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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*) / V = ( 1 / v ) ( 1 + W )

_{da}*where*

*v = *moist air specific volume* (ft ^{3} / 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