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Humid Air : Absolute air humidity

Definition of absolute humid air and associate air psychometric calculations

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1. Definition of air absolute humidity
2. Calculation of air absolute humidity
3. Absolute humidity at saturation

1. Definition of air absolute humidity

What is absolute air humidity ?

The 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.

What is the difference between absolute humidity and relative humidity ?

Air absolute humidity should not be confused with air relative humidity, which is representing a ratio of the partial pressure of water at the given condition vs the partial pressure of water at saturation.

2. Calculation of air absolute humidity

How to calculate air absolute humidity ?

ω = mw / ma = weight of water / weight of dry air

With :
ω = Absolute humidity of air (-)
mw = weight of water (kg water)
ma = weight of dry air (kg dry air)

The absolute air humidity can also be calculated from the molar fraction of water in the gas.

Calculation of air absolute humidity from molar ratio of water in air

With :
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 :

3. Absolute humidity at saturation

How to calculate absolute air 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 accommodate more water.



With :
ωsat = Absolute humidity of air at saturation at temperature T (-)
Pwatersat = saturation pressure of water at temperature T (Pa)
P = total pressure (Pa)