psychrometer [key], one of many instruments used for measuring the water vapor content or relative humidity of the atmosphere. It consists of two identical thermometers—the wet-bulb thermometer, so called because its bulb is covered with a jacket of tight-fitting muslin cloth that can be saturated with distilled water; and the dry-bulb thermometer. When the cloth is soaked and the thermometers are properly ventilated, the wet-bulb temperature will be lower than the dry-bulb temperature (actual air temperature) because of cooling due to the evaporation of water from the cloth. The drier the air is, the greater the evaporation and thus the more the wet-bulb temperature is depressed. Psychrometric tables list various humidity variables, such as relative humidity, according to dry-bulb temperature and wet-bulb depression at equilibrium. Ventilation is provided by whirling the thermometers at the end of a chain (sling psychrometer) or by a suction fan (aspiration psychrometer). Newer psychrometers use special electronic sensors. See hygrometer.
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