Utilisation of layered piezoelectric resonators for the online measurement of mass density and viscosity of fluids

For all processes in industry where liquids are intermediate or end products, there is a demand for online measurements of physical liquid properties, like density, viscosity and speed of sound. The continuous monitoring of those quantities is important for quality control and for enhancing productivity. In this paper layered resonant piezoelectric sensors for liquids are investigated and it is shown that they are advantageous because of their high resolution and accuracy and especially because of the analytically defined sensor function. The resonant sensor for liquids described in this paper is based on two bulk acoustic wave (BAW) piezoelectric plates with the liquid under investigation in between the vibrating plates. The density and the speed of sound sensors for liquids use BAW thickness extensional mode vibrators, while for the viscosity measurements BAW thickness shear resonators are used. In both cases sensor versions are available for low sample volumes of 250ml and 130ml, respectively. Further miniaturization of these sensor versions is feasible, which would allow their use also in microfluidic applications. For this sensor family a unique measurement electronics based on an electrical admittance measurement system has been developed. While the rather low accuracy currently obtained with the density sensors is still not competitive with established methods, the accuracy of monitoring the speed of sound and the viscosity with these resonant BAW sensors is already very close to, in some cases even better than, the accuracy obtained with more expensive offline laboratory measurement equipment.