How does a dehumidifier relative humidity sensor work?

Question

We've just acquired a dehumidifier. It gives you a digital display of the relative humidity of the air in the house. How does the electronic sensor work? Last time I worked as a weather forecaster, back in the 1980s, the only instrument I ever saw which gave you a direct readout of relative humidity, without the use of tables, used an inaccurate, fault-prone mechanical device which had a human hair in it. The length of the hair increased with increasing humidity.

Answer 1

There are chemical RH sensors used in radiosondes - HumiCap by Vaisala.

Basically, the capacitance of these things is related to the RH.

Go on - google HUMICAP

Answer 2

There are two types of sense elements that are being used in solid state humidity sensors, resistive and capacitive.  Each has advantages and disadvantages in terms of performance and circuit design.

In either case there is a moisture sensitive component that changes its electrical properties in the presense of moisture.

Electronic circuits make use of these well defined and predictable properties to convert a variable resistance or capacitance into a proportional voltage output.   For a given RH a simple transfrer function  such as   Voltage = (0.04 * RH in %) + .5    where voltage output would be 0.5V @ 0%RH and 4.5V @ 100%RH

Once  RH is converted to a voltage is easy to use microcontrollers  and ADC (Analog Digital Converters) to measure that voltage and mathematically compute RH and display is or take some action (like start/stop a dehumidifier)