Testing With Pressure Gauges

A/C Refrigerant System, Testing With Pressure Gauges

Due to the constant temperature/pressure relationship of R-134a, approximate high side system temperature can be determined based on system pressure.
Pressure gauges may have one or more temperature scales in addition to the pressure scale. Since various refrigerants have different temperature/pressure relationships, each gauge is specific for a particular refrigerant.

Measuring pressure and temperature in a "switched-on" refrigerant system

Use the A/C refrigerant high pressure gauge to measure the high side system pressure between the compressor and expansion valve (including condenser) with the A/C switched on.
The temperature display of the high pressure gauge applies only to a portion of the high pressure side. The high pressure gauge displays approximate temperature between the compressor outlet and expansion valve.


NOTE: The temperature/pressure relationship only holds true when the refrigerant system contains liquid and vapor, but not when it contains only gas. In the gaseous state, the temperature is approx. 10-30 °C (18-54 °F) higher than indicated on the pressure gauge.


Measuring pressure and temperature in a closed container or "switched-off" refrigerant system

Due to the constant temperature/pressure relationship of R-134a, approximate temperature of refrigerant in a closed container or in a switched-off A/C system can be determined based on its pressure.

If the temperature displayed on the gauge is lower than the actual temperature of the refrigerant, the refrigerant container or system is empty (discharged).


NOTE: The temperature/pressure relationship only holds true when the refrigerant system contains liquid and vapor, but not when it contains only gas. In the gaseous state, the temperature is approx. 10-30 °C (18-54 °F) higher than indicated on the pressure gauge.