As liquid refrigerant evaporates in the evaporator its temperature stays constant at the saturation temperature; once all the liquid has become vapour, the vapour continues to warm. The difference between the vapour temperature at the outlet and the saturation temperature is the superheat. Example: if the R404A suction pressure is 2 bar g, the saturation temperature is about −19 °C; if −10 °C is measured on the suction pipe, the superheat is 9 K.
Superheat is the proof that no liquid is reaching the compressor: 0 K superheat shows that the evaporator is flooded with liquid and the compressor is in danger of drawing in liquid (breaking valves, washing out oil). Very high superheat (above 15 K) shows that the evaporator is starved of refrigerant: low charge, expansion valve throttled or blocked, filter blocked; capacity falls and the compressor runs hot.
The thermostatic expansion valve is adjusted to hold superheat steady at 5–8 K; with an electronic valve 3–5 K is possible. Measurement needs a pressure gauge on the suction line and a temperature probe in good contact with the pipe. For zeotropic refrigerants (R449A) the saturation temperature is read from the "dew" column of the chart. Our refrigerant pressure–temperature chart gives the saturation temperature for the pressure you measure instantly.