Concentration measurement of refrigerant/refrigeration oil mixture by refractive index

Refrigeration oil having good miscibility with refrigerant is generally used in refrigeration units. Precise measurement of the mixing ratio of refrigerant to refrigeration oil is required for a sufficient understanding of the refrigeration cycle. In this paper, refractive index is chosen as a property which indicates the mixing concentration of the refrigerant/oil mixture. A laser displacement sensor is used to detect a change of optical path which changes according to the refractive index of test medium. The refractive indices of pure refrigerant, pure refrigeration oil and refrigerant/refrigeration oil mixture are measured with several combination of refrigerant/oil. It is found that the difference of refractive index between the refrigerant and the oil is sufficient for the measurement of the mixing concentration of refrigerant/oil mixture, and that the refractive index of the refrigerant/oil mixture changes almost linearly according to the mixing concentration. These data will be utilized for development of an in situ sensor in refrigerant compressors.