Psychrometrics
Dew point temperature from temperature and RH (Magnus)
- where
- intermediate Magnus term (dimensionless)
- relative humidity (%)
- dry-bulb temperature (°C)
- dew point temperature (°C)
Explain this equation
Gives the temperature at which water starts condensing out of the air. Use it to check insulation on chilled pipework and ducts, condensation risk on cold surfaces and coil behaviour.
γ combines the relative humidity RH with the temperature t in the Magnus form. t_d is the temperature at which the air's existing vapour pressure equals the saturation pressure.
The Magnus equation is easy to invert, so dew point follows directly without trial and error. The constants 17.625 and 243.04 °C fit it to measured saturation pressures.
Sense check: 20 °C at 50 % has a dew point of about 9.3 °C; at 100 % the dew point equals the air temperature. Lower RH means a lower dew point.
Watch out: keep cold surfaces a margin above the dew point; a surface just at it will still sweat in local humid pockets.
The temperature at which air starts to condense on a cold surface. Use it to check coil apparatus dew point, pipe or duct insulation and the risk of condensation on chilled surfaces.
Enter your values to see the result.
Good to know
Any surface colder than the dew point will collect condensation: insulate chilled-water pipework, refrigerant suction lines and cold ducts so their outer surface stays above it. At 20 °C and 50 % RH the dew point is about 9.3 °C.
Common mistake
Treating a dew point below 0 °C as a frost point. Below freezing this gives the dew point over water; frost forms at a slightly higher temperature.
Limits
Magnus form with Alduchov–Eskridge constants; within about 0.35 °C from −40 to 50 °C.
Sources
- 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)
- 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)
- 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.