Air saturation curve
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  4. Practical knowledge concerning dehumidifiers
  5. Basic knowledge regarding humidity

Basic knowledge regarding humidity

In terms of humidity relative is everything

Whether in the office, the workshop or at home – an optimum room climate is a basic prerequisite for the well-being and health of the inhabitants.

If nothing else, an agreeable room climate also promotes productivity and can help to reduce sick leave.

Two factors are decisive for these climate conditions: room temperature and relative humidity.

As illustrated in the comfort chart below, we feel most comfortable at temperatures between 20 and 22 °C and a relative humidity of 40 to 60 %. Climate conditions beyond these values are perceived as unpleasant by most people.

Moreover, too high humidity levels can cause the most varied types of damage. Often, the first visible warning signs are damp articles of clothing, a musty smell and stained walls (mould stains) or sprouting potatoes in basements.

For example, did you know that mould can already form at 70 % humidity – and rust at just 60 %?

If it is not controlled, the humidity level in a room can fluctuate significantly and rarely reaches the ideal values on its own – and it additionally also depends on the season and climatic conditions outdoors.

Comfort chart
Water vapour content of the air: At a temperature of 25 °C, one cubic metre of air can absorb max. 23 g of water; this would correspond to a humidity level of 100 %. If the air cools down to 10 °C due to contact with cold surfaces, it can only absorb 9.4 g. The excessive moisture then condenses to water on the cooler surfaces.
Water vapour content of the air
Dehumidifier equipment technology – practical knowledge from Trotec

Practical knowledge concerning dehumidifiers – summary of all chapters

Chapter 1: Basic knowledge regarding humidity – relative is everything

Chapter 2: Overview of dehumidification methods – condensation and desiccation

Chapter 2.1: Condenser dryers with compressor technology

Chapter 2.2: Condenser dryers with Peltier technology

Chapter 2.3: Desiccant dehumidifiers

Chapter 3: Which dehumidification method for which purpose?

Theory first, practice second

A basic knowledge of the subject of humidity is rather helpful for keeping your rooms ideally dry. The air cannot absorb an unlimited amount of water. There is a saturation limit, i.e. a maximum amount of water vapour that can be absorbed by the air in absolute terms. This is the absolute humidity, given in grams of water per cubic metre of air.

On that basis, the water vapour content that is actually dissolved in the air in relation to the water vapour quantity that the air could maximally absorb at the respective temperature is referred to as the “relative humidity” (RH).

This means that if the room air’s relative humidity amounts to 50 %, for instance, exactly half of the maximum amount of water possible for the current temperature is dissolved in the air.

It is just a matter of temperature

The water absorption capacity of the air therefore always depends on the prevailing temperature. The cooler the air, the less water it can absorb. This is illustrated in the following table on the basis of five temperature values.

Influence of the room temperature on the water absorption capacity of the room air

Room temperature 25 °C 20 °C 15 °C 10 °C 5 °C
Example 1
Constant relative humidity
Relative humidity 80 % 80 % 80 % 80 % 80 %
Water content of the room air 18.4 g/m³ 13.8 g/m³ 10.2 g/m³ 7.5 g/m³ 5.4 g/m³
Example 2
Constant water content
Water content of the room air 5.4 g/m³ 5.4 g/m³ 5.4 g/m³ 5.4 g/m³ 5.4 g/m³
Relative humidity 23.5 % 31.3 % 42.1 % 57.5 % 80 %

In example 1, the relative humidity remains 80 %, whereas the corresponding absolute water content in the air fluctuates considerably depending on the temperature.

In example 2, the amount of water contained in the air in absolute terms remains identical, which is why the relative humidity increases while the temperature drops.

We have to admit that this is quite a complex matter – and it becomes even more complicated in view of the fact that corrosion, rotting and mould formation are only promoted by the relative humidity, and never by the absolute water content of the air.

Only relative is absolutely relevant

So as demonstrated in example 2: at a room temperature of 5 °C and a water content of 5.4 g/m³ the relative humidity level would be 80 % – at which even metal corrodes – entailing the risk of mould formation, whereas the same water content at a room temperature of 25 °C leads to a relative humidity of only 23.5 %, which represents a much too dry room climate that irritates the respiratory system.

In this climate mould and rust would not stand a chance even though the room air still contains the same amount of water, namely 5.4 g/m³.

So all that matters is regulating the relative humidity level in a controlled manner. The absolute amount of moisture being withdrawn from the air is irrelevant; only the relative humidity level is important!

A basic understanding of these physical correlations is a crucial factor for implementing an effective dehumidification solution adapted to your application.

Influence of the relative humidity on the correlation between man and biology:

Infection of  respiratory organs
Allergies, asthma
Representations according to Scofield/Sterling diagram