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Viscosity SensorSensors and Ultrasonics |
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The utilization of thickness shear mode quartz crystals (e.g. AT-cut) for the determination of liquid parameters has shown that these quartz crystals not only generate a damped shear mode wave, where the sensor function is based upon, but also compressional waves in liquids with low damping in the liquid.
To overcome the problem described above, a symmetrically arranged double quartz cell geometry, containing the liquid sample in between, was created. The resonator (Fig. 1) consists of two crystallographically parallel orientated plano-convex 2,77 MHz AT-cut quartz crystals. The inner Au electrodes on the plane surfaces of the crystals are in contact with the liquid and are grounded. In order to prevent thermal stresses in the active crystals, the active crystal plates are spaced by a mounting consisting of passive plane-parallel AT-cut quartz crystal pieces with the same cristallographic orientation. The liquid volume between the active quartz plates is terminated by an inlet and an outlet pipe. The resonant sensor element is covered by an aluminum block housing that, in connection with a Peltier element and two platinum resistance microthermometers in the inlet and outlet, respectively, functions as thermostat. The sensor can be used in flow-through mode with continuous monitoring of the viscosity and the associated temperature of the liquid, or in batch mode.
Compared to density-viscosity product values from literature the average absolute deviation of the quartz sensor is 3%, the error in reproducibility (repeated measurements after refilling with the same liquid) is typically 1%, and the error in repeatability (repeated measurements with the filled sensor) is less than 1%.