difference between relative humidity and percentage saturation
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Relative Humidity
Relative humidity (Æ ) is defined as the ratio of the mole fraction of the water vapour (xw) in a given moist air sample to the mole fraction of water vapour in an air sample of saturated moist air (xws) at the same temperature and pressure.
By definition, the mole fraction of the water vapour (xw) is the ratio of the number of mole of water vapour in a given moist air sample to the total number of dry air and water vapour.
When the dry air and water vapour occupy the same volume and temperature, by applying the characteristic equation of state for perfect gas, Eqn. (3) becomes:
where Pw = partial pressure of water vapour in moist air
Pws = partial pressure of water vapour in saturated moist air
Relative humidity is usually expressed as percentage (%).
Degree of Saturation/Percentage Saturation
Degree of saturation (m ) is the ratio of the humidity ratio of moist air (w) to the humidity ratio of saturated moist air (ws) at the same temperature and pressure.
From Eqn. (2), Eqn. (6) becomes
The difference between relative humidity Æ and degree of saturation m is usually less than 2%.
Percentage saturation is degree of saturation when expressed in percentage.
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