Development of a micro thermal sensor for real-time monitoring of lubricating oil concentration

We report development of a micro thermal sensor and sensing techniques for monitoring the oil concentration in a refrigeration system. The sensor acquires the thermal response of the mixture to ac heating and then the concentration can be determined using one of the two different methods. In the first method, the thermal conductivity of the mixture is obtained using the three-omega method and the concentration is determined using the correlation between the mixture concentration and thermal conductivity. In the alternative fast-detection method, the concentration is determined by directly calibrating the sensor output signal to the mixture concentration. The performance of the sensor was tested using the R410A/polyvinyl ether (PVE) oil mixture. The uncertainty of the oil concentration measurement was estimated to be 5.8–7.3 wt.%, depending on the sensing scheme. Especially, the simple structure of the sensor makes the technique cost effective and adequate for miniaturization, i.e., for installation in a limited space.