Double membrane sensors for liquid viscosity and mass density facilitating measurements in a large frequency range

A resonating double membrane based sensor for viscosity and mass density facilitating measurements at different frequencies and two adjustable modes of vibration is presented. The sensor is designed to be suitable, e.g., for low cost handheld devices with inline capabilities and disposable sensor elements. In the sensor, a sample liquid is subjected to time harmonic shear stress induced by two opposed vibrating polymer membranes, placed in an external static magnetic field. These membranes carry two conductive paths each. The first path is used to actuate the membranes by means of Lorentz forces while the second acts as a pick-up coil providing an induced voltage representing the movement of the membrane. From the measured frequency response the liquid's viscosity and mass density can be deduced. This double membrane based setup allows to examine the test liquid at adjustable frequencies in the low kilohertz range from 1 kHz to 15 kHz. Two different designs are presented and the results obtained by experimental investigation are compared to previous theoretical findings.