Relative humidity is defined as the ratio of the partial pressure of water vapor in an air-water mixture to the equilibrium vapor pressure of water at a given temperature. It can be measured using various devices, including resistive, capacitive, crystal, thermal, gravimetric, and optical hygrometers. Resistive hygrometers measure changes in the electrical resistance of moisture-absorbing materials as humidity varies. Capacitive hygrometers detect changes in capacitance of a polymer film. Crystal hygrometers measure changes in mass of a hygroscopic crystal as it absorbs water from the air.