Thermo Hygrometer

Temperature and humidity are among the most commonly measured values in the environment of living beings, devices, materials and raw materials. Since the relative humidity is directly related to the air temperature, many hygrometers measure not only humidity but also temperature. These devices are therefore thermometers and hygrometers and are therefore also called thermo-hygrometers. In some methods of determining humidity, the temperature measurement is even a prerequisite for determining the humidity. Device types such as the dew point mirror hygrometer and the psychrometer are practically always thermo-hygrometers. The devices that determine the humidity without temperature comparisons are often additionally equipped with sensors for temperature measurement and are thus also thermo-hygrometers.

When giving information on air humidity, a distinction must be made between relative and absolute air humidity. Most thermo-hygrometers measure the relative humidity. However, the absolute humidity can also be determined from the values for relative humidity and temperature for constant pressure conditions. The absolute humidity is given as a concrete mass or volume value for a certain amount of air. Common units for humidity in the air and other gases are grams of water per cubic metre of air or, in the case of very small values, parts per million based on the volume proportion of water molecules in the gas mixture.

Relative humidity is the ratio of the actual amount of humidity to the saturation amount in percent. The saturation amount is the maximum amount of water vapour that the air can absorb. This quantity is dependent on the prevailing temperature and pressure conditions and thus the relative humidity is also dependent on temperature and pressure. Since warm air can absorb more water vapour than cold air, the saturation quantity increases as the temperature rises. At the same time, the relative humidity decreases, although the value of the absolute humidity does not change. Here, the usual fluctuations in the air pressure have far less effect than the changes in temperature. That is why the relative humidity is almost always given together with the temperature.

Measuring range and accuracy
Many thermo-hygrometers offer the maximum possible measuring range for the relative humidity from zero percent for completely dry air to one hundred percent for air saturated with water vapour. The others have a limited measuring range, for example from ten to ninety percent. The measuring range for humidity can be divided into subranges with different measuring accuracy. There are far greater differences between devices in the measuring range for temperature than for humidity. When comparing different thermo-hygrometers, it is important to consider which accuracy is relevant for the intended application.

Conditions of use
As with other measuring devices, at attention should be paid with the thermo-hygrometer that the device is suitable for the particular conditions of use. While there are many suitable devices for climate monitoring in common living and office spaces, for the measurements outdoors and in commercial or technical processes usually the thermo-hygrometers with special equipment are required. With these, it is often not only the deviating bandwidth for the temperature that must be taken into account, but possibly also the functionality when exposed to pressure, dust or aggressive media. Special filters have therefore been developed for some sensors.

Device models for the stationary or mobile use
Stationary thermo-hygrometers are installed, for example, in warehouses, museums, recreation rooms and production rooms if the climate conditions there need to be monitored or specifically influenced. These stationary thermo-hygrometers are available for many applications, for example with alarm functions, large display, data memory, interfaces for data transmission, switching or analogue interface.

Mobile thermo-hygrometers are not only used for individual measurements at different locations. They are also used as data loggers to document temperature and humidity conditions during transport, to check the effect of the settings of heating, air-conditioning and ventilation systems or to record values on the heating and ventilation behaviour of the users of the buildings.

Additional measurement and calculation variables
In addition to temperature and humidity, many models of the thermo-hygrometers can also determine and display the respective dew point. With some data loggers, the dew point is automatically determined in the PC software after the measurement data has been transmitted. Furthermore, some devices additionally offer the indication of the wet bulb temperature. This temperature is the limit value to which the air can be cooled down by the evaporation of water under the current conditions. Special thermo-hygrometers with an additional sensor for radiant heat can also determine the heat stress index. This value is intended to record the physical stress caused by heat, humidity and radiant heat as objectively as possible so that the additional stress caused by sport or physically strenuous work can be avoided if the values are high. Other climate values can also be recorded with additional sensors.

Here is a list of the common measured or calculated values

- Dew point
- Absolute humidity
- Wet bulb temperature
- External K-type temperature sensor
- Infrared temperature measurement
- Air velocity and volume flow
- Heat stress index
- Air pressure
- CO2 content
- Light intensity

If you need the additional values more often, you should check whether it is cheaper to use several individual devices or one thermo-hygrometer with additional sensors. The combination devices are often equipped in such a way that they are particularly suitable for a specific area of application. Examples are data loggers for machine monitoring or handheld devices for occupational health and safety or ventilation measurements.

Like most other measuring instruments, the thermo-hygrometers are calibrated before delivery. However, as some sensor types tend to drift over time or can be limited in their accuracy by other factors, the instruments should be recalibrated regularly. For some models, the user himself can carry out the calibration for the humidity measurement with suitable humidity calibration standards and for the temperature with a tested precise comparison thermometer. Other instrument types must be brought or sent to a calibration laboratory for calibration. In the calibration laboratory, the thermo-hygrometers are then tested either according to the ISO standard or according to the higher requirements of the German Calibration Service (DKD).

In the calibration laboratory, the thermo-hygrometers are usually tested for various reference points in the climatic chamber. The reference values are measured by a suitable comparison device with higher accuracy. The measurement results and measurement conditions of the calibration are certified in a calibration certificate, which is handed over or sent together with the calibrated device. With the help of these certificates, the accuracy of the device at the respective test time can be proven. If the calibration certificate is lost by the user, it can be requested again from the calibration laboratory.