R-503:
Very low temperature refrigeration systems typically operate in two or more stages (cascade type systems). It would be nearly impossible to achieve low temperatures in a single stage with an inexpensive compressor. Older cascade systems had a low temperature stage which used a lower boiling point gas, such as R-13 or R-503, and a high stage which traditionally used R-12, R-22, or R-502. Hydrocarbons are often added to very low temperature systems to make sure the oil stays fluid and returns from the very cold evaporator.
R-503 can be replaced by R-508B.