Refrigerant vapor compression cycles such as the A/C loop in automotive applications, need secure oil management to omit compressor failures due to lack of oil in the compressor. Future automotive A/C systems will have additional functionalities, will operate under different conditions and may become more complex with different parallel branches. Each of this will make oil management in the cycle more difficult. The determination of oil circulation ratio is one key indicator to judge the circulation and the feed back of the lubricant to the compressor. For R134a systems the measurement of OCR based on the acoustic velocity in the liquid line have been proven to be a sufficient method. Calibration of the oil refrigerant mixture is mandatory to get the sound velocity as a function of temperature, pressure and mass fraction. For the vehicle application and on road testing a special version with compact sensor has been developed, which allows normally an installation directly to the car without major modifications to the A/C-loop. A limitation of this sensor based on sound velocity is that it needs full solubility of the lubricant / refrigerant mixture. A phase separation, even if the fluid is perfectly mixed leads to failures. For R744 not only the refrigeration process is different but also the solubility of the lubricant R744 system is much more limited than that of lubricant R134a systems. A sensor based on the measurement of the sound velocity can not be used for transcritical cycles as there will normally always be two phases in any of the cycle states. A different sensor device is needed. Also R1234yf has a worse solubility with oil than R134a. A test loop has been set up, a sensor was tested with all three refrigerants. The results will be discussed.