While connected to a vehicle's A/C system, the static gauge reading should equal which value?

While connected to a vehicle's A/C system, the static gauge reading should equal which value?

• A. Equal to ambient temperature on the PT chart
• B. Equal to system operating pressure
• C. Equal to zero
• D. Equal to high-side pressure

Answer: (A)

When the A/C system is not running, there’s no refrigerant flow or compressor effect, so the pressure inside the system is just the saturated vapor pressure of the refrigerant at the surrounding air temperature. The PT chart for the refrigerant shows, for that pressure, what the corresponding ambient temperature would be. So a static gauge reading is interpreted as the ambient temperature on the chart. In other words, the pressure you read maps to the surrounding temperature on the pressure–temperature chart. This isn’t the operating pressure of a running system, nor is it zero, and it isn’t specifically the high-side pressure. The static condition ties the reading to ambient temperature via the PT relationship.

When the A/C system is not running, there’s no refrigerant flow or compressor effect, so the pressure inside the system is just the saturated vapor pressure of the refrigerant at the surrounding air temperature. The PT chart for the refrigerant shows, for that pressure, what the corresponding ambient temperature would be. So a static gauge reading is interpreted as the ambient temperature on the chart. In other words, the pressure you read maps to the surrounding temperature on the pressure–temperature chart.

This isn’t the operating pressure of a running system, nor is it zero, and it isn’t specifically the high-side pressure. The static condition ties the reading to ambient temperature via the PT relationship.