Viscosity sensor based on a symmetric dual quartz thickness shear resonator

The paper describes a novel quartz crystal sensor for the measurement of the density-viscosity product of Newtonian liquids. The sensor element consists of two circular quartz crystal plates with an air-gap in between and the liquid sample in contact with the outer plate surfaces. Plano-convex AT-cut quartz crystals arranged in mirror symmetric crystallographic orientation and vibrating in an even-symmetric thickness-shear fundamental mode at 2.77 MHz are utilized. The two outer plane sides of the crystals are fully covered by gold electrodes, which are both connected to ground potential. This special mirror symmetric set-up allows the compensation of spurious displacements in the circular clamping zones of the two crystals. The measurement values of the sensor are the fundamental resonance frequency f and the associated resonance Q-value, which are analytically dependent on the density-viscosity product of the liquid in contact with the sensing surfaces. In contrast to an earlier report about a sandwich resonator sensor, which entrapped the liquid sample between two quartz plates, the immersible sensor presented here is not restricted to low viscosity samples. The sensor covers a viscosity range from almost zero (air!) up to 2000 Pa.s, and is not restricted to electrically insulating liquids.