Hygrometers measure humidity levels in the air and come in various types, including the psychrometer, hair hygrometer, and chilled mirror dew point hygrometer. Electronic devices use changes in electrical resistance and capacitance to determine humidity. Hygrometers are used in various settings, including meteorology, saunas, museums, and HVAC.
A hygrometer is an instrument used to measure the relative humidity of the air or the amount of invisible water vapor in a given environment. These range from simple devices like the psychrometer and hair hygrometer to more complicated pieces like the chilled mirror dew point hygrometer, which uses the dew point temperature to measure humidity. Modern electronic equipment can also determine humidity by sensing changes in electrical resistance and electrical capacitance. In addition to meteorological use, hygrometers are useful in a variety of applications including saunas, museums, residential settings and the HVAC industry.
A psychrometer consists of two thermometers, one with a dry bulb and the other with a wet bulb, and uses evaporative cooling to measure humidity. As moisture evaporates from the wet bulb, its temperature decreases and the relative humidity is calculated from the temperature difference between the wet bulb and dry bulb thermometers using a psychrometric graph. The simplest hydrometer, a sling psychrometer, uses this wet/dry bulb method with thermometers mounted on a chain or rope that is swung in the air.
A condensing hygrometer or “chilled mirror dew point” is the most accurate instrument available. Electronic feedback controls the temperature of a cooled mirror exposed to open air, maintaining a dynamic balance between evaporation and condensation on the mirror. An optoelectronic mechanism detects condensation on the mirror surface and measures the dew point temperature – the point at which the air becomes so saturated that it can no longer hold water vapor and condensation occurs.
The hair hygrometer uses taut hair to measure humidity. When relative humidity increases, hair lengthens, and when hair falls out, hair becomes shorter. This tool uses strands of human or horse hair attached to levers that amplify small variations in length. A hygrograph, made up of an ink pen and a rotating cylinder, records the variations in humidity 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 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 changes in the amount of water in the air using an electrical signal between two plates. Resistive sensors use a polymer membrane, which registers changes in conductivity as water is absorbed.
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