AskDefine | Define hygrometer

Dictionary Definition

hygrometer n : measuring instrument for measuring the relative humidity of the atmosphere

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English

Noun

  1. An instrument that measures the humidity of the air or other gases, especially the relative humidity

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Extensive Definition

Hygrometers are instruments used for measuring humidity. A simple form of a hygrometer is specifically known as a "psychrometer" and consists of two thermometers, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure wet-bulb temperature. Evaporation from the wet bulb lowers the temperature, so that the wet-bulb thermometer usually shows a lower temperature than that of the dry-bulb thermometer, which measures dry-bulb temperature. When the air temperature is below freezing, however, the wet bulb is covered with a thin coating of ice and yet may be warmer than the dry bulb. Relative humidity is computed from the ambient temperature as shown by the dry-bulb thermometer and the difference in temperatures as shown by the wet-bulb and dry-bulb thermometers. Relative humidity can also be determined by locating the intersection of the wet- and dry-bulb temperatures on a psychrometric chart. One device that uses the wet/dry bulb method is the sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes.

Calibration

Accurate calibration of the thermometers used is of course fundamental to precise humidity determination by the wet-dry method; it is also important for the most accurate results to protect the thermometers from radiant heat and ensure a sufficiently high speed of airflow over the wet bulb. One of the most precise types of wet-dry bulb psychrometer was invented in the late 19th century by Adolph Richard Aßmann (1845-1918); in English-language references the device is usually spelled "Assmann psychrometer." In this device, each thermometer is suspended within a vertical tube of polished metal, and that tube is in turn suspended within a second metal tube of slightly larger diameter; these double tubes serve to isolate the thermometers from radiant heating. Air is drawn through the tubes with a fan that is driven by a clockwork mechanism to ensure a consistent speed (some modern versions use an electric fan with electronic speed control). According to Middleton, 1966, "an essential point is that air is drawn between the concentric tubes, as well as through the inner one."
One solution sometimes used for accurate humidity measurement when the air temperature is below freezing is to use a thermostatically-controlled electric heater to raise the temperature of outside air to above freezing. In this arrangement, a fan draws outside air past (1) a thermometer to measure the ambient dry-bulb temperature, (2) the heating element, (3) a second thermometer to measure the dry-bulb temperature of the heated air, then finally (4) a wet-bulb thermometer. According to the WMO Guide, "The principle of the heated psychrometer is that the water vapour content of an air mass does not change if it is heated. This property may be exploited to the advantage of the psychrometer by avoiding the need to maintain an ice bulb under freezing conditions." . Since the humidity of the ambient air is calculated indirectly from three temperature measurements, in such a device accurate thermometer calibration is even more important than for a two-bulb configuration.

References

hygrometer in Czech: Vlhkoměr
hygrometer in Danish: Hygrometer
hygrometer in German: Hygrometer
hygrometer in Modern Greek (1453-): Υγρόμετρο
hygrometer in Spanish: Higrómetro
hygrometer in French: Hygromètre
hygrometer in Indonesian: Higrometer
hygrometer in Italian: Igrometro
hygrometer in Hebrew: היגרומטר
hygrometer in Lithuanian: Psichrometras
hygrometer in Dutch: Hygrometer
hygrometer in Japanese: 湿度計
hygrometer in Norwegian: Hygrometer
hygrometer in Norwegian Nynorsk: Hygrometer
hygrometer in Polish: Wilgotnościomierz
hygrometer in Portuguese: Higrômetro
hygrometer in Russian: Психрометр
hygrometer in Slovak: Vlhkomer
hygrometer in Slovenian: Higrometer
hygrometer in Finnish: Kosteusmittari
hygrometer in Swedish: Hygrometer
hygrometer in Turkish: Higrometre
hygrometer in Ukrainian: Гігрометр
hygrometer in Chinese: 濕度計

Synonyms, Antonyms and Related Words

aerological instrument, aneroid barometer, aneroidograph, barograph, barometer, barometrograph, glass, hair hygrometer, humidor, hurricane-hunter aircraft, hygrodeik, hygrograph, hygroscope, hygrostat, hygrothermograph, psychrometer, radiosonde, recording barometer, sling psychrometer, vacuometer, weather balloon, weather instrument, weather satellite, weather vane, weatherglass
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