Humidity Sensor
Humidity sensors measure relative water-vapour content of air, used in HVAC, weather stations, greenhouse control, and indoor-air-quality monitors.
In a capacitive thin-film humidity sensor such as HUMICAP, a polymer layer absorbs or releases water vapour as humidity changes. This changes its dielectric properties and therefore its capacitance. The electronics measure that change and convert it to a humidity reading.
In plain terms
Five senses for your microcontroller. Each sensor is one of those senses translated into electricity.
Why designers use it
- Monitoring humidity in buildings, weather observations and industrial processes.
- Providing feedback for environmental control.
- Tracking moisture conditions during manufacturing and storage.
Best for
- Environmental monitoring
- Human-machine interfaces
- Industrial control loops
- Health and fitness wearables
Key specifications
- Humidity range: Use the exact device's specified range
- Accuracy: Check humidity and temperature conditions
- Response and recovery: Include installation, airflow and condensation
- Long-term behaviour: Check contamination, drift and calibration requirements
When not to use it
- Where a simple switch or threshold would do — sensors add complexity and cost.
- Where firmware can't handle the noise or calibration the sensor demands.
Common mistakes
- Assuming an accuracy figure applies across every temperature and humidity condition.
- Ignoring contamination, condensation and recovery behaviour.
- Using an unsuitable location or housing that prevents representative exposure to the air.
Where you will find it
- A resistive sensing element can be placed in a divider or bridge so its resistance change becomes a measurable voltage.
- A measurement chain may combine a sensor, conditioning electronics and an ADC. Each stage contributes its own limits.
A short history
Vaisala reports introducing HUMICAP at the CIMO VI congress in 1973. Its thin-film polymer sensor illustrates how changing moisture absorption can be converted into a measurable capacitance. This manufacturer account documents HUMICAP, not the specifications or invention history of every humidity-sensor family.
Good to know
- The polymer's changing dielectric properties are what the electronics measure.
- Condensation recovery can differ markedly between sensor designs.