289

Sensors and Materials, Vol. 28, No. 4 (2016) 289–294
MYU Tokyo
S & M 1179

Contactless Liquid Sensing TechniqueUsing a Quartz Oscillator
Tsubasa Susa, Takeru Watanabe, Masayuki Sohgawa, and Takashi Abe*
Niigata University, 8050, Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan(Received October 23, 2015; accepted December 16, 2015)

Keywords: contactless sensing, liquid concentration, quartz crystal resonator, oscillator, factory
automation

In this paper, a contactless sensor for the measurement of liquid concentration using a quartz
oscillator is reported. An out-of-plane electric field distribution from a planar sensing capacitor (SC)is used for the noncontact sensor measurement of liquid concentration on a spacer. This sensor
measures the changes in the capacitance of the SC by the changes in the frequency of the quartz
oscillator. Because the capacitance is dependent on the relative permittivity and conductivity of theliquid sample on the spacer, it is possible to measure the concentration of the sample. The response
is very stable because, in this sensor, the SC is incorporated into the quartz oscillator. Because theSC is in contact with the liquid sample instead of the quartz crystal resonator, the sensor has a highquality factor (approximately 30000). Moreover, this sensor can be used with a general integrated
circuit (IC) and an electronic component at a low cost. The sensor is used for the noncontact
measurements of liquid concentration.

1.

Introduction

Liquid sensing technology is used in many applications in industrial automation and waste
management for reducing environmental pollution. A typical example is the degradation state ofcutting oil and/or lubricating oil in a machining factory. The degradation state of oil is important
for reducing energy loss caused for tribological reasons. In such a case, contactless measurement
is desirable for practical purposes because the measured liquid is in bottles, tanks, or pipelines.
Contactless sensing is also important for hygienic applications.
Electromagnetic techniques with contactless and multifunctional sensing are used for thecontactless measurement of the concentration of electrolyte solutions. These techniques are a
powerful tool for contactless measurements.(1–5) However, the types of sensors used are relatively
expensive and have practical problems in industrial applications. To overcome the drawback ofproduction costs, less expensive sensors are required for industrial applications. In previous studies,a contact-type capacitive sensor for the measurement of liquid concentration with a quartz oscillator
had been proposed.(6,7) We proposed a new method for multichannel sensing at a time interval byintroducing sample solutions to multiple planer capacitors.(8) An out-of-plane electric field of theplanar capacitor was used in our previous study, for which the capacitance depends on the distance
between the liquid and the capacitor.
*Corresponding author: e-mail: memsabe@eng.niigata-u.ac.jp
ISSN 0914-4935 © MYU K.K.

290

Sensors and Materials, Vol. 28, No. 4 (2016)

This research focuses on a new contactless sensor for the measurement of liquid concentration
with a quartz oscillator. We have determined that the frequency change in a quartz crystal resonator(QCR) is sensitive to the liquid concentration on a spacer because an out-of-plane electric fieldfrom a planar capacitor is distributed in a liquid.

2.

Theory and Methods

2.1

Principle of the new method

Figure 1 shows the proposed contactless measurement method for liquid concentration. A planar
capacitor is connected in series with a QCR. The QCR was fabricated by patterning a Au electrodeon both sides of a crystal purchased from Seiko EG&G of Japan. The thickness of the crystal wasapproximately 100 μm. The electrode size was 2.6 mm. An out-of-plane electric field distribution
from planar sensing capacitor (SC) was used for the contactless sensor measurement of a liquidconcentration. The relationship between the capacitance change of the SC (ΔCL) and the frequencychange (Δf) can be given as∆ f = f (CL2 ) − f (CL1 ) = −∆CLC11×2(C0 + CL1 )(C0 + CL2 ) 2π LC1(1)where CL2 is the capacitance after placing a liquid sample on the SC, CL1 is the capacitance after
placing a reference liquid on a spacer. L is the reactance, C0 is the shunt capacitance, and C1 is thecapacitance of the QCR. The concentration of the liquid on the SC is evaluated from the frequencychanges because the capacitance depends on the relative permittivity and conductivity of the liquidsample on the spacer.
The relative permittivity of the liquid depends on the ions and ion pairs in the liquid.(9)Therefore, the conductivity changes are measured by the capacitive change in this method. The
absolute value of the frequency depends on the sensor packaging and oscillation circuit. Therefore,
the frequency response (∆F) for the liquid sample is normalized by the frequency response for
distilled water (∆Fw) at a spacer thickness of 1 mm. The normalized frequency response is given as

(a)

(b)

Fig. 1. (Color online) Proposed contactless measurement method for the liquid concentration: (a) a schematicillustration and (b) an equivalent circuit model.

291

Sensors and Materials, Vol. 28, No. 4 (2016)∆Fn = ∆F / ∆Fw(2)

In this study, the normalized frequency response was plotted against the concentration of theliquid. Incidentally, the resonant frequency decreased after putting a liquid sample on a spacer as
expected [Eq. (1)].
2.2

Fabricated sensor and its evaluation

Figure 2 shows an example of a fabricated sensor. The oscillating frequency of the sensor wasapproximately 16.2 MHz. The capacitors were composed of a pair of Au electrodes (width = 1.3mm, length = 2.24 mm, separation = 0.74 mm). The substrate used in the capacitor was a glass
plate (borosilicate glass). Spacers of different thicknesses (0.1–6.0 mm) were used to evaluate thefrequency response of the sensor against each thickness of the spacer that is modeled as a tank wall.
The samples used in this study were prepared by adding ethanol or NaCl to distilled water. The
distilled water is the solution with zero concentration. The capacitance changes were evaluatedusing an LCR meter (LCR HiTESTER 3535, Hioki, Japan). The conductivity of the NaCl solution
was evaluated with a conductivity meter (Twin-Cond., HORIBA, Japan).
To demonstrate the practical usefulness of the method, the concentration of a water-soluble
cutting and grinding fluid was measured. Commercially available fluids (NC-11, SX-542Y, TaiyuCorporation, Japan) were used, and these fluids were diluted 10–30-fold with distilled water.
All experiments were conducted at room temperature. Error bars from at least three separate
experiments are indicated by the symbols. The detection limits were given by multiplying thesensitivity by three times the standard deviation of the mean frequency variation during 1 min.

