Editing Hygrometer (section)

=== Psychrometer (wet-and-dry-bulb thermometer) ===
[[File:Stevenson screen interior.JPG|thumb|The interior of a [[Stevenson screen]] showing a motorized psychrometer]]
A psychrometer, or a wet and dry-bulb thermometer, consists of two calibrated thermometers, one that is dry and one that is kept moist with distilled water on a sock or wick.<ref name="Gorse Johnston Pritchard 2012 p. 960">{{cite book |last1=Gorse |first1=C. |last2=Johnston |first2=D. |last3=Pritchard |first3=M. |title=A Dictionary of Construction, Surveying, and Civil Engineering |publisher=OUP Oxford |series=Oxford Quick Reference |year=2012 |isbn=978-0-19-104494-6 |url=https://books.google.com/books?id=Z6tGBAAAQBAJ&pg=PT960 |access-date=13 September 2018 |page=960}}</ref> At temperatures above the freezing point of water, [[evaporation]] of water from the wick lowers the [[temperature]], such that the wet-bulb thermometer will be at a lower temperature than that of the dry-bulb thermometer. When the air temperature is below freezing, however, the wet-bulb must be covered with a thin coating of ice, in order to be accurate. As a result of the heat of sublimation, the wet-bulb temperature will eventually be lower than the dry bulb, although this may take many minutes of continued use of the psychrometer.
[[File:Daniell hygrometer-MHS 2191-P4070317-gradient.jpg|thumb|left|upright=0.8|Psychrometer probably made in Switzerland circa 1850 by Kappeller ([[Musée d'histoire des sciences de la Ville de Genève|MHS Geneva]])]]

[[Relative humidity|Relative humidity (RH)]] is computed from the ambient temperature, 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 [[Psychrometrics|psychrometric chart]]. The dry and wet thermometers coincide when the air is fully saturated, and the greater the difference the drier the air.  Psychrometers are commonly used in [[meteorology]], and in the [[heating, ventilation, and air conditioning]] (HVAC) industry for proper [[refrigerant]] charging of residential and commercial air conditioning systems.

==== Sling psychrometer ====
[[File:Sling psychrometer.JPG|thumb|A sling psychrometer for outdoor use]]
A sling psychrometer, which uses thermometers attached to a handle, is manually spun in free air flow until both temperatures stabilize. This is sometimes used for field measurements but is being replaced by more convenient electronic sensors. A whirling psychrometer uses the same principle, but the two thermometers are fitted into a device that resembles a [[Socket wrench|ratchet]] or football rattle.