Pressure Gauge Uses

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Pressure and Temperature Measurement at Refrigerant Circuit

High pressure gauge measures pressure and temperature, which expand uniformly from outlet of A/C compressor via the condenser up to constriction (restrictor, or expansion valve) with A/C system switched on.
Low pressure gauge measures pressure and temperature, which expand uniformly from constriction (restrictor, or expansion valve) via evaporator up to input of A/C compressor with A/C system switched on.

NOTE:

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 gauge.

Verification of Refrigerant in a Closed Conrainer

Refrigerant R134a is present in a closed container or in a refrigerant circuit when temperature indicator on the pressure gauge matches the refrigerant temperature (standing fluid adopts the ambient temperature).

A closed container or a refrigerant circuit which has been switched off is empty when temperature indication on the pressure gauge is below the temperature of the refrigerant.

NOTE:

The relationship between pressure and temperature indicated on the gauges no longer applies if no liquid is present and the pressure is built up solely by gas.