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Humidity Sensor Types

Humidity sensors measure the amount of water vapor in the atmosphere. There are three ways to measure humidity.



Absolute humidity is the percentage of water vapor per volume of air.



The dew point is the temperature at which water vapor condenses into liquid, given the current air pressure and humidity. (The oldest type of instrument used a chilled mirror to measure when condensation occurred.)



Relative humidity compares the moisture content in the air with the saturated moisture at the same temperature and pressure. In the oldest measurement devices, a human hair connected to a dial expanded and contracted with the relative humidity. These devices were calibrated to a dry atmosphere.



Today, humidity sensors work somewhat differently, in that they measure the difference in electrical conductivity or temperature between moist and dry air.
  1. 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).


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