Using Pressure Gauge

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Fig 1: Indicators On Pressure Gauge

Courtesy of VOLKSWAGEN UNITED STATES, INC.

Pressure gauge scales

Temperature scale for refrigerant R134a CF3 CH2F or CH2F CF3
Pressure scale

NOTE:

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

In addition to the pressure scale, pressure gauges may have one or more temperature scales. The scale values for R134a are assigned according to the vapor pressure table. As different refrigerants develop different vapor pressures at the same temperature, each temperature scale is marked for the appropriate refrigerant.

a Refrigerant circuit pressure and temperature measurements

The high-pressure gauge measures the pressure and temperature distributed evenly from the compressor outlet via the condenser to the constriction (restrictor or expansion valve) when the air conditioning system is switched on.

The low-pressure gauge measures the pressure and temperature distributed evenly from the constriction (restrictor or expansion valve) via the evaporator to the compressor inlet when the air conditioning system is switched on.

NOTE:

The relationship between pressure and temperature indicated on the gauges only exists in a refrigerant circuit containing liquid or vapor, but not gas. In the gas state, the temperature is approx. 10 C to 30 C higher than the gauge reading.

b Verification of refrigerant in a closed vessel

Refrigerant R134a is present in a closed vessel or refrigerant circuit if the temperature reading on the gauge corresponds to the temperature of the refrigerant (standing liquid assumes the temperature of its surroundings).

A closed vessel or deactivated refrigerant circuit is empty if the temperature indicated on the gauge is below that 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.

Pressure Gauges permit the following tests and measurements