How to measure humidity?

Air humidity is an important measurement variable for a pleasant climate in rooms. If the air is too humid, there is the risk of mold. But the room climate is not only of great importance in offices and living rooms. Humidity is also controlled in industry to ensure quality criteria for products and installations.

Various factors influence the humidity measurements. This part of our “Measuring Know-How” series tells you about these factors and everything to look out for when using hygrometers.

Physical basis of humidity as a measured variable

Atmospheric air always contains a certain amount of water vapor. This content varies over time and location and is referred to as air humidity. Like any other substance, air has a limited capacity to absorb water. This limit is called saturation. Below saturation, the difference of humid air and dry air cannot be seen with the naked eye. Above the saturation limit, excess water becomes visible in the form of fine water droplets (condensate): fog forms.

The moisture absorption capacity

The moisture absorption capacity of the air depends on the air temperature. The warmer the air, the more water it can absorb.

Imagine a sponge whose absorption capacity varies with temperature. At 0 ° C, the sponge can absorb 4.8 g of water (corresponds to a humidity of 100 %). If there is more than 4.8g of water, the sponge will drip. If the sponge has absorbed 2.4 g of water, this corresponds to a relative humidity of 50 % at 0 ° C.

At 20 ° C, the sponge can absorb 17.3 g of water (so at 17.3 g of water, the sponge has a relative humidity of 100 %). If the sponge has therefore absorbed 2.4 g of water at 20 ° C, this corresponds to a relative humidity of approx. 14 %.

To note:

If the temperature drops, the relative humidity increases (for the same water content)!

The characteristic quantities of humidity

• Absolute air humidity

The absolute humidity of the air (habs) is calculated by dividing the mass of water contained in the air by the volume occupied by the quantity of air concerned. Absolute humidity is generally expressed in g / m³.

• The relative humidity of the air

Relative air humidity describes the relationship between the actual absolute humidity of the air at the given time and the maximum possible absolute humidity. Relative humidity is expressed in percent.

Max humidity air and dew point temperature

• Maximum air humidity

Corresponds to the maximum possible absolute humidity (hmax) at a determined temperature. It is reached when the partial pressure of water vapor in the air reaches the saturated water vapor pressure at the respective temperature. In this state, the relative humidity of the air corresponds to 100 %.

If the maximum humidity is exceeded, the excess water vapor collects in the form of condensate (formation of droplets). Saturating humidity is absolute humidity and is expressed in g / m³.

• Dew point temperature

The dew point temperature is defined as the temperature at which the current water vapor content of the air is at the maximum (100 % relative humidity). That is, the current water vapor pressure corresponds to the saturated vapor pressure. The dew point temperature is therefore a variable independent of the current temperature.

The dew point temperature is determined from the ambient temperature and the relative humidity of the air or the relative humidity of the air is determined from the ambient temperature and the dew point temperature . In addition, the absolute humidity of the air can be calculated with this quantity.

• Info

In nature, water vapor which condenses is deposited on the surfaces of solid bodies. Man feels most comfortable in a range of about 30 % to 65 % relative humidity for light to medium activities.

Devices for measuring humidity

Humidity measurements are important in different areas. Thermo-hygrometers, for example, allow

• guarantee a healthy room climate in the offices
• determine the humidity of wood or masonry
• monitor air humidity in server rooms
• carry out control measures in the ventilation ducts.

Labomat offers you the right device for these and many other applications in the field of humidity measurement.

How to avoid measurement errors

Like temperature, humidity is preferably measured in the middle of the room. Moisture in the corners of the room, behind cabinets etc. can be determined in a complementary manner and this should be noted separately in the report.

Just opening a door or staying in the room for a long time changes the humidity in the room. That is why you have to enter the room quickly, close the doors and take the measurements without delay.

With humidity probes, the humidity in the air must first "diffuse" into the plastic layer of the probe. The length of the response time depends to a large extent on whether the sensor “bathes” well in the humidity of the air or not. It shortens if the probe is moved in air.

The influence of temperature

• A typical situation

The measuring case is transported in winter at an ambient temperature of 0 ° C. At the measuring point, the measuring device arrives in a room at 20 ° C and 50 %HR.

• What's going on ?

The humidity sensor fogs up: In the “microclimate” of the sensor at 0 ° C, the air saturation is max. 4.8 g / m3. But the air in the room has a humidity of approx. 9 g / m3.

• To measure correctly

Move the probe at 1.5 m / s with light wrist movements. Thus, the "microclimate" which surrounds the probe (0 ° C) is "pierced" and the actual ambient climate of 20 ° C and 50 %HR enters the sensor.

The influence of the person taking the measurements

• A typical situation

When adjusting the measuring device, the probe is in contact with the breathing air.

• What's going on ?

By rejecting the inhaled air saturated with humidity on the probe, the relative humidity (%HR) and therefore the temperature increases in the measuring area of the measuring device.

• To measure correctly

Hold the hygrometer as far away from your mouth as possible, arm straight, and move it approx. 1.5 m / s by moving the wrist.

The right place to measure

• A typical situation

A moisture content of 50 % relative humidity is determined in the middle of the room at 25 ° C. In the same room, the meter displays a relative humidity of 90 % near the wall at 15 ° C. Which value is correct?

• To measure correctly

Like temperature, humidity is preferably measured in the middle of the room at a height of 1.1 m. If you do not move the probe or hardly move it, you should expect a response time of ten minutes.

Avoiding measurement errors: all tips at a glance

• Measure in the middle of the room.
• Measure at chest height.
• Hide powerful radiation sources from the body.
• Hold the measuring device as close as possible to your body, arm straight.
• Move the humidity probe in the air by moving your wrist about 2 times per second.