How to Use the P-T Chart Calculator
Select the refrigerant installed in the system. The chart rebuilds with that refrigerant's dew point and bubble point at every 10 PSIG.
Type the reading in PSIG or kPa gauge. The result bar shows the exact dew point and bubble point, and the nearest table row is highlighted.
Switch between °F and °C. The table, the result bar and the exports all follow the unit you choose.
Take the dew point for superheat and the bubble point for subcooling. Send the saturation temperature to the superheat or subcooling calculator.
How to use a P-T chart on the job site
A P-T chart converts gauge pressure into saturation temperature for a specific refrigerant. You find the pressure in the left column and read the temperature across, and that saturation temperature is the baseline for superheat on the suction side and subcooling on the liquid side.
Why every refrigerant needs its own chart
At 100 PSIG, R-22 saturates at 59.1°F, R-410A at 31.5°F, R-32 at 30.4°F and R-134a at 87.7°F. Those gaps are far too large to ignore, and using the wrong chart gives the wrong superheat and subcooling and then a wrong charging decision. The saturation temperature calculator gives the same conversion for a single reading.
Dew point, bubble point and glide
Blends do not boil at one temperature. At 100 PSIG, R-407C has a dew point of 61.6°F and a bubble point of 51.1°F, a glide of 10.5°F, and R-454B has 36.4°F and 33.7°F. Superheat is measured from the dew point and subcooling from the bubble point, so the chart shows both columns for every refrigerant. For R-410A the two columns differ by only a fraction of a degree. The superheat calculator and subcooling calculator apply the same convention.
Gauge pressure and altitude
Gauges read pressure above the local atmosphere, and the tables add 14.696 psi, the standard atmosphere at sea level, to get absolute pressure. At altitude the local atmosphere is lower, so a gauge that reads zero in open air is already a little below the standard reference. If you work in kPa, the chart shows the gauge value beside each PSIG row, and the pressure converter handles other pressure units.
Reading pressures in cold weather
Pressure follows temperature, so a low reading in winter is not a fault by itself. An R-410A suction of 75 PSIG is a dew point of 18°F, and the same pressure in summer would point to a very cold evaporator. Always convert the reading to a saturation temperature and compare it with the air temperature across the coil. For heat pumps in Canadian winters, Natural Resources Canada lists resources for HVAC professionals on selecting and installing cold climate units.
Setting pressure controls from the chart
Low-pressure and high-pressure cut-outs are set from saturation temperatures, not from remembered pressures. Pick the lowest evaporating temperature and the highest condensing temperature the system should see, read the pressures from the chart, and add the control differential. The pressure switch calculator works through the settings.
Checking the chart against NIST
The tables are generated with CoolProp and interpolated, and for pure refrigerants you can check any row yourself. The NIST Chemistry WebBook lists saturation properties for single fluids such as R-134a and R-32, in temperature or pressure steps. It does not list blends such as R-410A, so compare those with the manufacturer's chart for the same refrigerant.
Frequently Asked Questions
A P-T chart gives the saturation temperature of a refrigerant at each pressure. A technician reads a gauge pressure, finds the matching temperature on the chart for that refrigerant, and uses it as the baseline for superheat and subcooling. On an R-410A system, 118 PSIG saturates at 39.8°F. If the suction line reads 51.8°F, the superheat is 12°F. The same gauge reading on R-22 would mean a different temperature, so the chart must match the refrigerant in the system.
Not a single one, because the pressure follows the saturation temperature and that moves with the load and the weather. An R-410A evaporator at 40°F is at 118.4 PSIG and one at 45°F is at 130.1 PSIG, while a condenser at 100°F is at 317.6 PSIG and one at 120°F is at 418.3 PSIG. Convert the reading to a temperature on the chart and judge that against the superheat, subcooling and temperature split, not the pressure alone.
Use the dew point for superheat and the bubble point for subcooling. A blend such as R-454B or R-407C changes temperature as it boils or condenses, so one pressure has two saturation temperatures. For R-454B at 100 PSIG the dew point is 36.4°F and the bubble point is 33.7°F. For R-410A the two differ by less than half a degree.
The pressures come from CoolProp, an open-source property library, and the lookup interpolates between them to within 0.01°F of the library values. The gauge pressure is converted with 14.696 psi of atmospheric pressure, so readings taken at altitude carry a small offset. You can check a pure refrigerant such as R-134a or R-32 against the NIST Chemistry WebBook.