The compressor’s superheated discharge temperature can be measured by placing an insulated thermistor or thermocouple on the discharge line about 3 to 4 inches from the compressor. Today’s technology allows service technicians to use an infrared-temperature-sensing device that can be purchased at a reasonable price. The compressor’s discharge temperature is a measure of the superheated refrigerants vapor temperature. Since the compressor’s discharge temperature is a superheated vapor temperature measurement, a pressure/temperature relationship does not exist, and a pressure gauge cannot be used for its measurement.
Pressure gauges can only be used for a pressure/temperature relationship when a saturation temperature (evaporating and/or condensing temperature) is needed. Since the compressor’s discharge temperature is a reflection of what is going on inside the compressor, it must be monitored very closely. The back of the compressor’s discharge valve (Figure 1) is actually the hottest part of the system, but it is impossible to measure by a service technician. The next closest place, however, is the discharge line of the compressor.