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A hygrometer is an instrument used to measure the relative humidity of air, or the amount of invisible water vapor in a given environment. They range from simple devices like the psychrometer and the hair hygrometer to more complicated pieces like the cooled mirror dew point hygrometer, which uses the temperature of condensation to measure humidity. Modern electronic equipment can also determine humidity by detecting changes in electrical resistance and electrical capacitance. In addition to meteorological use, hygrometers are helpful in a range of applications, including saunas, museums, residential settings, and the HVAC industry.
A psychrometer consists of two thermometers, one with a bulb that is dry and the other with one that is wet, and uses evaporative cooling to measure humidity. As moisture evaporates from the wet bulb, it's temperature goes down, and relative humidity is computed from the difference in temperatures between the wet- and dry-bulb thermometers using a psychrometric chart. The simplest hydrometer, a sling psychrometer, uses this wet/dry bulb method with thermometers mounted on a chain or a rope that is swung in the air.
A condensation or "cooled mirror dew point" hygrometer is the most precise instrument available. Electronic feedback controls the temperature of a chilled mirror exposed to the open air, keeping a dynamic equilibrium between evaporation and condensation on the mirror. An optoelectronic mechanism detects condensation on the mirror surface and measures the dew point temperature, or the point at which air becomes so saturated it can no longer hold its water vapor and condensation occurs.
The hair hygrometer uses hair under tension to measure humidity. When relative humidity increases, hair becomes longer, and when it drops, hair becomes shorter. This instrument uses strands of human or horse hair attached to levers that magnify small changes in length. A hygrograph, consisting of an ink pen and a rotating cylinder, records humidity variations throughout the day. Hair hygrometers are not as accurate as their counterparts and register significant errors at very high and very low relative humidity.
Electronic devices measure humidity by using materials with electrical resistance that varies with the amount of moisture absorbed. The two most common are capacitive and resistive sensors. Capacitive sensors work by measuring the changes in amount of water in the air using an electrical signal between two plates. Resistive sensors use a polymer membrane, which registers changes in conductivity when water is absorbed.