Refrigerant PT chart

A refrigerant pressure-temperature (PT) chart is a quick way to connect pressure readings with a saturation temperature (or vice versa). It is widely used in HVAC and automotive A/C for field interpretation, but it is easy to misuse if you ignore units, pressure basis, or blend behavior.

What a PT chart actually represents

Most PT charts are a lookup for the saturation relationship:

Tsat(P) and Psat(T)

If the refrigerant is at saturation (boiling/condensing), pressure and temperature are tightly linked. Away from saturation (superheated vapor or subcooled liquid), pressure alone does not tell you the temperature, and vice versa.

The most common pitfalls

• Gauge vs absolute pressure: many gauges read pressure relative to ambient. Property correlations typically require an absolute pressure basis. If you convert, you need the correct local atmospheric pressure (altitude matters). See Gauge vs absolute pressure (psig vs psia).
• Wrong refrigerant / wrong blend convention: for blends, "saturation temperature" can depend on bubble vs dew point and temperature glide. Charts, apps, and procedures may not use the same convention.
• Not actually at saturation: PT charts are only directly meaningful when the refrigerant at the measurement point is near saturation. Superheat/subcooling shifts the temperature away from Tsat.
• Pressure drop: a sensor at the wrong location (or with significant line pressure drop) can produce a misleading Tsat(P) if you pair it with a temperature measured elsewhere.

Blends: bubble point, dew point, and glide

Many common refrigerants are mixtures. If a blend has noticeable temperature glide, then a PT chart is not a single curve -- it depends on whether you mean the bubble or dew endpoint of saturation.

Learn more: Bubble vs Dew (Temperature Glide) and Zeotropic vs Azeotropic.

Using FluidTool to verify a PT chart

Instead of relying on a static PDF chart, you can use FluidTool to compute Tsat(P) or Psat(T) for the exact refrigerant you selected (and explore endpoints for blends):

1. Select a refrigerant.
2. Use a Two-phase input pair (P + Q or T + Q).
3. Compare Q=0 and Q=1 to see bubble vs dew endpoints (if applicable).
4. If you have measured pressure and temperature, use Superheat/Subcooling concepts to interpret how far you are from saturation.

Quick links: Try R134a, Try R410A, Try R407C, Try R22.

Related concepts

• Saturation pressure vs temperature: the core Psat(T)/Tsat(P) relationship.
• Subcooling (Delta Tsc): the liquid-side "distance from saturation" concept.
• Superheat & Subcooling: what "distance from saturation" means in practice.
• Two-phase quality (Q): how Q parameterizes states inside the saturation region.
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