A sensor for the measurement of liquids density

Density is one of the characteristic properties of a substance, that can be used to understand other physical and chemical properties. On-line density sensors are of interest in many applications: the radioactive monitoring in waste storage tanks, in pipelines for process control in the petrochemical industry, in the production of chemical reagents, in food processing, in the production of paper and textiles and so on. In this paper, a piezoelectric density sensor is proposed and analyzed with FE methods. The idea at the base of the present work relies in the well-known property of piezoelectric structures to vary their resonant behavior depending on load conditions. The active element of the proposed density sensor is a cheap piezoceramic bimorph, widely used in buzzers and telephone receivers. The bimorph is clamped all around by tightening its border to the edge of a rigid open chamber in which the liquid is poured; the flexural resonance frequency of the membrane is modified by the liquid mass. The liquid volume is the volume of the chamber and therefore the membrane resonance frequency can be related to the liquid mass density. We analyze the proposed sensor by a FEM commercial code (ANSYS®) to test the device capability and to design the chamber in which the liquid is poured, in order to maximize the device sensitivity; we computed the sensor resonance frequency by varying the chamber height and for three different values of the chamber radius. The obtained results demonstrate that best sensitivity is obtained with smaller values of both height and radius.