Application of piezoelectric tuning forks in liquid viscosity and density measurements

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Abstract

Low frequency commercial piezoelectric tuning forks (QTFs) were applied to the measurement of physicochemical properties of liquids. Viscosity and density of the test liquids were correlated with the resistance and resonance frequency of the QTF equivalent electric circuit fitted to the measured impedance spectra. Sensor calibration procedure based on the theoretical model was performed. The applied measurement technique based on the analysis of the QTF equivalent electric circuit proved its usefulness for determining liquid viscosity in the range up to 16 cP.

Introduction

Quartz tuning forks (QTFs) are commonly used as a frequency standard in Real Time Clock circuits in battery powered consumer electronics. QTFs have also been reported to be applicable as sensors for measurement of various physicochemical quantities. These devices can be used as mass sensors, in which the resonant frequency is a function of mass of particles adhered at the sensor surface. After the specific surface functionalization, the QTF devices can be utilized as biosensors [1], [2] or humidity sensors [3]. Giessibl et al. [4] and Grober et al. [5] used them as force sensors for atomic force microscopy (AFM). Tuning forks were also applied as liquid viscosity [6], [7], [8], [9] and liquid density [7], [8], [9], [10] sensors. Each of the above mentioned QTF applications is based on the measurement of its oscillation parameters such as resonance frequency, quality factor or oscillation amplitude. Owing to the piezoelectric quartz properties, measurement can be performed fully electrically using a self-excited circuit [1] or by measurement of the QTF current as a function of frequency of the actuation signal [8]. Integrated, piezoelectric actuation and detection, next to a very low price (ca. 0.1 USD), low dimensions and a very good temperature stability of resonance frequency stimulate the interest in these mass produced electronic devices. Unfortunately, the above-mentioned methods of tuning fork resonance frequency measurement (the self-excited circuit and the admittance modulus measurement) are not suitable for QTF operating in a liquid environment. This is mainly due to the rapid increase of resistance R in the electric equivalent RLC circuit modeling the QTF (Fig. 1b). The increase in resistance R represents the growth of damping caused by the surrounding medium. This results in a significant decrease of the Im current flowing through the RLC equivalent circuit of the QTF. The parasitic current Ip, forced by the electric actuation signal, flowing through the QTF parasitic capacitance C0 (Fig. 1) is much bigger than the Im current. As a consequence, measurement of the Im current, which is a function of the resonator oscillation amplitude, becomes difficult. One of the solutions of this problem is using a custom made tuning fork fabricated, so that the parasitic capacitance was significantly reduced [9]. However, the non-standard construction results in increased sensor cost.

Another solution is the use of an additional mechanical actuator and measure how the current forced by the piezoelectric charge at the tuning fork electrodes changes [2], [11]. In this case no Ip current is forced through the parasitic capacitance, so that it does not influence the Im current measurement. However, this method has a number of drawbacks. Using an additional actuator generates costs. The dimensions of the sensor head integrating the auxiliary piezoactuator are much bigger and the system architecture is more complicated. Moreover, the mechanical drive of the QTF can excite parasitic resonances which are related to the more elaborated mechanical setup.

We use another, fully electric measurement method, recently used by Liu et al. [9], suitable for operating a standard unmodified QTF in low conductive liquids. Tuning fork conductance G and susceptance B were measured and the analysis of the electric equivalent circuit of the QTF was performed. We analyzed the influence of medium density and viscosity on the RLC components forming the QTF equivalent electric circuit. It is shown that the method presented is suitable for the QTF operation in a relatively high viscosity liquid environment and can be applied for measuring liquid density and viscosity.

Section snippets

Measurement of QTF resonance characteristics in liquid using QTF electric conductance and susceptance measurement

The main problem encountered during measurement of the QTF mechanical parameters in high damping conditions is the influence of the parasitic current Ip on the RLC equivalent circuit admittance modulus measurement. For frequencies near to the mechanical resonance, the reactance of the parasitic capacitance C0 (Fig. 1) becomes of the same range or smaller than the resistance R and strongly influences the measurement of Im current. This phenomenon has been described for a typical quartz crystal

Calibration of a QTF based viscometer

In order to estimate the relationship between the QTF viscous damping coefficient R(η) and viscosity of the surrounding fluid η, a number of resonance characteristics measurements in liquids with defined viscosity were performed. Measured admittance spectra were fitted using equivalent circuit modeling. Exemplary measurement results of the QTFs admittance are shown in Fig. 3. For the clarity of the figure, each second measurement point was shown.

For each curve the resonance of the QTF was

Conclusions

It is shown that measurement of the QTF resonance frequency and equivalent electrical resistance allowed to determine viscosity and density of the liquid, in which the oscillator was immersed. However, these quantities cannot be measured independently. The applied electric measurement technique was appropriate for the measurements performed in nonconductive and poorly conductive liquids. QTFs resonance measurements in liquids exhibiting viscosity up to 16 cP were performed. Calibration of a QTF

Acknowledgements

This work was partially financed by the project: Detectors and sensors for measuring factors hazardous to environment–modeling and monitoring of threats, POIG.01.03.01-02-002/08-00.

K. Waszczuk was born in 1984 in Wodzislaw Slaski, Poland. He graduated from the Faculty of Microsystems Electronics and Photonics of Wroclaw University of Technology in 2007, receiving the MSc degree in electronics and telecommunications. Since then, he has been working at Research and Development Department for EBS Inkjet Systems Poland. He is pursuing his PhD degree at Wroclaw University of Technology, working on biochemical and microbiological applications of resonant mass sensors.

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    K. Waszczuk was born in 1984 in Wodzislaw Slaski, Poland. He graduated from the Faculty of Microsystems Electronics and Photonics of Wroclaw University of Technology in 2007, receiving the MSc degree in electronics and telecommunications. Since then, he has been working at Research and Development Department for EBS Inkjet Systems Poland. He is pursuing his PhD degree at Wroclaw University of Technology, working on biochemical and microbiological applications of resonant mass sensors.

    T. Piasecki was born in 1976 in Wroclaw, Poland. He received the MSc degree from the Faculty of Electronics of Wroclaw University of Technology in 2000. His PhD thesis concerning the computer modeling of semiconductor devices based on AIII-N heterostructures was defended in 2005 at the Faculty of Microsystems Electronics and Photonics of Wroclaw University of Technology. He continues his work at the Faculty of Microsystem Electronics and Photonics. His current field of expertise is the methods for impedance measurements and use of impedance spectroscopy in research of material properties as well as impedimetric sensors and biosensors. He is an author or co-author of 32 scientific papers.

    K. Nitsch is PhD, DSc, Professor at the Faculty of Microsystems Electronics and Photonics of Wroclaw University of Technology. His research interests are in area of measurement and analysis of electrical properties of materials, component and sensors. He specializes on the various applications of impedance spectroscopy to microelectronics materials and structures characterization. He has authored over 180 scientific publications.

    T. Gotszalk was born in Wroclaw, Poland. He received the MSc degrees from the Faculties of Electronics and of Electrical Engineering of Wroclaw University of Technology in 1989 and 1991, respectively. In 1996, he received the PhD degree from the Institute of Electronic Technology of the Wroclaw University of Technology. He has been honored with Siemens research award (2000) and the prize of Polish Science Foundation FNP (1997) for his scientific work. He is the head of the Division of Micro- and Nanostructures Metrology at the Faculty of Microsystems Electronics and Photonics of Wroclaw University of Technology. He has authored over 100 scientific publications.

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