How superheat and subcooling are calculated
Superheat is vapor-line temperature minus saturation temperature at the measured suction pressure. Subcooling is saturation temperature at the measured liquid pressure minus liquid-line temperature. Both measurements must refer to the same part of the system.
Subcooling uses bubble saturation temperature minus liquid-line temperature. For zeotropic blends the dew and bubble temperatures differ. The bundled pressure-temperature data is a CoolProp 7.2.0 snapshot from our app, with linear interpolation between table points. Pressure is gauge pressure at sea level; altitude correction is not included in this web version. It does not prescribe adding or removing refrigerant.
References: CoolProp high-level API and saturation conventions · National Refrigerants pressure-temperature guidance.
Example: R-410A superheat at 118 psig
For R-410A at 118 psig, the bundled dew-pressure table gives a saturation temperature of about 39.8°F. A vapor-line reading of 55°F therefore gives about 15.2°F of superheat. Take pressure and temperature readings at corresponding locations. This example does not establish a target charge or tell you to add refrigerant.
Try this example in the calculatorSupported refrigerants: R-22, R-32, R-134a, R-410A, R-407C, R-449A, R-454B and R-502. Use dew saturation for superheat and bubble saturation for subcooling. Each fluid has a supported pressure range; the tool rejects values outside its table.
Frequently asked questions
Why do blends have bubble and dew temperatures?
A refrigerant blend can change phase over a temperature range. Use dew temperature for superheat and bubble temperature for subcooling. This calculator selects the correct side automatically.
What does a negative result mean?
A negative value is inconsistent with a normally superheated vapor or subcooled liquid at the measurement location. Check mode, refrigerant, probe placement and readings before drawing a conclusion.
Can I use this to decide how much refrigerant to add?
No. A temperature difference alone does not determine charge quantity. Follow the equipment manufacturer’s charging procedure and target, and verify airflow and operating conditions.
Which pressures are supported?
The calculator reports an error outside each refrigerant’s bundled pressure-temperature table. It never extrapolates beyond that range.