Psychrometrics
Moisture content (humidity ratio) from relative humidity
- where
- saturation vapour pressure, Magnus (kPa)
- dry-bulb temperature (°C)
- partial pressure of water vapour (kPa)
- relative humidity (%)
- moisture content (g/kg dry air)
- atmospheric pressure (kPa)
Explain this equation
Converts a thermometer and hygrometer reading into moisture content, the actual amount of water in the air. Use it before latent load or mixing calculations.
p_ws is the most vapour the air could hold at temperature t. RH/100 times that gives p_w, the vapour pressure actually present. p is atmospheric pressure, and p − p_w is the pressure of the dry air. W is grams of water per kilogram of dry air.
The ratio of partial pressures is the ratio of molecules; 0.621945 converts that to a mass ratio because water molecules are lighter than air, and the 1000 gives g/kg.
Sense check: 20 °C at 50 % is about 7.2 g/kg; 30 °C saturated is about 27 g/kg.
Watch out: lower pressure at altitude gives a slightly higher moisture content for the same RH.
Converts a temperature and relative humidity reading into moisture content (grams of water per kilogram of dry air), the quantity used for latent loads and mixing.
Enter your values to see the result.
Good to know
621.945 is 1000 × 0.621945, the ratio of the molar masses of water and dry air, giving g/kg. Standard pressure is 101.325 kPa; it falls about 1.2 kPa per 100 m of altitude. At 20 °C and 50 % RH the moisture content is about 7.2 g/kg.
Common mistake
Treating RH as a measure of how much water is in the air. Warm air at 40 % can hold more water than cool air at 90 %.
Limits
Magnus saturation pressure (within about 0.4 % from −40 to 50 °C) and no enhancement factor, so results are about 0.5 % below ASHRAE table values.
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)
- 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.