Humidity Ratio of Air

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Humidity ratio can be expressed by mass of water or by the vapor partial pressure in the moist air.

Humidity Ratio by Mass

Humidity ratio can be expressed as the ratio between the actual mass of water vapor present in moist air - to the mass of the dry air. Humidity ratio is normally expressed in kilogram or pounds of water vapor per kilogram or pounds of dry air.

Humidity ratio expressed by mass:

x = m_w / m_a         (1)

where

x = humidity ratio (kg_water/kg_air, lb_water/lb_{dry_air})

m_w = mass of water vapor (kg, lb)

m_a = mass of dry air (kg, lb)

Humidity Ratio by Vapor Partial Pressure

Humidity ratio can also be expressed with the partial pressure of water vapor:

x = 0.62198 p_w / (p_a - p_w)         (2)

where

p_w = partial pressure of water vapor in moist air (Pa, psi)

p_a = atmospheric pressure of moist air (Pa, psi)

The maximum amount of water vapor in the air is achieved when p_w = p_ws the saturation pressure of water vapor at the actual temperature. (2) can be modified to:

x_s = 0.62198 p_ws / (p_a - p_ws)         (3)

where

x_s = specific humidity at saturation (kg_water/kg_air, lb_water/lb_{dry_air})

p_ws = saturation pressure of water vapor

Since the water vapor pressure is small regarding to the atmospheric pressure, the relation between the humidity ratio and the saturation pressure is almost linear.

Maximum specific humidity at some common temperatures:

Note that the saturation pressure of water vapor, - and the maximum humidity ratio, increases dramatically with the air temperature. This important for the capacity of drying processes.

Example - Humidity Ratio of Moist Air

The specific humidity for saturated humid air at 20^oC with water vapor partial pressure 2333 Pa at atmospheric pressure of 101325 Pa (1013 mbar, 760 mmHg) can be calculated as:

x = 0.62198 (2333 Pa) / ((101325 Pa) - (2333 Pa))

= 0.0147 (kg/kg)

= 14.7 (g/kg)

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Related Topics

• Air Psychrometrics - The study of moist and humid air - air condition - psychrometric charts, Mollier diagrams, air temperature, absolute and relative humidity, moisture content and more .

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