How to Calculate Superheat and Subcooling

Superheat equals the suction line temperature minus the saturated suction temperature (the saturation temperature for the suction pressure, from the refrigerant PT chart). Subcooling equals the saturated liquid temperature (from the liquid pressure) minus the liquid line temperature. The subtraction order is opposite: superheat subtracts the saturation temperature from the measured line temperature, and subcooling subtracts the measured line temperature from the saturation temperature. Both come out positive on a healthy system.

For superheat, convert the suction (low-side) pressure to a saturation temperature. For subcooling, convert the liquid (high-side) pressure to a saturation temperature. They come from different points in the cycle, so you read the PT chart at two different pressures. On a zeotropic blend you also use two different columns: the dew point for superheat and the bubble point for subcooling.

The formula is the same, but a blend has temperature glide, so one pressure maps to two saturation temperatures. Use the dew point saturation temperature for superheat and the bubble point saturation temperature for subcooling. Reading the wrong column shifts the result by the glide. On R-407C the glide is large, so the error is big; on R-454B the glide is small (it is a near-azeotrope), so the error is minor but still worth getting right.

Yes. Superheat and subcooling both start from a saturation temperature, and the only way to get it is to convert the measured pressure through the refrigerant pressure-temperature relationship, whether from a printed PT chart, a digital gauge, or a calculator. A pressure alone is not enough; the same pressure gives a different saturation temperature for every refrigerant.

Evaporator superheat is measured at the evaporator outlet and reflects how well the coil is fed. Total (or compressor) superheat is measured at the compressor inlet and includes the heat the suction line picks up on the way back, so it reads higher. The formula is identical, only the measurement point moves. For charging a fixed-orifice system you use total superheat at the condensing unit.

A superheat at or near zero means the suction line temperature has reached the saturation temperature, so liquid refrigerant is still boiling at the measurement point and leaving the evaporator. That is floodback, and it can slug the compressor. A superheat in the low single digits is already the danger zone: recover charge and do not add. A negative result usually means a measurement or PT-lookup error, or a pressure that is off the bottom of the chart, so recheck the reading before trusting it.