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Understanding humidity and Rosahl

A brief review of humidity and dew point when selecting Rosahl dehumidifiers.



Water vapour is a gas and mixes freely with the nitrogen and oxygen in the air.  Like most gases it also has a liquid form-water.

The amount of water vapour in air is referred to as humidity and this is affected by factors such as available moisture, temperature and atmospheric pressure.  There are several different forms of humidity: absolute; specific, and relative humidity.  We are considering only relative humidity (RH).


Relative humidity (RH)

Relative humidity (RH) is the relationship between actual water density and saturated water density as follows:

RH=actual water vapour density ÷ saturation water vapour density x 100%

From this equation it is possible to calculate any value given the other two.  For example, if the temperature is 20ºC and the RH is 57.8%, then the actual water vapour density is  57.8 x 17.3/100=10g/m³


Dew point

If air is gradually cooled the relative humidity will rise to 100%, or its saturation point.  If the temperature continues to fall water is released as condensation: this is called the Dewpoint.  For a dew point calculator try www.dpcalc.org

To minimise condensation when dehumidifying with Rosahl or other devices it is best to reduce the relative humidity in the enclosure before reducing the temperature otherwise the dew point will be too high and condensation will occur.


Saturated Water Vapour density

The saturation point of air is maximum level of water that it can carry (100%), and this is dependant on the temperature of the air.  Water densities in air are usually given in g/m³: saturated air at 10 ºC will hold 9.4g of water per m³ whereas at 40ºC it will hold 51.1g (see graph)