Humid Air : Absolute air humidity
1. IntroductionThe absolute humidity is defined as the weight of water in the air per weight of dry air. From a given state, if there is no water / air transfer the absolute humidity is not changing.
ω = mw / ma = weight of water / weight of dry airWith :
ω = Absolute humidity of air (-)
mw = weight of water (kg water)
ma = weight of dry air (kg dry air)
3. Other relationsThe absolute air humidity can be calculated from the molar fraction of water in the gas.
Mwater = molar weight of water = 18 g/mol
Mair = molar weight of air = 29 g/mol
y = molar ratio of water in air = Pwater / Ptotal
Pwater = partial pressure of water
If y is small, which is quite often the case, then the absolute humidity can be approximated easily :
4. Absolute humidity at saturation
When the water partial pressure in the air reaches the saturation pressure at the corresponding temperature, then the air is saturated - it cannot accomodate more water.
ωsat = Absolute humidity of air at saturation at temperature T (-)Pwatersat = saturation pressure of water at temperature T (Pa)
P = total pressure (Pa)