Absolute humidity is an amount of water vapor, usually
discussed per unit volume. The mass of water vapor,per unit volume of total air
and water vapor mixture, can be expressed as follows:
Absolute humidity in air ranges from zero to roughly 30 grams
per cubic meter when the air is saturated at 30 °C. The absolute humidity
changes as air temperature or pressure changes. This is very inconvenient for
chemical engineering calculations, e.g. for clothes dryers, where temperature
can vary considerably. As a result, absolute humidity is generally defined in
chemical engineering as mass of water vapor per unit mass of dry air, also
known as the mass mixing ratio, which is much more rigorous for heat and mass
balance calculations. Mass of water per unit volume as in the equation above
would then be defined as volumetric humidity. Because of the potential
confusion, British Standard BS 1339 suggests avoiding the term "absolute
humidity". Units should always be carefully checked. Most humidity charts
are given in g/kg or kg/kg, but any mass units may be used.
