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How to Use the Saturation Temperature Calculator
Choose pressure to temperature or temperature to pressure, then select the refrigerant from the unit nameplate. Choose evaporating for superheat work or condensing for subcooling work.
Choose psig and degrees Fahrenheit or kPa and degrees Celsius. Enter the site elevation if it is well above sea level, because gauges read pressure above the local atmosphere.
Type the gauge pressure from the manifold, or the saturation temperature you want a pressure for, then press Calculate. The page uses the dew point on the evaporating side and the bubble point on the condensing side.
Subtract the saturation temperature from the line temperature to get superheat or subcooling. Add the project details, then use PDF, CSV, Copy or Print.
Reading saturation temperature from a refrigeration gauge
A gauge gives pressure, and a thermometer gives line temperature. Saturation temperature is what connects them. It is the temperature at which the refrigerant in the system is boiling or condensing at the pressure you measured, and every superheat and subcooling check starts from it. This page converts in both directions, from the gauge reading to the temperature and from a target temperature to the pressure you should see.
What the number tells you
On the low side, the saturation temperature at suction pressure is the refrigerant's boiling temperature in the evaporator. A suction line warmer than that is carrying superheat. On the high side, the saturation temperature at liquid line pressure is the condensing temperature, and a liquid line cooler than that is carrying subcooling. A pressure reading by itself says little, because the same 118 psig means 39.8°F on R-410A and 68.4°F on R-22. When a reading looks wrong, the saturation temperature is the first value to rebuild, and the refrigerant fault diagnosis tool uses it to work out which fault fits the readings.
Gauge pressure, absolute pressure and elevation
A gauge reads the difference between the system pressure and the air around it, but refrigerant properties are tabulated against absolute pressure. The calculator adds the local atmospheric pressure to your gauge reading before it looks anything up. It estimates that pressure from the site elevation with the troposphere formula in NASA's Earth atmosphere model, which gives 101.4 kPa at sea level and 89.5 kPa at 1,045 m, about the height of Calgary. The effect is modest. A reading of 118 psig on R-410A is 39.8°F at sea level and 39.1°F at 1,045 m. Leave the elevation blank for sea level, or enter it in feet or metres to match the unit you picked. A gauge that is not zeroed to the local air before use adds its own error on top.
Same pressure, different refrigerant
Each refrigerant boils at its own temperature for a given pressure, so the nameplate decides which table applies. At 118 psig and sea level the dew point is 38.7°F for R-32, 39.8°F for R-410A, 44.9°F for R-454B, 57.1°F for R-404A, 68.4°F for R-22, 70.3°F for R-407C and 97.0°F for R-134a. The three closest, R-32, R-410A and R-454B, still differ by up to 6.1°F, which is enough to turn a correct superheat into a wrong one. Going the other way, 40°F of saturation is 118.3 psig on R-410A but only 107.5 psig on R-454B and 68.5 psig on R-22. On a retrofit, confirm which refrigerant is in the system before you trust a pressure.
Dew point, bubble point and glide
A blend does not always change state at one temperature. At a fixed pressure it begins to boil at the bubble point and finishes at the dew point, and the gap between them is the glide. Sporlan's guide to refrigerant blends tells technicians to use the bubble point to calculate subcooling and the dew point to calculate superheat, and it puts R-404A at just over 1°F of glide. The tables here are close to that: R-404A at 60 psig has 0.8°F between its two points, R-410A about 0.2°F and R-454B at 100 psig 2.6°F. R-407C is the large case, with 11.0°F of glide at 70 psig, a dew point of 44.5°F against a bubble point of 33.6°F. That is why the side selector uses the dew point when you choose evaporating and the bubble point when you choose condensing, and why the table always shows both.
Where the pressure data comes from
The pressures come from CoolProp 8 using its multiparameter equations of state. R-32, R-134a, R-22 and R-410A use the library's own models for each, and R-407C and R-454B are built as mixtures from the mass fractions in their ASHRAE designations (23% R-32, 25% R-125 and 52% R-134a for R-407C, and 68.9% R-32 and 31.1% R-1234yf for R-454B). R-404A and R-410A use the library's own blend models. The page holds a table in 1 K steps from -60 to 65°C and interpolates between the steps in the logarithm of pressure, so the result is smooth. If you need a reading in bar or MPa for a metric gauge, the pressure converter changes units without touching the refrigerant.
Frequently Asked Questions
Add the local atmospheric pressure to the gauge reading to get absolute pressure, then look up the temperature at which the refrigerant boils or condenses at that pressure. The calculator does both steps. At sea level, R-410A at 118 psig has a dew point of 39.8°F, and the same 118 psig on R-22 reads 68.4°F, so the refrigerant must match the nameplate. Use the evaporating side for superheat and the condensing side for subcooling.
At a saturation temperature of 40°F, R-410A is at about 118.3 psig at sea level. For the same 40°F, R-22 is at about 68.5 psig, R-134a at about 35.0 psig, R-404A at about 85.4 psig and R-32 at about 120.9 psig. These are dew-point values for the evaporating side. Enter a temperature in the calculator for any of the seven refrigerants and it gives the gauge pressure at your elevation.
Use the dew point for superheat and the bubble point for subcooling. A blend that boils over a range of temperatures has a bubble point, where the first vapour forms, and a dew point, where the last liquid is gone. Sporlan's blends guide says to use bubble point to calculate subcooling and dew point to calculate superheat. At 70 psig, R-407C has a dew point of 44.5°F and a bubble point of 33.6°F, so the choice matters by about 11°F. R-410A and R-32 show almost none.
Yes, a little. A gauge reads pressure above the local atmosphere, which is 101.4 kPa at sea level and 89.5 kPa at 1,045 m. A reading of 118 psig on R-410A therefore means 39.8°F at sea level and 39.1°F at 1,045 m, a difference of 0.8°F. That is small next to the glide of a blend or a gauge error, but it adds up when you are checking a tight superheat target.