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Humidity Ratio of Air

Heat Transfer Engineering
Thermodynamics

Humidity Ratio of Air

Humidity ratio W is the ratio of the mass of water vapor to the mass of dry air:

Eq. 1
W = M_w / M_da

W equals the mole fraction ratio x_w / x_da multiplied by the ratio of molecular masses ( 18.01528 / 28.9645 = 0.62198 ), thus

Eq. 2
W = 0.62198 x_w / x_da

Specific humidity γ is the ratio of the mass of water vapor to the total mass of the moist air sample:

Eq. 3
γ = M_w / ( M_w + M_da )

In terms of humidity ratio:

Eq. 4
γ = W / ( 1 + W )

Absolute humidity d_v , or water vapor density, is the ratio of the mass of the water vapor to the total volume of the sample:

Eq. 5
d_v = M_w / V

Humidity density ρ of a moist air mixture is the ratio of total mass to total volume:

Eq.6
ρ = ( M_w + M_da ) / V = ( 1 / v ) ( 1 + W )

where

v = moist air specific volume (ft³ / lb_da )

Saturation humidity W_s (t, p) is the humidity ratio of moist air saturated with respect to water (or ice) at a specified temperature t and pressure p.

Degree of humidity saturation m is the ratio of the air humidity ratio W to the humidity ratio of saturated moist air W_s at the same temperature and pressure:

Eq. 7
u = ( W / W_s )|_{t, p}

where the air saturation humidity ratio W_s is

Eq. 8
W_s = 0.622 p_ws / ( p - p_ws)

p_ws = saturation pressure of water (psia) in the absence of air, at the given temperature t.

• Absolute Humidity, Relative Humidity, Specific Humdity Thermodynamics
• Water Vapor Permeability
• Wet Bulb Temperature Equation
• Water Vapor Released by Humans
• Water Vapor Density
• Water - Density Viscosity Specific Weight

Source

• Heat Transfer, A Practical Approach Yunus A. Cengel