Psychrometer (Wet-And-Dry Bulb Hygrometer)

Psychrometer (from ancient Greek ψυχρός - cold and μέτρον - measure) is a device for measuring relative air humidity, based on the difference in readings of two thermometers - dry and wet.

The operating principle of the psychrometer is based on the dependence of the evaporation temperature on the relative humidity of the air. One of the thermometers (wetted) has a reservoir wrapped in gauze moistened with water. Water evaporating from the surface of the gauze cools the thermometer reservoir. The rate of evaporation, and therefore cooling, is higher the drier the air (i.e., the lower the relative humidity).

The relative humidity of the air is determined by the difference in temperature shown by dry and wet bulb thermometers. For this purpose, special psychrometric tables or nomograms are used.

Thus, a psychrometer is a type of hygrometer - a device for measuring air humidity.