Relative Humidity Measurement Principle of Rotronic Devices

This article describes the measurement principle of the relative humidity measurement based on a water sensitive capacitance.

Generally, an electrical capacitance depends on

• The surface A,
• The distance between the electrodes d and
• The permittivity εof the dielectric

Water content in the dielectric influences the permittivity and by this way the capacitance as well. This effect is used to get a rate of humidity of the gas (e. g. air) surrounding the sensor.

How the sensor works

• The electrodes are permeable to water in gaseous state. Water molecules can enter into and leave out of the dielectric while the humidity of the gas surrounding changes. If the humidity of the gas becomes stable, a humidity equilibrium between the dielectric and the gas surrounding is built.
• Influence of the temperature: Beside the water content, also the temperature influences the permittivity. To compensate the temperature effect, a temperature sensor is used close to the capacitive humidity sensor.
• The capacitance and the temperature are measured. Both measurements give a rate of relative humidity.

Further influences

• There are various factors that can distort a measurement. Stress factors as dust, pollution, microorganism or spores result in a long term drift. Similar effects can be caused by the sunlight, by chemical substances as e. g. detergents or mechanical stress as icing.
• Depends on the applications, there are several solutions as special filter, housing, sealing, special cable etc.. For any question, please don’t hesitate to contact our support team (Link) or mailto Support-rh@rotronic.ch

Area of application
The following picture shows some area of application exemplarily: