An acoustic sensor for simultaneous density and viscosity measurements in liquids

We studied theoretically the properties of a vibrating sensor, constituted of a cylindrical tip partly immersed in a liquid. The tip is driven axially by a stepped horn and by a piezoelectric element. Equations of the fluid flow around the tip are solved and show that longitudinal and transverse waves are emitted in the fluid. This allows the device to be sensitive to the density and viscosity of the fluid. It is shown that the properties of the fluid can be deduced by measuring the frequency shift at resonance and the corresponding electric impedance. The precision of the actual device is still low for several reasons, which are discussed. Then our apparatus seems to be more convenient to in situ reaction monitoring rather than for rheological precise measurements.