Shortcomings in the present standard method of determining the circulating oil concentration in a refrigeration system have led to the current ASHRAE-sponsored research, wherein a continuous, in-line method of measuring the flowing oil concentration is sought.

A literature survey and preliminary property measurements examined properties of oil-refrigerant mixtures that could be measured to infer the oil concentration in the liquid line of a refrigeration system. Four measurement methods were selected for development into oil concentration sensors: a vibrating U-tube densimeter, a new type of in-line viscometer, a prototype acoustic velocity probe, and an optical fiber refractometer.

The performance test results were statistically analyzed to determine the oil concentration measurement uncertainty. The three sensors tested were found to attain the desired +1 weight-percent uncertainty under a variety of conditions. Application guidelines are presented for the use of the densimeter, viscometer, and acoustic velocity as oil concentration sensors.