Psychrometer

A psychrometer has two built-in thermometers to determine humidity. One thermometer (the dry bulb) measures the ambient (room) temperature, and the other thermometer (the wet bulb) is wrapped in an absorbent material moistened with distilled water. To obtain a reading, air is passed through the psychrometer to evaporate moisture on the wet bulb. The rate or speed of evaporation is conditioned by the amount of moisture in the air. When the moisture has evaporated, a reading of the temperature on the wet bulb is taken, as the process of evaporation will have cooled it. The readings on the dry bulb thermometer and the wet bulb thermometer are then compared and used to determine the actual humidity.

There are several types of psychrometer, the most popular for libraries and archives being the sling version, which is the least expensive and easiest to operate. The instrument is rapidly spun for several minutes to impel air through it. Psychrometers must themselves be calibrated from time to time, and procedures must be followed to the letter in order to obtain reliable information. Misreadings can be caused by over-wetting the bulb, under-rotating the sling, exposing the instrument to strong light, and handling the instrument without gloves.

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