Pressure Gauge Uses
Pressure and Temperature Measurement at Refrigerant Circuit
- The high pressure gauge measures pressure (and temperature), which expands uniformly from the A/C compressor outlet via the condenser up to the constriction (restrictor or expansion valve) when the A/C system is switched on.
- The low pressure gauge measures pressure (and temperature), which expands uniformly from the constriction (restrictor or expansion valve) via the evaporator up to the A/C compressor inlet when the A/C system is switched on.
The relationship between pressure and temperature indicated on the gauges only exists in a refrigerant circuit that contains liquid or vapor, but not gas. In a gaseous state, the temperature is approximately 10 °C to 30 °C (50 °F to 86 °F) higher than indicated on the pressure gauge.
Verification of Refrigerant in a Closed Reservoir
Refrigerant R744 is present in a closed canister or in the refrigerant circuit when the pressure indicator on the pressure gauge matches the refrigerant temperature (approximately 64 bar (928.24 psi) at 25 °C (77 °F) for refrigerant R744, the temperature of standing fluid is equal to the ambient temperature). Refer to REFRIGERANT VAPOR PRESSURE TABLE .
A closed cylinder or a refrigerant circuit that has been switched off is empty when the temperature indicator on the pressure gauge is below the temperature of the refrigerant.
The relationship between pressure and temperature no longer exists when there is no liquid and the pressure is only built up by gas or the ambient temperature is greater than 31.1 °C (88 °F). Refer to REFRIGERANT VAPOR PRESSURE TABLE .
The correlation between pressure and temperature is also only applicable when all refrigerant circuit components have the same temperature. For example, if the gas cooler (condenser) is warmer than the surrounding area when the motor is hot, then the refrigerant circuit pressure increases.