Audi A4: Pressure Gauge, Tests and Measurements

General Information

Indicators on pressure gauge

1 - Temperature scale for refrigerant R134a CF3-CH2F or CH2F-CF3.

2 - Pressure scale

  Note

Pressure is measured in different units: 1 MPa (145 psi) corresponds to 10 bar (145 psi) positive pressure. 1 bar (14.5 psi) absolute pressure corresponds to 0 bar/psi positive pressure and thus to the ambient pressure (atmospheric pressure).

The pressure gauge may have one or more temperature scales next to the pressure scale. The R134a scale values are allocated respectively in the vapor pressure table. Since various refrigerants create different vapor pressures at the same temperature, each temperature scale is identified for the respective refrigerant.

Pressure Gauge Uses

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 ºC to 30 ºC (50 ºF to 86 ºF) higher than indicated on the gauge.

Verification of Refrigerant in a Closed Container

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.

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