Testing A/C Refrigerant System with Pressure Gauges

Due to the constant temperature/pressure relationship of R-12 and 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 high pressure between the compressor and restrictor (including condenser) with the A/C switched on.

Use the A/C refrigerant low-pressure gauge to measure low pressure between the expansion valve and the compressor inlet (including evaporator) with the A/C switched on.

The temperature displays of the low-and high-pressure gauges apply only to a portion of the low and high-pressure sides. The low-pressure gauge displays approximate temperature between the expansion valve and evaporator outlet. 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 (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-12 and 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.