Guide · By DueTrack Studio · Updated 2026-10-29
How to read a refrigerant PT chart
A PT chart lists the saturation temperature of a refrigerant at each pressure. Find the measured pressure, read across to the temperature at which the refrigerant boils or condenses at that pressure, and compare it with the actual line temperature. For zeotropic blends with temperature glide, use the dew point for superheat and the bubble point for subcooling.
What does a PT chart show?
Each row pairs a pressure with the temperature at which that refrigerant is saturated — a mix of liquid and vapor. Different refrigerants have very different charts, so always use the chart for the refrigerant on the nameplate.
How do you use it in the field?
- Read the gauge pressure.
- Find that pressure for the correct refrigerant.
- Read the saturation temperature.
- Compare with the measured line temperature to get superheat or subcooling.
What are bubble and dew points?
Blends such as many 400-series refrigerants do not boil at a single temperature; the range is called glide. The bubble point is where liquid starts to boil; the dew point is where the last liquid has evaporated. Use dew for superheat calculations and bubble for subcooling.
Questions
Why doesn't my PT chart reading match my thermometer?
Because the refrigerant isn't saturated at that point — the difference is your superheat or subcooling. That gap is exactly what you are measuring.