Psychrometrics
Saturation vapour pressure of water (ASHRAE Hyland–Wexler and Magnus)
- where
- absolute temperature (K)
- dry-bulb temperature (°C)
- saturation pressure over water, ASHRAE Hyland–Wexler (kPa)
- saturation pressure, Magnus form (kPa)
Explain this equation
Gives the pressure of water vapour in fully saturated air at a given temperature. It is the base for relative humidity, moisture content and dew point.
t is temperature in °C and T the same in kelvin. p_ws is the ASHRAE Hyland–Wexler result, a curve fitted to the steam tables. p_M is the simpler Magnus form used by the other psychrometric calculators here. The 1000 turns pascals into kPa.
Saturation pressure rises roughly exponentially with temperature because warmer water molecules escape more easily; both forms describe that curve, the longer one more precisely.
Sense check: about 0.61 kPa at 0 °C, 2.34 kPa at 20 °C, 7.4 kPa at 40 °C and 101.4 kPa at 100 °C, where water boils at sea level. Every 10 °C rise nearly doubles it.
Watch out: below 0 °C this is over supercooled water, not ice.
The pressure of water vapour in air that is fully saturated at a given temperature. It is the starting point for relative humidity, moisture content and dew point.
Enter your values to see the result.
Good to know
The first result uses the Hyland–Wexler equation from ASHRAE Fundamentals, which reproduces the steam tables closely from 0 to 200 °C. The second is the simpler Magnus form (Alduchov and Eskridge constants), used by the other calculators here because it can be turned round for dew point; it agrees within about 0.4 % from −40 to 50 °C. Pressures in kPa; 1 kPa = 10 mbar.
Common mistake
Using the °C value where the equation needs kelvin (T = t + 273.15), or the other way round.
Limits
Over liquid water. ASHRAE gives a separate equation over ice below 0 °C. Real moist air has a small enhancement factor (about 1.004) not included here.
Sources
- ASHRAE Handbook — Fundamentals (2017), chapter 1 Psychrometrics: Hyland–Wexler saturation pressure (eq. 6), humidity ratio W = 0.621945 pw/(p − pw), enthalpy h = 1.006t + W(2501 + 1.86t), specific volume v = 0.287042(t + 273.15)(1 + 1.607858W)/p, adiabatic mixing
- O. A. Alduchov, R. E. Eskridge, Improved Magnus form approximation of saturation vapor pressure, Journal of Applied Meteorology 35(4), 1996, pp. 601–609 (es = 610.94 exp(17.625t/(t + 243.04)) Pa)
- M. G. Lawrence, The relationship between relative humidity and the dewpoint temperature in moist air, Bulletin of the American Meteorological Society 86(2), 2005, pp. 225–233 (Magnus form with constants 17.625 and 243.04 °C)
- CIBSE Guide C: Reference data (2007), chapter 1 (properties of humid air) and chapter 4 (flow of fluids in pipes and ducts)
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.