Application of quartz tuning forks and extensional microresonators for viscosity and density measurements in oil/fuel mixtures

Real time monitoring of the physical properties of liquids is of great concern in the automotive industry. For example, tracking the viscosity of lubricating oils is of great importance because they are exposed to dilution with diesel fuel as result of late-injection processes, which are essential for regenerating diesel particulate filters. Here we describe two in-plane movement based resonators and their capability to assess oil dilution with diesel and biodiesel fuels. One of the resonators is a state-of-the-art micron-sized AlN-based rectangular plate, actuated in the first extensional mode in the MHz range. The second resonator is a commercially available millimeter-sized quartz tuning fork, working at 32.7 kHz. Electrical impedance measurements were performed to characterize the performance of both resonators in various liquid media over a wide range of viscosities. These measurements were compared with the results obtained with low-cost electronic circuits also developed in this work. In order to track density and viscosity of different fluids we have measured two parameters by various techniques: the resonance frequency and the quality factor.