Lowry, Brian J. Department of Chemical Engineering, University of New Brunswick, Fredericton, Canada.
- Additional Readings
In the most general sense, a hygrometer is any device used to measure humidity in air or other gases. The more modern definition is that hygrometers are only devices that measure humidity via changes in material properties. Humidity may also be measured by other methods, chiefly psychrometry and dewpoint, both of which rely on temperature changes: due to evaporation for psychrometers, and due to condensation for dew point. In contrast, hygrometers are generally isothermal. Primitive hygrometers relied on mechanical changes, such as the slackening of hair with increasing humidity. However, the vast majority of modern hygrometers are based on changes in the electrical resistance or capacitance of a material with changes in humidity. Most hygrometers are designed and calibrated to indicate relative humidity, which is the percentage of water in air (or another gas) relative to the saturation condition. When air is saturated with water, further water content appears as mist or frost on exposed surfaces. Many commercial hygrometers are combined with thermometers, sometimes in the same electrical circuit, and these are known as thermohygrometers. See also: Dew point; Humidistat; Humidity; Moisture-content measurement; Psychrometer; Psychrometrics
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