Online Condition Monitoring of Lubricating Oil based on Resonant Measurement of Fluid Properties

Two resonant sensor systems for fluid analysis and condition monitoring are described and applications in industrial environment are discussed. The presented sensors are based on a vibrating piezoelectric tuning fork element for simultaneous measurement of mass density and shear viscosity and are suitable for monitoring lubricating and hydraulic oils. The two systems are built as an easy to install screw-in solution with passive temperature tracking, and a measurement cell with precise temperature control using a thermoelectric element. The paper addresses the high temperature stability requirement for oil condition monitoring applications, where the oil degradation should be monitored without being obscured by temperature changes. The presented experimental results comprise monitoring the dilution of motor oil by minute amounts of Diesel fuel, and measurements of the pressure dependency of viscosity in hydraulic applications. It is shown that the introduced sensors can be operated at elevated pressure and, due to the high measurement rate, yield excellent results at varying pressure situations.