The right temperature and optimum humidity ensure healthy living and working. However, creating a healthy indoor climate requires permanent monitoring of the room humidity. Therefore, the use of a hygrometer or a climate meter (combination of thermometer and hygrometer) is essential to maintain a good indoor climate. In order to be able to reliably monitor your room climate, you should calibrate your measuring device regularly.

Table of Contents

What is a calibration?
When should calibration be done?
How is a hygrometer calibrated?
How is the hygrometer corrected?
High-quality processed hygrometer from FISCHER

What is a calibration?

A calibration is initially only a comparison measurement: The displayed value of a measuring device to be tested is compared with the displayed value of a reference measuring device. During the calibration itself, there is no intervention in the measuring device.

However, the comparison measurement can show that the display of the measuring device deviates so much from the reference measurement that an adjustment is necessary. It is important to note that every measurement is associated with a measurement uncertainty (tolerance). This applies to the measuring device itself, but also to the reference measuring device. As long as the deviation of the two display values ​​is within this tolerance, no adjustment of the measuring device is necessary.

If both the hygrometer to be calibrated and the reference hygrometer have a measurement accuracy of (for example) ± 3% relative humidity, deviations in the displays of up to 6% relative humidity are still within the permissible tolerance range.

The differences at different measuring points can also be of different sizes. For example, the difference could be 2% at 30% relative humidity, and 4% or 5% at 75% relative humidity. An always constant difference is possible, but not probable.

When should calibration be done?

Measurement deviations outside the permissible tolerance range can have various causes. Long-term storage of the hygrometer in relatively low humidity leads to a shift in the display. This is especially true for human hair hygrometers. However, strong vibrations during transport can also change the factory adjustment of the hygrometer to such an extent that the displayed measured value is outside the tolerance range. Furthermore, the measuring element of a hygrometer is subject to natural aging processes, which can lead to a shift in the display over time.

We therefore recommend calibrating hygrometers once or twice a year. If the hygrometer is used in the room climate, this should be done at the end of the heating period, since the hygrometer was exposed to very low humidity during this time. However, calibration is also useful if the measuring device has been transported and the displayed value does not seem plausible.

How is a hygrometer calibrated?

Before calibration, the hygrometer should be regenerated in high humidity in order to reset any display drift that may have occurred. To do this, the hygrometer is exposed to several changes in air humidity (alternation between saturation humidity and room humidity) and/or placed in a wet pack (see below) overnight.

Comparison measurement with a reference hygrometer

The relative humidity is a temperature-dependent variable. It is therefore important that both hygrometers show the same temperature. This is best achieved with an even flow of air (forced ventilation) using a fan (30 to 60 minutes) at a constant room temperature. Proximity to heating sources should be avoided, as well as other sources of radiation such as direct sunlight. It is also important to ensure that both hygrometers are in the operating position, i.e. in an upright position. Lying on the back of the case usually leads to a slight change in the position of the hands.

After the hygrometer has been adjusted to the existing temperature for a sufficient period of time (about one hour), you can carry out the comparison measurement. The temperature should also be kept as constant as possible during the calibration (temperature change <1 °C per hour). Please consider that a temperature difference of only 1 °C at room temperature (depending on humidity) creates an actual difference of 2-7% relative humidity.

In the private sector, one often does not have a reference measuring device that can be used for the comparison measurement. However, there are several pragmatic methods that can be used to create a reference value for home use. Below are the practices we recommend:

Wet pack

Place your hygrometer in a damp cloth for at least 30 minutes. A tightly closing, transparent plastic bag is also suitable, in which the hygrometer is placed together with a damp cloth. This has the advantage that the display can be observed at any time. After a dwell time of at least 30 minutes, the reading of the hygrometer should be in the range of 95-98% relative humidity. The cloth may only be opened for the reading, because the hygrometer pointer will suddenly move to a lower value as soon as the hygrometer is removed from the damp cloth. It should also be considered that the wet pack does not generate a defined moisture value, so the range of 95-98% is only a guide value. A display value in the range of 92-100% is still within the tolerance range.

A value of over 100% relative humidity (oversaturation) is physically not a stable condition and is therefore only possible for a short time at best. Nevertheless, the display of the hygrometer can also be just over 100%, because the tolerance also applies to the higher value: If the relative humidity is 99% and the measurement uncertainty is ± 3% relative humidity, a displayed value of 102% relative humidity is still within the tolerance range . A correction of the hygrometer is possible and certainly useful, but not absolutely necessary.

