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3. Principles of Humidification
Relative and absolute humidity
How does excessively dry air come about and what affects it? Air consists of a large number of gases, one of which is hydrogen. The quantity of hydrogen in air determines its humidity. We talk about absolute humidity when describing the total quantity of water contained in one kilogram of air in the form of gas, e.g. 8 g/kg. An important factor in the air/water ratio is the air's constant endeavour to absorb water (in the form of vapour) until it is saturated. For this reason, there is another key value in assessing ambient humidity and defining an optimum humidity value: relative humidity!
Relative humidity describes the quantity of water contained in air in relation to the maximum quantity of water that can be absorbed. This "percentage saturation" of the air with water vapour is affected by air temperature: hot air can absorb more water vapour than cold air.
Humidity and temperature
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This example explains the link between temperature and relative humidity: If a room is hermetically sealed against the outside air, it has a constant absolute humidity. This means that water can neither be absorbed from the environment, nor can it be given off into that environment. If inside temperature rises due to heating, relative humidity would drop because the warmed air from the heating system would be able to absorb more water vapour than the sealed room has available. If, in the winter, windows or doors are opened for ventilation, the inside air becomes even drier than before. The higher vapour pressure of the hot, dry internal air means that the residual moisture remaining escapes outside. Relative humidity continues to fall. This explains why ambient air is often so extremely dry - especially in the winter - if air is not actively humidified.
How to counteract dry air?
1) Ventilate properly
Especially during the winter months, windows in rooms that are not centrally ventilated should be opened wide for short periods (intermittent ventilation). Otherwise, the increased thermal pressure in heated rooms causes humidity to be continuously emitted outside when windows are opened for longer periods. Windows and skylights in rooms ventilated by a central air conditioning system should remain closed at all times.
2) Do not overheat rooms
Warm air can absorb more moisture than cool air. If a room's internal temperature increases and the absolute humidity remains constant, relative humidity drops. The air becomes dry. The optimum room temperature is between 20° and 22°C.
3) Take regular measurements
It makes sense to measure relative humidity on a regular basis using a hygrometer. The recommended room humidity for living and working areas is between 40 and 60% (at a room temperature of 20°C).
4) Introduce additional moisture
If the air remains below the desired or indeed required relative humidity for a prolonged period, consideration should be given to installing air humidification systems. Nozzle systems for direct room humidification can also be retrofitted into old buildings without any problems.
5) Heed the "Optimised air humidification" certificate
The "Optimised air humidification" certificate is an aid to selecting suitable air humidification systems. Manufacturers holding this certificate have demonstrated that their systems operate hygienically and safely.
