In Refrigeration as well as Air Conditioning the refrigerant is feed to the evaporator by the "Metering Device" be it Capillary Tube, Expansion Valve, or Fixed Orifice Tube.
The expansion valve is adjusted by Super Heat measurement using a pressure gauge and a digital thermometer.
Start the refrigeration system and run it for at least 10 minutes to establish steady-state temperatures. Connect the refrigerant manifold gauges to the suction service valve between the evaporator and the compressor. Connect the pipe clamp thermocouple to the suction line near the suction service valve. Connect the thermocouple to a digital thermometer.Measure the suction line pressure using the refrigerant pressure gauge. Record this pressure and the suction line temperature from the thermometer. Use the refrigerant temperature/pressure chart to convert the pressure gauge reading to evaporator saturation temperature.Subtract the evaporator saturation temperature from the thermocouple temperature. This difference is the system superheat. This shows the temperature rise above the bubble point temperature of the system. Review the operating specifications for your system to determine the proper superheat. Inadequate superheat can cause liquid refrigerant to return to the compressor, resulting in compressor damage. Incorrect superheat can also indicate improper refrigerant charge, thermal expansion device problems, a clogged filter-drier or a dirty condenser coil.