How to Measure SubCooling

Subcooling is measured to adjust the refrigerant charge on AC/Heat Pump Systems with a TXV expansion device.

  • Run the condensing unit until pressures and temperatures stabilize.
  • Read and record the liquid pressure at the liquid line pressure port fitting at the condensing unit.
  • Place a digital temperature probe against the liquid line near the liquid gauge pressure port.  Read and record the temperature.
  • Reference the face of your high pressure gauge or use a pressure / temperature chart and convert the measured liquid pressure to the coresponding condenser coil saturation temperature for the refrigerant being used.  Record this value.
  • Next, subtract the measured liquid line temperature from the saturation temperature.  The result is your actual liquid subcooling level.


  • Refrigerant: R-22
  • Liquid pressure:  200 PSIG
  • Corresponding condenser saturation temperature:  100F
  • Liquid line temperature:  90F
  • 100F – 90F =Liquid subcooling
  • Liquid subcooling = 10F