The pressure-temperature (P/T) chart is one of the most-used tools in refrigeration. It lets you convert a measured pressure into the refrigerant’s saturation temperature, which underpins superheat and subcooling calculations.
What a P/T chart shows
For a given refrigerant, the chart lists the saturation (boiling/condensing) temperature that corresponds to each pressure. Each refrigerant has its own chart because they all behave differently.
How to use it
- Measure the pressure on the relevant side of the system
- Find the matching saturation temperature on the P/T chart for that exact refrigerant
- Compare with the actual line temperature to calculate superheat (low side) or subcooling (high side)
Watch out for blends (glide)
Zeotropic blends like R407C and R448A have ‘temperature glide’ — they boil and condense over a range of temperatures rather than a single point. P/T charts for these show separate bubble and dew point values; use the correct one for superheat (dew) and subcooling (bubble).
FAQ
Why does each refrigerant need its own P/T chart? Because the pressure-temperature relationship is unique to each refrigerant’s properties.
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