How does a humidity sensor work?

[Ichinisan]

How does it actually sense humidity levels? Using what principle?

A unit like this one: https://www.amazon.com/gp/product/B0013BKDO8/

 

[Paperdoc]

Found this white paper from Silicon Labs, who manufacture one type of Relative Humidity sensor.

http://www.silabs.com/Support Documents/TechnicalDocs/cmos-solution-for-humidity-sensing.pdf

VERY briefly, such devices depend on the fact that certain materials change some of their properties in a predictable fashion depending on the Relative Humidity of the air that surrounds them. For example, the older RH gauges with round dials often used some type of natural hair, which stretches longer in higher Humidity. Today's electronic units usually measure either the resistance or the capacitance of a small piece of selected material, plus the temperature, and use electronic circuits to calculate the Relative Humidity and then transmit the result out to a display device. The Silicon Labs products described in the paper measure the capacitance of a special thin polyimide film. The paper describes many of the requirements to build the device, protect the sensor and maintain calibration of the system.

 

[sm625]

http://www.sensorsmag.com/sensors/h...midity-sensor-a-review-three-technologies-840

 

[cholley]

conductivity of the air would make most sense to me, some clothing dryers with auto shutoff work in that fashion

 

[Billb2]

Some of the most accurate ones in the past were made from horse hair.

 

[Rubycon]

Capacitive sensors have been used for years.
The cheap ones don't have long term repeatable accuracy particularly if they have been exposed to ambients at their operating/non operating extremes. Aneroid devices typically used horse hair and were pretty accurate! The reference is a sling pyschrometer. Anyone in the (HVAC) trade should be familiar with its proper use as well as the psychometric chart!