3.

Results and Discussion

3.1

Evaluation of basic sensor performance

The sensor was tested in an ethanol/water mixture or NaCl aqueous solution to evaluate its
responses to the relative permittivity and conductivity of the liquid sample.

Fig. 2(Color online) Example of the fabricated device.

292Sensors and Materials, Vol. 28, No. 4 (2016)

Figure 3 shows the dependence of the normalized frequency change [Eq. (2)] on the ethanol
concentration for a spacer thickness of 1 mm. The frequency change (ΔFn) decreases monotonicallywith increasing ethanol concentration. As suggested by Eq. 1, the sensor frequency response
depends on the capacitance change; in fact, the frequency response corresponds to the capacitancechange.
Figure 4 shows the dependence of the frequency change on NaCl concentration. The value ofΔFn increases with increasing NaCl concentration.
The effect of the thickness of the spacer that is modeled as a tank wall on the frequency response
was evaluated, because the tank (or pipe) wall thickness suitable for the evaluation depends on eachuser. Figure 5 shows the relationship between the spacer thickness and the normalized frequency
response of the sensor to NaCl or ethanol concentration. The detection limits are shown in Fig.
6. The detection limits can be improved with thinner spacers. In the case of ethanol, the detection
limit for a spacer thickness of 5 mm is less than 1 wt%. This detection limit can be improved by
optimizating the size of the QCR because the frequency response depends on the capacitance of theQCR.(10)Fig. 3. ( C o l o r o n l i n e ) D e p e n d e n c e o f t h enormalized frequency response on the ethanol
concentration.

Fig. 4. ( C o l o r o n l i n e ) D e p e n d e n c e o f t h enormalized frequency response on the NaClconcentration.

Fig. 5. (Color online) Relationship between thespacer thickness and the normalized frequency
response of NaCl or ethanol concentration.

293

Sensors and Materials, Vol. 28, No. 4 (2016)

Fig. 6. (Color online) Relationship between thespacer thickness and the detection limit of NaCl orethanol concentration.

Fig. 7. (Color online) Relationship between thenormalized frequency response and the concentrationof glucose.

Fig. 8. (Color online) Relationship between the normalized frequency response and the concentration of thegrinding fluid sample.

The results in Fig. 7 suggest that the frequency response depends on the glucose concentration.
The proposed sensor can measure the glucose concentration because the capacitance changes withthe variation in the concentration. The relative permittivities of glucose and distilled water areapproximately 3 and 80 at room temperature, respectively. Thus, the relative permittivity of theaqueous solution of glucose depends primarily on the concentration of glucose.
3.2

Demonstration showing the practical usefulness

Figure 8 shows the relationship between the normalized frequency change and concentrationof the grinding fluid samples for a spacer thickness of 1 mm. The value of ΔFn increases withincreasing fluid sample concentration. The frequency response corresponds to the capacitancechange. An ionized surfactant is used in this fluid sample. The detection limit is less than 0.01
wt%. On the other hand, the required detection limit for the industrial application is 3 wt%.
Although these measurements were taken in a laboratory setup, the sensor is expected to be robustin an industrial environment. Comparing the measured data with the database that is created fromsolutions of known concentration, the proposed contactless sensor is expected to obtain an accurate
measurement of liquid concentration.

294

4.

Sensors and Materials, Vol. 28, No. 4 (2016)

Conclusions

In this paper, a new contactless sensing method for the accurate measurement of liquid
concentration is proposed for industrial applications. The proposed sensing method is useful for
accurately estimating the concentration of an aqueous solution that contains a single chemicalspecies.

Acknowledgements
This work was supported in part by the Japan Science and Technology Agency and NAGAI NS
Promotion Foundation for Science of Perception.

References
12345678910T. Tohyama and H. Yamashita: J. Instrum. Technol. 3 (1957) 146.
H. Wakamatsu: Hewlett-Packard Journal 48 (1997) 37.
A. Bakar, Md. Ismail, and K. Shida: Sens. Actuators, A 69 (1998) 152.
T. Kume, T. Nagano, T. Watanabe, and T. Mitsuno: Trans. JSIDRE 227 (2003) 105.
A. Tura, S. Sbrignadello, D. Cianciavicchia, G. Pacini, and P. Ravazzani: Sensors 10 (2010) 5346.
S. Ishii, K.Emura, and T Abe: IEEJ Trans. Sens. Micromachines 133 (2013) 96.
S. Ishii, K. Emura, and T. Abe: Jpn. J. Appl. Phys. 52 (2013) 127101.
S. Toyama and T. Abe: IEEE Sens. J. 14 (2014) 135.
T. Sato and R. Buchner: J. Phys. Chem. A 108 (2004) 5007.
T. Watanabe, S. Toyama, M. Sohgawa, and T. Abe: IEEJ Trans. Sens. Micromachines 134 (2013) 224.