R. Thalhammer, S. Braun, B. Devcic-Kuhar, M. Gröschl, F. Trampler, E. Benes, H. Nowotny, M. Kostal:

"Viscosity sensor utilizing a piezoelectric thickness shear sandwich resonator";

IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control,45(1998), 5; 1331 - 1340.

This paper describes a novel quartz crystal sensor for measurement of the density-viscosity product of Newtonian as well as non-Newtonian liquids. The sensor element consists of two plano-convex AT-cut quartz crystals vibrating in a thickness-shear mode with the liquid sample in between. This special set-up allows suppression of disturbing resonances in the liquid layer. Such resonances are generated in the common single-plate arrangements due to compressional waves caused by spurious out-of-plane displacements of the shear vibrating finite plate. The primary measurands of the sensor are the fundamental resonance frequency and the associated resonance Q-value, which are influenced by the viscously entrained liquid contacting the quartz surfaces. If the real and imaginary part of the specific acoustic shear impedance z of the sample fluid turns out to be equal, the fluid behaves Newtonian and as a consequence the fundamental resonance frequency and the Q-value become linearly dependent. In the case of Re(z)!=Im(z) more complex fluid models such as the Maxwell spring/damper model must be used to determine characteristic parameters such as relaxation times. The sensor allows the measurement of samples with viscosities from almost zero (air) up to 200cP with a sample volume of about 130 microliters.

http://dx.doi.org/10.1109/58.726459

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