Capacitive Sensors
Sensors that measure relative humidity comprise a thin film of polymer or metal oxide, deposited on a glass or ceramic substrate between two electrodes. The relative humidity is proportional to changes in the capacitance of this film, measured as the dielectric constant.
These sensors can also calculate the dew point. They typically take between 30 and 60 seconds to produce a measurement, and operate at temperatures up to 392 degrees Fahrenheit.
Today, the same methods used in producing semiconductors are used to produce capacitive sensors.
Resistive Sensors
When a substance gets wet, its electrical resistance changes. This is the concept behind resistive humidity sensors, which measure the change in impedance of a specific medium---such as a salt or conductive polymer---in response to changes in humidity.
Resistive sensors comprise noble metals (gold, silver, platinum or palladium) wound around a plastic or ceramic tube, or deposited on a substrate using photoresistance. The sensors respond in 10 to 30 seconds, and can operate at temperatures up to 212 degrees Fahrenheit. The main disadvantage of resistive sensors is that they take some time to recover when they get wet.
Thermal Conductivity Sensors
Thermal conductivity sensors are based on the fact that a dry mix of gases conducts heat more efficiently than gas that is saturated with water vapor. These sensors measure the difference in radiated heat between a resistor that is exposed to the atmosphere, and one that is encapsulated in a controlled environment.
This kind of sensor reacts quickly, and works at much higher temperatures than other types of sensors (i.e., up to 575 degrees Fahrenheit).