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This is the ratio of the mass of the water vapor to the volume of the complete gas mixture containing the water vapor.

 

The vapor concentration (density of water vapor in a mixture) - or absolute humidity - is defined as the ratio of the mass of water vapor Mv to the volume V occupied by the mixture.

 

Dv = Mv / V , expressed in grams/m3 or in grains/cu ft.

 

This can be derived as follows from the equation PV = nRT:

 

a) Mv = n x mw

 

n = number of moles of water vapor present in the volume V.

mw = molecular mass of water.

 

b) Dv = Mv /.V = n x mw / V = mw x p / RT, where:

 

mw = 18.016 gram

p = partial pressure of water vapor [Pa]

R = 8.31436 Pa x m3 / °K x mole

T = temperature of the gas mixture in °K

 

Dv [g / m3] = p / 0.4615 x T

 

1 gr (grain) = 0.0648 g (gram)

1 cu ft = 0.0283168 m3

 

Dv [gr / cu ft] = 0.437 x Dv [g / m3]