

Conclusions

In this paper, a probe type CPE sensor was developed for the measurement of liquid level in a storage tank. The sensor was fabricated by depositing thin PMMA film on each side of the copper-clad polyimide strips. Three different sensor probes were fabricated having a single layer, two layers, and three layers of PMMA film. The pore morphology of the films was studied. The tap water level was varied from 0–4 cm to determine the electrical parameters of the sensors. Well defined constant phase

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Mohammad Zia Ur Rahman received the M. Tech. degree in instrumentation and control systems from Aligarh Muslim University, Aligarh, India. He completed his Ph.D. degree in the Department of Electrical Engineering, Faculty of Engineering and Technology, Jamia Millia Islamia Central University, New Delhi, India in 2019. His research interest is in instrumentation and measurement.

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  • Cited by (5)

    Mohammad Zia Ur Rahman received the M. Tech. degree in instrumentation and control systems from Aligarh Muslim University, Aligarh, India. He completed his Ph.D. degree in the Department of Electrical Engineering, Faculty of Engineering and Technology, Jamia Millia Islamia Central University, New Delhi, India in 2019. His research interest is in instrumentation and measurement.

    Omar M. Al-Dossary obtained his Ph.D. degree from University of Essex, United Kingdom in 1994 entitled “Macroscopic Polar Optical Lattice Vibrations and Electron–Phonon Interaction in Layered Semiconductor structures. At present, he is working professor in Physics, theory of electron–phonon interaction at Department of physics and Astronomy, College of Science, King Saud University (KSU), Riyadh, Saudi Arabia. Professor Al-Dossary is an editor in chief of the Journal of King Saud University Science, He has published more than 75 research papers in reputed international journal. He holds various administrative assignments as a Dean/chairman/consultant/ board secretary in various universities. He was also the member of the council of various universities. His professional interest motivates him to work with the teams of researchers from King Saud University (KSU), KACST, Najran University, Korea University, Shanghai university, Glasgow university, Fritz-Haber-Institute der Max-Planck-Gesellschaft and Cambridge University in the field of Nano science and technology (spintronics and sensors devises), photoionization and associated quantum Phenomena and Quantum optics.

    Tarikul Islam (M’16-SM’18) was born in Murshidabad, West Bengal, India. He received the M.Sc. Engg. Degree in Instrumentation and Control system from A. M. U. Aligarh, U.P. in 1997 and the Ph.D. (Engg.) degree from Jadavpur University, Kolkata, India, in 2007. From 1997 to 2006, he was Assistant Professor and from 2006 to 2012, he was Associate Professor with the Electrical Engg. Department, Jamia Millia Islamia (J.M.I.), a Central University. Since, 2012, he is working as Professor with the same University. He has over 20 years of teaching and research experiences. He has authored/co-authored 6 book chapters, two edited books, filed two Indian patents and published more than 160 papers in peer reviewed journals and conferences. His research interests include sensing technologies, and electronics instrumentation. He is a life member of IETE (India), ISTE (India). He is associated editor, IEEE Sensors journal, IEEE TIM. He is a co-editor of a special issue of an International Journal of Electronics, MDPI.

    ACKNOWLEDGEMENT: Authors would like to thank Ministry of Minority affairs, Govt of India for granting fellowship to complete a part of the work. The authors would also like to extend their sincere appreciation to the researchers supporting project number (RSP-2020/61), King Saud University , Riyadh, Saudi Arabia. Authors acknowledge the valuable inputs from Prof Siddathartha Sen, IIT Kharagpur.

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