Detection of Nonmetallic Contaminants in Lubricating Oil Using a Microwave Rectangular Cavity Resonator Sensor

Evaluation of lubricating oil is critical to the operation and maintenance of industrial engines. Here a microwave rectangular cavity resonator sensor is developed for condition monitoring. An oil-filled quartz tube is placed at the position of maximum electric field intensity, where any impurity in the oil perturbs the resonance frequency and quality factor. This arrangement enables the detection capability of nonmetallic contaminants. Particle types are readily differentiated from the resonance parameters due to the different permittivity characteristics of the mixtures. The effect of the contamination on the permittivity is also revealed, providing an alternative analysis approach. From the measurement of standard liquids, the optimal permittivity calculation algorithm for the system developed is found, and estimation errors within ±5% are achieved. The use for oil adulteration detection is also demonstrated. The multifunctional sensor is low-cost, scalable, easy to implement, and capable of on-site and online monitoring.