Refrigerant PT Charts (Pressure-Temperature)

PT charts for each refrigerant

How do you read a refrigerant PT chart?

Pick the chart for the refrigerant on the nameplate, then go from what you know to what you need. If you have a gauge pressure, find it in the pressure column and read the saturation temperature on that row. If you have a temperature, find its row and read the pressure. Between rows, interpolate, or use the refrigerant PT calculator, which does it for you.

1. 1

   Confirm the refrigerant.

   Read the nameplate. R-410A, R-32 and R-454B equipment can look alike, and their charts are not interchangeable.

2. 2

   Choose the column.

   On a blend, use dew (vapor) for anything on the suction side and bubble (liquid) for the liquid line. On R-32 or R-22 there's one column.

3. 3

   Convert the pressure.

   Read the saturation temperature at your gauge pressure. Correct for altitude first if you're well above sea level.

4. 4

   Compare with the pipe.

   Suction line temperature minus the dew point is superheat. Bubble point minus liquid line temperature is subcooling.

5. 5

   Judge against the target.

   The target comes from the unit's charging chart or installation instructions, not from the PT chart.

Bubble point, dew point and glide

A single-component refrigerant boils at one temperature for each pressure. A blend of refrigerants with different boiling points doesn't: it starts boiling at the bubble point and finishes at the dew point, a little warmer. That spread is temperature glide. R-454B has about 2.7°F of it at typical suction pressure, enough to throw superheat off by that much if you read the wrong column. R-410A has about 0.2°F, which is why older R-410A charts printed a single column.

Do PT charts change with altitude?

The physics doesn't change, but your gauge does. PT charts are printed in psig relative to a sea-level atmosphere of 14.696 psia. A gauge zeroed at altitude reads relative to thinner air, so for the same saturation temperature it shows a higher number. Using NASA's standard-atmosphere model, the air is about 12.2 psia at 5,000 ft and 10.9 psia at 8,000 ft.

What a PT chart can't tell you

• Whether the charge is right. Pressure alone moves with indoor load and outdoor temperature. You need the line temperature too, and the manufacturer's target.
• Which refrigerant is in the system. A system topped off with the wrong refrigerant, or a mix, won't follow any chart. Recover it and recharge with the nameplate refrigerant.
• Normal operating pressures. A chart gives saturation pressure at a temperature. What a healthy system runs at depends on its design, airflow and conditions.
• Whether it's safe to retrofit. That's an EPA listing and manufacturer question, covered on the R-22 page.

How we build and check these charts

We don't copy manufacturer charts. Each table is computed from published equations of state with CoolProp 8.0.0, the open-source property library, from −40°F to 150°F in 5°F steps, and stored with the library version and date. Blends are modeled at their ASHRAE 34 compositions. An automated test suite then checks 20 temperatures per refrigerant against the Daikin A1 vs. A2L chart, plus the Arkema R-454B chart and the Arkema wall chart. R-22, R-32 and R-410A agree within 0.1 psi. R-454B agrees with Daikin within 1.4 psi and with Arkema within 1.5%, a smaller spread than the up-to-3-psi gap between those two manufacturers' own R-454B charts.