Pressure Gauge Uses

WARNING: This page is about a different car, the 2023 Audi SQ8, 2023 Audi RS Q8, and 2023 Audi Q8. However, it is still accessible from the selected car via links, so may be relevant.

Pressure and Temperature Measurement at Refrigerant Circuit

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

Verification of Refrigerant in a Closed Reservoir

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

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.

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.