A liquid density sensor based on longitudinal wave effect of a piezoelectric quartz crystal in liquid phase

We report a new response of a piezoelectric quartz crystal (PQC) sensor in a liquid phase. In the case of strong longitudinal wave effect, the resonant peak of PQC was split into two peaks. With gradually increasing liquid density, an additional resonant peak appeared, moved from low to high frequency region and disappeared periodically in the conductance–frequency spectrum. The frequency of the additional resonant peak was sensitive to the change in liquid density. A linear relationship was found between the frequency and liquid density in a cycle. The sensitivity to liquid density was 2.6 × 106 Hz g−1 cm3 for a 5 MHz PQC with a reflection distance of 16 mm. As the distance between the two resonant peaks reduced, the intensity of the additional resonant peak increased while that of the primary resonant peak reduced. The additional resonant peak and primary resonant peak had similar intensity in the situation of overlap. By switching a reflection plate and longitudinal wave eliminator, the changes in surface mass loading on PQC, density and viscosity of the liquid were monitored simultaneously by a PQC sensor in an impedance analysis method.