Psychrometer

Psychrometer (wet-and-dry-bulb thermometer)

A psychrometer, or a wet and dry-bulb thermometer, consists of two calibrated thermometers, one that is dry and one that is kept moist with distilled water on a sock or wick. At temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, such that the wet-bulb thermometer will be at a lower temperature than that of the dry-bulb thermometer. When the air temperature is below freezing, however, the wet-bulb must be covered with a thin coating of ice, in order to be accurate. As a result of the heat of sublimation, the wet-bulb temperature will eventually be lower than the dry bulb, although this may take many minutes of continued use of the psychrometer.