How to Read a PT Chart: Saturation, Dew Point, Bubble Point and Glide

Every superheat and subcooling reading starts with a saturation temperature, and that comes from a pressure-temperature (PT) chart, either printed or built into your digital manifold.

What the chart tells you

Inside the evaporator and condenser, refrigerant is boiling or condensing. While liquid and vapor exist together, the refrigerant's temperature is locked to its pressure. The PT chart lists that relationship. If you know the pressure, you know the saturation temperature.

Example for R-410A: about 118 psig corresponds to about 40 °F saturation, a typical evaporator in cooling. About 365 psig corresponds to about 110 °F, a typical condensing temperature on a warm day.

How to use it

1. Pick the right refrigerant. Check the nameplate. Using the wrong column is a common mistake, and with R-410A, R-454B and R-32 all in the field, it's an easy one.
2. Use gauge pressure (psig) if the chart is in psig. Most field charts are. Absolute pressure (psia) is about 14.7 psi higher at sea level.
3. Read across to the saturation temperature.
4. Compare to the actual line temperature to get superheat (suction side) or subcooling (liquid side). The superheat & subcooling calculator does the subtraction for you.

Blends, glide, dew and bubble

Some refrigerants are blends whose components boil at different temperatures. During a phase change their temperature slides across a range at constant pressure. That range is the glide.

• Bubble point: the temperature where liquid starts to boil (all liquid, first bubble).
• Dew point: the temperature where the last liquid boils off (all vapor, last droplet).

The rule:

Reading	Use
Superheat (evaporator outlet, vapor)	Dew point
Subcooling (condenser outlet, liquid)	Bubble point

R-410A is a near-azeotrope with a very small glide, so its chart usually shows one column. R-454B has a small glide, and older blends like R-407C have a large one. Pure refrigerants like R-32 and R-22 have no glide at all.

Common mistakes

• Reading the wrong refrigerant column.
• Using a mid-glide or wrong column on a high-glide blend, which throws superheat and subcooling off by several degrees.
• Reading pressure while the system is still settling. Give it 10–15 minutes.
• Mixing psig and psia. Use the unit converter if your data is in kPa or bar.

FAQ

Why does my digital manifold show two saturation temperatures?

For blends with glide, it calculates the dew point on the low side and the bubble point on the high side. That's correct.

Is the PT chart accurate for superheated vapor or subcooled liquid?

No. It only holds where liquid and vapor are both present (saturation). That's why you measure saturation at the coil pressures and compare it to actual line temperatures.

Does altitude change the PT chart?

The pressure-temperature relationship is based on absolute pressure. Gauges read relative to the local atmosphere, so at high altitude psig readings shift slightly. Most digital manifolds let you set altitude or measure barometric pressure.

Sources

• HVAC School: how similar are A2Ls to R-410A
• HVACProSales: R-454B pressure and charging characteristics