Whether at the workplace, in residential buildings or in production processes: Air humidity is measured and controlled everywhere. But why is the measurement of relative humidity so important? In relation to humans, humidity is an important factor influencing their comfort. In building technology, comfort defines a climatic condition range in which room users feel comfortable. Depending on the temperature, the comfortable humidity range is between 40%rh and 60%rh. Since people subjectively perceive the climatic conditions differently, an objective measurement of this variable is necessary in order to control the room air conditioning systems according to requirements. Another important reason for measuring relative humidity is that materials behave differently under different humidity conditions. Depending on the application, an individual ambient humidity is required, e.g. in clean rooms, greenhouses or production halls.

Relative humidity is measured, for example, in ventilation technology, clean room technology or in industrial applications. In the field of ventilation technology, humidity measurements are used to control room humidity, whereby the measured values can be recorded in the duct, for example for supply air, or in rooms, such as industrial halls or office buildings. For clean rooms, laboratories or operating theaters, certain humidity ranges are defined that must be permanently maintained to ensure process reliability. To record the relative humidity, measuring systems are used that monitor compliance with the critical range and transmit the data to higher-level systems. Thus, the measurement data can be logged or alarms can be triggered when limit values are exceeded or not reached.

FSM humidity transmitters are based on a capacitive measuring principle. The sensor element consists of a capacitor whose dielectric is made of a polymer. This absorbs or releases water in proportion to the relative ambient humidity. The resulting change in the dielectric constant alters the capacitance of the capacitor, which can be measured with an electronic circuit. The temperature measurement is made possible by bipolar transistors.