Saline solution

All salt is more or less hygroscopic. This means it attracts water. If salt is present in a saturated solution, a very constant "equilibrium humidity" is established above this salt solution. With an air humidity above the equilibrium humidity, water is absorbed, with an air humidity below this, water evaporates. Therefore, in a closed, temperature-constant room, the equilibrium moisture content will be established, which can be used for comparison purposes. In order to create such a "normal climate" you need pure salt, distilled water and a closed, temperature-constant vessel.

A readily available and safe salt is sodium chloride. It is available with high purity as regeneration salt for dishwashers or as table salt. It is important to ensure that as few additives as possible (iodine, flour, anti-caking agents, etc.) are included. Highly pure (>99.9%) but also quite expensive, sodium chloride is available in pharmacies.

A saturated solution is prepared from this salt with water (distilled if possible). This requires about 40-50 g of salt for 100 ml of water. Too much salt doesn't hurt as long as all the salt crystals are well wet. The accuracy that can be achieved depends on the purity of the materials used and the constancy of the temperature and can be assumed to be in the following order of magnitude:

Table salt without additives in tap waterabout 75% relative humidity ±5% rel. humidity
Regenerating salt in distilled waterabout 75% relative humidity ±2% rel. humidity
Sodium chloride >99.9% in distilled water
(at constant temperature <0.1K/h@25°C)
75.3% relative humidity <±0.5% rel. humidity

Magnesium chloride is also a salt that is well suited for comparative measurements and is harmless. It is commercially available as a dietary supplement E511, in road salt or as an additive for marine aquariums. In the required purity > 99%, however, it is only available in pharmacies. For the saturated solution you need about 60-70 g per 100 ml of water.

Magnesium chloride hexahydrate AR
in distilled water <0.1K/h@25°C)
32.8% relative humidity ±0.5% relative humidity

For the comparison measurement, the hygrometer and the saturated solution must be in the same vessel and adjusted at a constant temperature for at least 1-2 hours. The vessel should be kept as small as possible, but also allow a view of the hygrometer reading. Saline solution and hygrometer must not touch each other under any circumstances, otherwise the hygrometer can be damaged. With consecutive use of both "normal climates" (sodium chloride 75.3% relative humidity, magnesium chloride 32.8% relative humidity), it is possible to check the linearity of the hygrometer (calibration at two different measuring points). "Humidity Packs" from BOVEDA (or comparable) are also very practical for calibrating a hygrometer, which also use a saturated salt-water solution to create an equilibrium moisture content. These packs are available for different relative humidities and are placed in a tight-fitting, transparent plastic bag with the hygrometer. However, a tolerance of ±5% relative humidity must also be taken into account with these “humidity packs”.

You can use the methods described to determine whether the reading on your hygrometer is within the permissible tolerance range. If this is not the case, it is necessary to adjust the display. You should proceed as follows:

How is the hygrometer corrected?

The display is corrected by simply adjusting the pointer position (zero point adjustment). The adjusting screw is usually located in one of the lateral housing openings. These hygrometers can be easily recognized by the two screws on the dial. Adjustment screwSometimes the adjustment is also made through an opening on the back of the housing. If the supplied operating instructions do not provide you with any clarity, please contact our customer service.

Wet pack

It is very important that the hygrometer is left in the damp cloth. This may only be opened so far that access to the adjusting screw is possible. Correct the hygrometer reading to 98% relative humidity.

After the display correction, close the cloth again and wait a few more minutes. You can then check whether the display actually adjusts to 98% relative humidity. A second minor correction may be necessary. In this case, proceed as described again. If you use the plastic bag mentioned above, you can poke through the film with a screwdriver and set the required value at your leisure. A light tap will loosen the slight friction in the measuring mechanism after the adjustment and a more precise setting is possible.

Saline solution

First note the display difference. For example, if the equilibrium humidity in the vessel was 75% relative humidity and the hygrometer reading was 81% relative humidity, there is a deviation of +6%. Take the hygrometer out of the jar and let it adjust to the room climate for at least an hour. The correction then takes place in the opposite direction. In the example given, there is an adjustment of -6% relative humidity. Then repeat the calibration described above. In this way you can check whether the hygrometer display adjusts to the value above the salt solution. If necessary, a correction must be made. To do this, proceed as described here again.

High-quality processed hygrometer from FISCHER

Humidity plays a major role in our health and well-being. Therefore, the use of a hygrometer or climate meter is essential to maintain a good indoor climate. To get accurate readings, these gauges should be calibrated regularly. You can use the methods described above to do this.