Vaisala HUMICAP® is a capacitive thin-film polymer sensor consisting of a substrate on which a thin film of polymer is deposited between two conductive electrodes. The sensing surface is coated with a porous metal electrode to protect it from contamination and exposure to condensation. The substrate is typically glass or ceramic.
The thin-film polymer either absorbs or releases water vapor as the relative humidity of the measurement environment rises or falls. The dielectric properties of the polymer film depend on the amount of absorbed water. As the relative humidity around the sensor changes, the dielectric properties of the polymer film change, and so does the capacitance of the sensor. Capacitance is converted into a humidity reading. The HUMICAP sensor is complemented by a temperature sensor, which is bonded together with the capacitive polymer on some of the sensor variants. Additional output parameters such as dew point are calculated from the readings measured by the sensors.
HUMICAP sensors with bonded temperature sensors are also known as composite sensors. Probes with composite sensors can use the sensor purge function to maintain sensor performance. They also implement condensation prevention functions that are a key element in achieving an excellent stability and good condensation resistance in high humidity applications.
The integrated sensor arrangement on the HMP1 and HMP9 probes is different from the composite sensors used in the other HMP probes. However, they are also capable of using condensation prevention heating and sensor purge.