The migration of the refrigerant into a non-functioning refrigeration system is one of the main causes of oil dilution.

The refrigerant vapors migrate to the lower pressure area (the coldest) of the circuit and condense at this point. This migration phenomenon continues until the pressure-temperature ratio of the refrigerant becomes the same throughout the system.

The migration is also favored by the affinity between oil and coolant. The compressor, being a large mass of cast iron, after many hours of shutdown, is often the coldest part of the refrigeration system. At the level of the engine-compressor compartment the refrigerant vapors condense in a particular way. Furthermore, since the oils usually used have an affinity with the refrigerant, a dilution is rapidly produced if there is no way to reduce the affinity between oil and refrigerant.

To reduce this dilution phenomenon, most compressors are equipped with resistors on the cup, resistances designed to maintain an oil temperature of 10-20 ° C higher than room temperature; this temperature increase, decreases the miscibility of the oil with the refrigerant.

The resistance on the oil cup does not serve to prevent the oil from being diluted by the refrigerant, which migrates to other areas of the refrigeration system, nor does it serve to evaporate large quantities of refrigerant that are already diluted in the compressor.

Migration problems can be solved in a valid way by installing an intermittent "pump down" system.