Humid Air : Absolute air humidity definition and calculation
Definition of absolute humid air and associated air psychometric calculations
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1. Definition of air absolute humidity
2. Calculation of air absolute humidity
3. Absolute humidity at saturation
1. Definition of air absolute humidity
What is absolute air humidity ?
The absolute humidity is defined as the mass of water in the air per mass of dry air. From a given state, if there is no water / air transfer the absolute humidity is not changing. The absolute air humidity is independent of the temperature.
In process engineering, understanding and controlling absolute air humidity can be very important as it directly impacts the performances of the process but also of the final product in many industries.
Absolute humidity vs relative humidity : What is the difference between absolute humidity and relative humidity ?
Air absolute humidity should not be confused with air relative humidity, which is representing a ratio of the partial pressure of water at the given condition vs the partial pressure of water at saturation.
2. Calculation of air absolute humidity
How to calculate air absolute humidity ?
Absolute humidity formula :
ω = mw / ma = weight of water / weight of dry air
With :
ω = Absolute humidity of air (-)
mw = weight of water (kg water)
ma = weight of dry air (kg dry air)
The absolute air humidity can also be calculated from the molar fraction of water in the gas.
With :
Mwater = molar weight of water = 18 g/mol
Mair = molar weight of air = 29 g/mol
y = molar ratio of water in air = Pwater / Ptotal
Pwater = partial pressure of water
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If y is small, which is quite often the case, then the absolute humidity can be approximated easily :
3. Absolute humidity at saturation
How to calculate absolute air humidity at saturation ?
With :
P = total pressure (Pa)
4. Air Humidity measurement
It is very important to be able to measure (automatically if possible) air humidity. The following instruments can be used for this purpose :
- Hygrometer : A hygrometer measures humidity in the air. It uses methods like hair tension or electrical resistance to provide accurate humidity readings. For example, a hair tension hygrometer has a hair strand that expands or contracts with humidity changes. This movement is then translated into a humidity value. Electrical resistance hygrometers rely on moisture-induced changes in electrical conductivity to determine humidity levels.
- Psychrometer : A psychrometer assesses relative humidity. It works based on evaporative cooling, where water evaporation causes air temperature to drop. A typical psychrometer has two thermometers: a dry bulb thermometer for air temperature and a wet bulb thermometer covered in a wet wick. As the water evaporates from the wick, it cools the wet bulb thermometer. By comparing the temperatures recorded by the dry and wet bulb thermometers, engineers can calculate relative humidity. Larger temperature differences indicate drier air, while smaller differences mean higher humidity.
5. Absolute air humidity calculator Excel
MyEngineeringTools.com has developed a
free Excel calculator that allows you to calculate the absolute
humidity of air, knowing the pressure and the partial pressure of
water, it allows also many other calculations related to humid air :
Humid air enthalpy calculator
Excel (click here)
Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.
