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