Refrigerant concentration measurement at compressor oil sump by refractive index (concentration of R410A in PVE oil)

The dissolution of refrigerant into refrigeration oil has great influence on oil viscosity. In this study, a refractive index measurement is applied to measure the refrigerant concentration in the oil at a compressor oil sump. Although the refractive index of the oil/refrigerant mixture is correlated with the refrigerant concentration using the refractive indices of the oil and the refrigerant, the temperature of the oil in the compressor is so high that the refractive index of the refrigerant cannot be defined because the temperature surpasses the critical temperature of the refrigerant. Therefore the correlation under such high temperature conditions is examined. It is found to be reasonable to use the refractive index of the refrigerant derived by an extrapolation of the refractive indices of the saturated liquid refrigerant under a sub-critical condition. In addition, a transient measurement of the refrigerant concentration in the oil was carried out in a practically operated compressor. Although the output signal of the sensor is disturbed by bubbles generated during the separation of the refrigerant from the oil, a data processing procedure which eliminates the over-ranged signal and averages the output within a certain time period is proposed for eliminating the influence of the bubbles.