The pressure-temperature chart is the first tool every tech learns and one of the most misread. It is accurate, but only in one condition: where liquid and vapor exist together at saturation. Use it anywhere else and it will give you a confident, wrong answer.
Where the chart applies
Inside a running system, refrigerant is saturated in two places: the evaporator, where it is boiling, and the condenser, where it is condensing. In both, the pressure fixes the temperature. R-410A at about 118 psig is boiling at 40°F. R-22 boils at the same 40°F at only about 69 psig. Read the pressure, look it up, and you have the saturation temperature.
Where it doesn't
The suction line carries superheated vapor that has warmed past saturation. The liquid line carries subcooled liquid that has cooled below it. Neither line's temperature can be read off the chart. That gap is the whole point: superheat and subcooling measure how far the actual line temperatures sit from the saturation temperatures the pressures predict.
Blends and glide
Some blends, such as R-454B, boil across a small range of temperatures at a given pressure. Their PT charts list two values: the dew point, where the last liquid boils off, and the bubble point, where the first vapor appears. Use the dew point for superheat and the bubble point for subcooling, or let a digital manifold set to the correct refrigerant handle it.
Using the chart on a system that is off
A system that has been off long enough for everything to reach the same temperature, with liquid still inside, settles at the saturation pressure for that temperature. That makes a quick check possible before you start anything:
- Close to the chart pressure for the surrounding temperature: liquid is still present.
- Well below it: little or no liquid is left, so suspect a leak.
- Above it: suspect non-condensables, such as air, or the wrong refrigerant.
The most common mistake
Using the wrong refrigerant scale. Confirm the refrigerant on the data plate, set your manifold or app to match, and double-check after you connect. Every number you calculate afterwards depends on it.
Test yourself with the Saturated Refrigerant Quiz.