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

 

The mixing ratio r of moist air is the ratio of the mass Mv of water vapor to the mass Ma of dry air with which the water vapor is associated:

 

r = Mv / Ma

 

Mv = n x mw = mw x p V / RT

 

Ma = n x ma = ma x pa V / RT = ma x (Pb – p) / RT

 

mw = 18.016 gram

ma = 28.966 gram

p = partial pressure of water vapor [Pa]

pa = partial pressure of dry air [Pa]

Pb = total or barometric pressure [Pa]

R = 8.31436 Pa x m3 / °K x mole

T = temperature of the gas mixture in °K

 

r = mw p / ma (Pb – p)

 

r = 621.97 x p / (Pb – p) [g / kg]

 

1 gr (grain) = 0.0648 g (gram)

1 lb = 0.4535923 kg

 

r [gr / lb] = 7 x r [g / kg]