Use the superheat and subcooling calculator

The superheat and subcooling calculator works out superheat from a suction pressure and a suction line temperature, and subcooling from a liquid line pressure and a liquid line temperature. It runs in your browser, on a phone as well as a desk, and needs no account. Either half works on its own.

A gauge reads against the air around it. The saturation tables are in absolute pressure, so the calculator adds a standard atmosphere, 101.325 kPa, to a gauge reading. Where the air around you is far from a standard atmosphere, enter the absolute pressure if you have it, in kPa abs.

A single-component refrigerant, such as R-32 or R-134a, boils at one temperature at a given pressure. A zeotropic blend, such as R-454B or R-407C, boils across a range: its liquid starts to boil at the bubble point and the last of it boils off at the dew point. The difference is its glide.

So for a blend the calculator reads superheat from the dew point, because only above it is the refrigerant all vapour, and subcooling from the bubble point, because only below it is the refrigerant all liquid. The display names the point it used, and the line under the cells gives the blend's glide at that pressure.

Once a half shows a saturation temperature, choose Show the working under it. The working is the three steps you'd take with a P–T chart:

1. 1

   The absolute pressure: the gauge reading plus a standard atmosphere, after converting bar or psi to kPa.

2. 2

   The saturation temperature, or the dew or bubble point for a blend: the two rows of the refrigerant's table either side of your pressure, with your reading between them.

3. 3

   The subtraction that gives the superheat or subcooling.

The working's figures are rounded, so working by hand from them can differ in the last digit. Results are shown to 0.1 K.

The calculator reads only between the rows of a refrigerant's table, never past them. When it can't give a saturation temperature, the display says why instead:

• –“Below the table” or “Above the table”: the pressure is outside the rows of the refrigerant's public table, and Kelvin doesn't extrapolate.
• –Above the refrigerant's critical point: liquid and vapour are no longer distinct there, so there's no saturation temperature and nothing to measure superheat or subcooling from.
• –Below R-744's triple point: R-744 can't be liquid there.
• –“Not solved here”: CoolProp didn't solve the table's rows at that pressure, and Kelvin doesn't interpolate across the gap.
• –“Below a perfect vacuum”: the reading works out below zero absolute. Check the reading and the unit.
• –“Not a number”: the cell holds something other than a figure.

A negative result is still shown, with a warning. Negative superheat means the suction line reads colder than the dew point or saturation temperature: liquid may be reaching it, or a reading or the refrigerant chosen is off. Negative subcooling means the liquid line reads warmer than the bubble point or saturation temperature: the liquid may be flashing to vapour, or a reading or the refrigerant chosen is off.