How do you measure the relative humidity

Measuring humidity - How to determine the humidity in your home

If you want to do something good for your health and optimize the humidity in your home, you must also know ways and means of measuring it in the first place. Nowadays, nobody has to carry out complicated experiments and dig through reference tables. Good hygrometers work reliably and do this work for you.

Why should you know the humidity in your home and how can you determine it? What do you have to consider if you want to buy and use a hygrometer? This article explains how to measure humidity.

How is humidity defined?

The humidity describes how much water there is in the air and is usually given relative to the maximum value. The numerical values ​​of the air humidity are therefore between 0% with dry air and 100% when it is completely saturated with water. A further increase beyond 100% is not possible; since the air cannot hold the excess water, it condenses and precipitates as mist or dew drops.

How humidity and temperature are related

The ability of air to absorb water depends heavily on its temperature. Cold air can hold less moisture than warm air: at a temperature of 10 ° C only about half as much water can be absorbed as at room temperature of a good 20 ° C. Not least because of this dependency, the temperature should always be measured in addition to the air humidity in order to be able to correctly assess the room climate.

Measure absolute and relative humidity

The absolute humidity indicates how much water there is in the air, usually given as g / m³ (grams of water per cubic meter of air). Typical values ​​are around 10 g / m³. Without the addition of “absolute”, however, air humidity means the relative value, based on the maximum amount of water that the air can hold at a certain temperature.

At 20 ° C, this maximum value for saturated air is, for example, 17.3 g / m³. An absolute humidity of 10 g / m³ corresponds to a relative humidity of 58% at the temperature. This value is within what is considered ideal: for a pleasant living climate, the humidity in the room air should be between 40% and 60%.

Most measurement methods are designed to measure the relative humidity in your home, as this determines the indoor climate. To determine the absolute humidity, you can calculate it from the relative humidity and the room temperature, for example with the help of a Online tools.

How can humidity be measured?

There are various ways in which you can determine the humidity in your living space. The methodologically simplest procedure does not require high-tech equipment, but in practice it is also the most cumbersome and imprecise: it is the dew point procedure, which uses the relationship between temperature and relative humidity described above. For regular humidity control, however, you should invest in a hygrometer, a device for measuring air humidity that provides more accurate values ​​than the dew point method and is less cumbersome to use.

For tinkerers: the dew point method

To measure room humidity with the dew point method, you only need a mirror or a metal plate that you can slowly cool down (e.g. place ice cubes or cool packs on the underside). You also need to be able to hold a thermometer to the surface to get an accurate reading of the temperature.

While the metal plate cools down, the air humidity rises locally: the air cools down directly above the metal surface and the relative humidity rises. The colder the metal becomes, the higher the humidity rises until it finally reaches 100% - the air is now saturated. Further cooling results in condensation on the surface: the metal plate steams up.

The moment you observe the surface fogging, the so-called dew point is reached. Since the temperature and maximum water capacity of the air are closely related, the temperature then measured is a clear indicator of the amount of water present in the air: the more humid the air was at the beginning, the earlier the dew point is reached, i.e. the higher the measured temperature, if Condensation can be observed. In order to determine the original humidity from the temperature at the dew point, you have to use a dew point curve as an aid [1-4].

For professional measurements: hygrometer

If you don't feel the urge to experiment yourself, you should get a suitable measuring device. Hygrometers take advantage of the fact that certain substances can absorb water from the air and thereby change their chemical and physical properties.

Historically, the hair hygrometer is a prominent example: the length of a hair can change by up to 2.5% when the air humidity increases from 0 to 100%. Such a change can already be transferred mechanically to a pointer and thus made visible. Even today there are hair hygrometers based on this method, but many have electrical displays. Hair hygrometers are some of the most accurate, but also one of the most expensive devices with which you can measure the humidity in the house. In addition, regular maintenance of the devices is necessary in order to be able to guarantee accurate measurement results over a longer period of time.

A high level of accuracy is not required to monitor the indoor climate, and cheaper electric hygrometers are sufficient. These consist of circuits in which the properties of individual elements are sensitive to air humidity. This can be, for example, resistors, the value of which changes with moisture, or capacitors, the capacity of which depends on the water content in the air].

Where does the hygrometer belong?

If you carry out a humidity measurement that should be representative of the room or even your apartment, you have to place the hygrometer as centrally as possible. In any case, keep enough distance from radiators and windows, as other temperatures and thus also humidity values ​​can quickly arise in their vicinity.

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