P/T Chart

The AC Service Tech Quick Guide Pressure / Temperature chart. Pressures are shown in PSIG at sea level, with the saturated liquid and saturated vapor temperature for each refrigerant.

Use the P/T chart to convert a measured pressure into its saturation temperature, which is the starting point for both the superheat and the subcooling calculation. Subtract the saturation temperature from the actual vapor line temperature to get superheat; subtract the actual liquid line temperature from the saturation temperature to get subcooling.

The chart is also the fastest way to identify an unknown refrigerant. Let the system sit off for an hour or more so the pressures equalize, then compare the equalized pressure and the ambient temperature against the saturation values for each refrigerant.

See also Charging Procedures, Target Superheat, and the HVAC Calculators.

If you want to learn more about refrigerant charging methods, procedures, and troubleshooting, read the Refrigerant Charging and Service Procedures book by Craig Migliaccio.

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