Electromechanical resonators for sensing fluid density and viscosity—a review

The resonance characteristics of vibrating structures change when they are immersed in fluids and these changes can be related to the parameters density and viscosity of Newtonian liquids. The various suitable structures, vibration modes, transduction principles, and signal analysis methods lead to an immense variety of sensor systems reported in the literature. In this review, we focus on the basic similarities between various electromechanical transducers and their evaluation methods and show that despite the apparent differences, a common design route can be followed. It is outlined that single vibration modes can be approximated by damped harmonic oscillators where the hydrodynamic loading is accounted for by a general model. The two common classes of sensors, the piezoelectric and the electrodynamic, can be described by dual circuits such that the same resonance estimation method can be applied. Also, the limitations associated with this unified approach are discussed. Furthermore, aspects concerning interface circuits and accuracy-limiting factors, such as noise and interference, are reviewed.