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Understanding humidity and Rosahl

Rosahl electric micro-dehumidifier offer a new way of protecting equipment from moisture damage. They are long lasting low energy devices. This article deals with understanding humidity and dew point, how they affect the use and selection of Rosahl dehumidifiers, and importantly how to avoid unexpected problems in use



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. Humidity refers to the amount of water vapour in air.

Factors that affect humidity include available moisture, temperature and atmospheric pressure. Many of these factors are changing constantly. This is important when selecting dehumidifier capacity for a specific application.

Interestingly, there are several different forms of humidity: absolute; specific, and relative humidity.

For this comparison, 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%

Coupled with 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

Under normal conditions, air contains water in the form of water vapour (relative humidity). The density of water vapour changes with temperature. Firstly, this means the higher the temperature, the more water vapour it can hold. Conversely, cooling the air reduces the water vapour it can hold. The result of continued cooling increases its relative humidity until the water vapour density reaches its saturation point (100%). Consequently, at saturation point, the water vapour condenses into liquid water, usually on a cool surface. This is the Dewpoint.

For more information, there is a helpful dew point calculator 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.

See more information here

Saturated Water Vapour density

The saturation point of air is maximum level of water that it can carry (100%), importantly, this is dependent 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. This means at 40ºC it will hold 51.1g (see graph)







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