System checks
Saturation temperature from pressure (interpolate your P-T data)
- where
- saturation temperature (°C)
- saturation temperature at P₁ (°C)
- measured pressure (bar)
- lower table pressure (bar)
- saturation temperature at P₂ (°C)
- upper table pressure (bar)
Explain this equation
Reads a saturation temperature between two lines of a refrigerant P-T table. Use it when your reading falls between table values and you need a more precise saturation temperature for superheat or subcooling.
P₁, T₁ and P₂, T₂ are two neighbouring rows of the table, either side of your reading P. The fraction (T₂ − T₁)/(P₂ − P₁) is how many degrees the temperature changes per bar over that interval. Multiplying by how far your pressure is above P₁ and adding to T₁ gives the answer.
Linear interpolation: it treats the curve as a straight line between two close points.
Sense check: a reading exactly halfway between the table pressures gives a temperature halfway between the two table temperatures.
Watch out: keep everything on one basis, all gauge or all absolute. The P-T curve bends, so use table points close together and never extrapolate far outside them.
Estimates saturation temperature for a pressure reading by straight-line interpolation between two points you enter from the refrigerant manufacturer’s pressure-temperature table.
Enter your values to see the result.
Good to know
Enter all three pressures on the same basis (all gauge or all absolute) and in the same unit. Pick the two table points either side of the reading and as close together as the table allows. For zeotropic blends use the dew-point column for suction (superheat) and the bubble-point column for liquid (subcooling).
Common mistake
Mixing gauge and absolute pressures (about 1 bar out), or using points far apart: the P-T curve is not straight, so wide spacing gives errors of a kelvin or more.
Limits
Linear interpolation of a curved relationship. With points 1–2 bar apart at evaporating pressures the error is typically a few tenths of a kelvin; closer points are better. No refrigerant data is built in: use the published table for your refrigerant.
Sources
- NIST Chemistry WebBook: Thermophysical properties of R-134a (saturation pressure and temperature data)
- ASHRAE Handbook – Refrigeration, Chapter 1 Halocarbon refrigeration systems (Table 1 velocities: suction 900–4000 fpm, discharge 2000–3500 fpm, liquid below 300 fpm; superheat and subcooling in line sizing)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.