| Viscosity sensors are the subject of increasing interest as part of a multisensor array for fluid condition sensing (FCS) systems. Quartz sensors have exhibited insufficient dynamic range. An isolated-electrode thickness shear mode (TSM) sensor using langasite was shown to have suitable dynamic range; however practical applications require the ability to withstand higher ambient pressures. The present work explores the pressure-viscosity limitations of hermetic TSM sensors and demonstrates two derived sensor geometries that overcome this limitation. Sensor operation in a nominally 50 mPa-s hydraulic fluid is demonstrated at 20 bars for one system and at 600 bars for another. In contrast the previously reported sensor operates to 6 bars and has vanishing dynamic range of viscosity beyond 60 bars while the proposed technologies are projected to maintain high dynamic range well past the tested ranges. |