Liquid Density Measurement in High-Pressure Region Using Quartz Crystal Resonators

Numerous attempts have been made to measure the viscosity of liquids under high pressure by analyzing the response of a quartz crystal resonator. However, because the response of the resonator yields the product of density and viscosity, separating each value is necessary. A procedure was devised to measure the density changes of a liquid under high pressure by considering the fact that the response of a TiO₂-coated quartz crystal resonator is correlated with density. The resonance frequency shift of the TiO₂-coated quartz crystal resonator is the sum of terms that depend on \(\rho \) and \(\sqrt{\eta \rho }\). Each effect can be separated using plane equation fitting. By applying ethyl laurate, densities up to 300 MPa were obtained at 313 K and 333 K. These values agreed with previously reported values within ± 1 %, thereby demonstrating the effectiveness of this method. Since the pressure dependence of \(\sqrt{\eta \rho }\) is also obtained in the process of obtaining density data, the pressure dependence of the viscosity \(\eta \) can be estimated. The viscosities of ethyl laurate at 313 K and 333 K were calculated. Although the viscosity values differed significantly from the reported values and the measurements are still inaccurate, the possibility of using this method as a density measurement method under high pressure was demonstrated. Therefore, this study introduces a method with the potential to conveniently measure high-pressure physical properties.