Testing With Pressure Gauges

- Due to the constant temperature/pressure relationship of R-134a, approximate temperatures 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 restrictor (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 restrictor.


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 (50 - 86 °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 temperature display on 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 (50 - 86 °F) higher than indicated on the pressure gauge.