SIMULTANEOUS MEASUREMENT OF MASS AND VISCOSITY USING PIEZOELECTRIC QUARTZ CRYSTALS IN LIQUID-MEDIA

An inexpensive mass and viscosity sensor can be constructed from a quartz crystal microbalance using an automatic gain control oscillator. The gain control voltage, which maintains a constant oscillation amplitude, is shown to be affected only by the energy loss to the liquid medium and the crystal mounting. The frequency shift between a loaded and an unloaded crystal operating in the same medium is shown to closely follow the Sauerbrey equation. A viscosity or density correction based on the gain control voltage can be applied to the measured frequency shift to obtain the mass loading. Errors due to the oscillator phase shift and the crystal mounting losses can be removed by calibration.