Hygrometer

Hygrometer, device to measure humidity (the amount of water vapor the air holds). Usually, hygrometers measure relative humidity, the amount of moisture as a percentage of the saturation level at that temperature. In the hair hygrometer, which is of limited accuracy, the length of a hair (usually human) increases with increase in relative humidity. This length change is amplified by a lever and registered by a needle on a dial. The wet- and dry-bulb hygrometer (psychrometer) has 2 thermometers mounted side by side, the bulb of one covered by a damp cloth. Air is moved across the apparatus (e.g., by a fan) and evaporation of water from the cloth draws latent heat from the bulb. Comparison of the 2 temperatures, and the use of tables, gives the relative humidity. In the dewpoint hygrometer, a polished container is cooled until the dew point is reached, giving a measure of relative humidity. The electric hygrometer measures changes in the electrical resistance of a hygroscopic (water-absorbing) strip.

See also: Humidity.