Subcooling is the temperature decrease of a liquid refrigerant below its saturation (condensing) point at a given pressure. Measuring subcooling at the condenser outlet verifies that refrigerant has fully condensed to liquid before reaching the metering device — flash gas (vapor) entering the metering device reduces efficiency and capacity.
For systems with TXVs, subcooling is the primary method for verifying proper refrigerant charge. Manufacturer specifications typically call for 10-20°F of subcooling at the condenser outlet. Low subcooling indicates undercharge (not enough refrigerant to fill the condenser); high subcooling indicates overcharge.
Subcooling is measured by comparing the actual refrigerant temperature at the liquid line service valve to the saturation temperature corresponding to the measured high-side pressure (obtained from refrigerant pressure-temperature charts). Accurate subcooling measurement requires proper probe placement and stable system operation.