Simultaneous measurement of density and viscosity in gases with a quartz tuning fork resonator by tracking of the series resonance frequency

Quartz crystal tuning fork resonators are suitable sensors for separable, simultaneous measurement of the density and the viscosity of a surrounding fluid. In this contribution we present the application of a frequency tracking interface for measurement of gas density and viscosity with a single quartz crystal tuning fork resonator. By frequency tracking, we obtain equal accuracy for the density and viscosity estimation as by evaluation of the admittance spectrum in the vicinity of the resonance frequency (which is typically used) but with a significantly reduced measurement time (one measurement per second compared to about 50 seconds per admittance spectrum). Furthermore, sophisticated fitting algorithms are not required.