A Hygrometer is an instrument used for measuring the moisture content in the environmental air, or humidity. Humidity is difficult to measure accurately. Most measurement devices 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.
From calculations based on physical principles, or especially by calibration with a reference standard, these measured quantities can lead to a measurement of humidity. Modern electronic devices use temperature of condensation, changes in electrical resistance, and changes in electrical capacitance to measure humidity changes.