3. Functional description
3.1 HUMICAP technology
Vaisala HUMICAPâ 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 chemical 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.
3.2
Chemical purge
Chemical purge is a process where sensors are heated to remove possible contamination.
Purge is essential for long-term performance and accuracy of the probe. Purge takes up to
6 minutes to complete, during which time measurements are not available.
Depending on the selected sensor, the purge is automatically performed:
• At probe start-up
• At probe start-up and after every 24 hours
Disabling the automatic purge is not possible.
HUMICAPâ 180L2 sensor does not support chemical purge.
You can trigger a purge manually at any time with Vaisala Insight software or Modbus, or by
closing the external purge trigger on the component board.
Chapter 3 – Functional description
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