Three layer thickness extensional mode piezoelectric resonator for determining density and sound velocity of liquids

The principle of calculating two unknown physical parameters of a layered resonator out of two quasiharmonic series resonance frequencies is applied to the determination of density and sound velocity of liquids. The resonance frequencies are measured using a three layer sensor vibrating in thickness extensional resonance modes. The three layers are formed by two piezoelectric plates and the liquid sample in between. Knowing the behaviour of the two piezoelectric single resonators from measurements which have to be done first, we have calculated the sound velocity and the mass density of the liquid between these two plates out of two series resonance frequencies of the composite resonator.