Measurement of density and viscosity of dodecane and decane with a piezoelectric tuning fork over 298–448 K and 0.1–137.9 MPa

A novel downhole density-and-viscosity sensor has been developed utilizing a lithium niobate (LNB) piezoelectric tuning fork resonator with two embedded electrodes. The mechanical flexural tuning fork resonator can be described by an electrical equivalent circuit model when it is immersed in a fluid. Based on this model, it is shown that a simple algorithm can be derived to calculate the fluid density and viscosity simultaneously by using experimentally determined peak frequencies of the real and imaginary components of the admittance spectra of the tuning fork. Tests conducted on the tuning fork resonator using a variety of Newtonian fluids at room temperature and atmospheric pressure revealed that the calculated fluid densities and viscosities closely agreed with known fluid values. In addition, at high pressures and temperatures the measured viscosities and densities of two reference hydrocarbon fluids, dodecane and decane, agreed reasonably well with the scientific literature values over a temperature range of 298–448 K (25–175 °C) and a pressure range of 0.1–137.9 MPa (14.5 psi–20 kpsi), which are typical of downhole environments.