The compressor’s discharge line temperature is important because it is an indication of the amount of heat absorbed in the evaporator and suction line and any heat of compression generated by the compression process.
When refrigeration systems are exposed to low-ambient conditions, the condensing pressure (head pressure) will fall. If the head pressure gets too low, the metering device will not have enough pressure drop across it to operate properly.
Many service technicians experience service calls where the compressor has both a low head pressure and a high suction pressure. Often, the refrigeration equipment is still running, but the product temperature is suffering about 7-10°F.
It is the system check sheet that will tell the service technician whether a system is overcharged or not. Service technicians must install pressure gauges and thermistors — or some other sort of temperature-sensing devices — in order to systematically troubleshoot a refrigeration system correctly.