The plates of ST-quartz with Euler angles 0°, 132.75°, 0° and 0°, 132.75°, 90° are used as a weak piezoelectric material. The measurements are carried out at room temperature and atmospheric pressure. For this purpose it is possible to use new structures like phononic crystals with various periodic inclusions as well. The resonators with a lateral electric field excitation have recently been suggested for measurement conductivity and viscosity of liquids. Till know Rayleigh surface acoustic waves (SAW), shear-horizontal (SH) SAW, leaky SAW, bulk acoustic waves, slot acoustic wave, SH plate acoustic waves (PAW) of zero order, and PAW of higher order propagating in isotropic and piezoelectric and plates were used for the purpose. Performance of the sensors depends evidently on the type of acoustic wave exploited in device. The acoustic sensors are potentially attractive because they do not require large fluid samples, do not introduce significant distortions into the probe, allow remote data collection via radio channel, and have two independent responses (phase and amplitude) to each action. Acoustic devices are relay upon the changes in the wave amplitude and velocity when a sample of liquid loads propagation medium. One of the most attractive approaches for developing such sensors is the use of acoustic wave propagation. To measure this parameter a need was recognized for microsensing devices with small size, high precision, good reliability, and multiple usages. The knowledge on viscosity is also necessary in medicine for characterizing rheological bio-liquids (blood, sperm). The measurement of fluid viscosity is an important problem for various applications in some industries (machine, instrument, chemical, food, pharmaceutical), agriculture, bio-engineering, ecology, etc. Temperature responses are almost zero in air, but when plate is coated with liquid they increase depending on liquid properties, allowing measurements of the temperature dependence of the liquid viscosity. Viscosity responses of the prototypes become comparable with their electric outputs only for η < 2 cP. Because of operation at ultrasonic frequency (tens MHz) the prototypes have different sensitivities in various η-ranges: 0.3 dB/cP for 1–20 cP, 0.12 dB/cP for 20–100 cP, and 0.015 dB/cP for 100–1500 cP. Basing on the modes, the sensor prototypes with selective operation are developed and tested for η from 1 to 1500 cP, σ from 0 to 1.2 S/m, and t from 0 to 55 ☌. It is found that all modes possessing the property are characterized by small surface-normal displacement, avoiding wave radiation into adjacent liquid, large in-plane displacements, enhancing viscous coupling the modes and liquids, and small electro-mechanical constant, reducing electro-acoustic interaction. The search of the modes with the selective properties is accomplished by varying plate thickness h, crystal orientation, wave length λ, and mode order n. Using plates of weak piezoeletcric crystal (quartz) loaded with various liquids, it is shown that along with common modes, whose sensitivity towards different liquid parameters comparable with each other, there are some uncommon modes, whose amplitude responses towards viscosity η are much larger than towards temperature T and electric conductivity σ.
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