Superheat is the temperature increase of a vapor above its saturation (boiling) point at a given pressure. In HVAC, measuring superheat at the evaporator outlet verifies that refrigerant has completely evaporated before reaching the compressor — liquid refrigerant entering the compressor causes severe damage.
For systems with fixed-orifice metering devices (pistons), target superheat is calculated using the outdoor ambient temperature and indoor wet bulb temperature (Superheat Calculator or manufacturer charts). Typical target range is 8-28°F superheat. Low superheat indicates overcharge or restriction; high superheat indicates undercharge.
For systems with TXVs, superheat is typically 8-12°F at the evaporator outlet (controlled by the TXV itself). Subcooling measurement at the condenser outlet is more useful for diagnosing TXV system charge. Both superheat and subcooling measurements require a manifold gauge set and temperature clamps.