Relative Humidity
A humidity sensor, meter or transmitter is an instrument used for measuring the moisture content in the atmosphere. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity. Instruments use temperature of condensation (the dew point), or changes in electrical capacitance or resistance to measure humidity differences.