<p>Most air conditioning and refrigeration systems that employ the vapor compression cycle rely on oil circulating with refrigerant to lubricate the bearings and other contact surfaces in the compressor. The lubricant acts as a sealant to reduce leakage losses during the compression process and it also helps to absorb some of the excess heat that is generated in the compression chamber. However, this oil circulation results in oil retention in various other components outside the compressor depending on the physical interaction between lubricant and refrigerant and their transport properties. Other factors, such as the geometry and orientation of connecting lines, and the system operating conditions, such as refrigerant flow rate and oil circulation ratio, also impact the oil retention. Because of oil retention, the oil level in the compressor reduces, which may ultimately affect its efficiency and life span. In addition, the effectiveness of heat exchangers (evaporators and condensers) decreases. The current line sizing rules reported in the ASHRAE Handbook on Refrigeration have only limited consideration of the effects of oil in the system. With the increasing development of variable-speed systems as well as future use of newer HFO refrigerants, there is a need in the industry for upgrading the line sizing recommendations, especially the connecting gas lines of unitary split systems, which consider the effects of oil retention. To develop these rules, measuring oil retention at different operating conditions is important. A test setup has been built to measure oil retention in horizontal and vertical lines of different diameters at different refrigerant and oil flow conditions. Based on the collected data, a physics-based semi-empirical model is developed which can predict oil retention in gas lines for some of the commonly used refrigerant-lubricant combinations in the HVAC&R industry.</p><p>Oil Circulation Ratio (OCR) is one of the input parameters to the model which predicts oil retention. A non-invasive, in-situ method to measure OCR in real time, which involves minimal human intervention, is developed. This method is based on oil separation and is implemented on the suction line. The approach has been validated with two different methods, one of which is an ASHRAE standard. The results of the study offer clear evidence that the method is as accurate as a standard method and it involves less human intervention as the measurement process is automated.</p>