Editing Hygrometer (section)

== Classical hygrometer ==
=== Ancient hygrometers ===
Crude hygrometers were devised and developed during the [[Shang dynasty]] in Ancient China to study weather.<ref name="Hamblyn 2010 16–17">{{Cite book |title=The Invention of Clouds: How an Amateur Meteorologist Forged the Language of the Skies |url=https://books.google.com/books?id=HbajgFGBrTEC |last=Hamblyn |first=Richard |publisher=Pan Macmillan |year=2010 |isbn=978-0-330-39195-5 |publication-date=June 4, 2010 |pages=16–17}}</ref> The Chinese used a bar of charcoal and a lump of earth: its dry weight was taken, then compared with its damp weight after being exposed in the air. The differences in weight were used to tally the humidity level.

Other techniques were applied using mass to measure humidity, such as when the air was dry, the bar of charcoal would be light, while when the air was humid, the bar of charcoal would be heavy. By hanging a lump of earth on one end of a staff and a bar of charcoal on the other end and attaching a fixed lifting string to the middle point to make the staff horizontal in dry air, an ancient hygrometer was made.<ref>{{Cite book |title=Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures |url=https://archive.org/details/encyclopaediahis00seli |url-access=limited |last=Selin |first=Helaine |publisher=Springer |year=2008 |isbn=978-1-4020-4559-2 |edition=2nd |publication-date=April 16, 2008 |page=[https://archive.org/details/encyclopaediahis00seli/page/n752 736]}}</ref><ref name="Hamblyn 2010 16–17" />

=== Metal-paper coil type ===
The metal-paper coil hygrometer is very useful for giving a dial indication of humidity changes. It appears most often in inexpensive devices, and its accuracy is limited, with variations of 10% or more. In these devices, water vapor is absorbed by a salt-impregnated paper strip attached to a metal coil, causing the coil to change shape. These changes (analogous to those in a [[Bimetallic strip|bimetallic thermometer]]) cause an indication on a dial. There is usually a metal needle on the front of the gauge that points to a scale.

=== Hair tension hygrometers ===
[[File:Whalebone hygrometer-MHS 1085-P4070321-gradient.jpg|thumb|upright=0.8|[[Jean-André Deluc|Deluc]]'s hair tension whalebone hygrometer ([[Musée d'histoire des sciences de la Ville de Genève|MHS Geneva]])]]
These devices use a human or animal hair under some tension. (Whalebone and other materials may be used in place of hair.) The hair is [[hygroscopy|hygroscopic]] (tending toward retaining moisture); its length changes with humidity, and [[indicator (distance amplifying instrument)|the length change may be magnified by a mechanism and indicated on a dial or scale]]. Swiss physicist and geologist [[Horace Bénédict de Saussure]] was the first to build such a hygrometer, in 1783. The traditional folk art device known as a [[weather house]] also works on this principle.

{{blockquote|It consists of a human hair {{convert|8|or|10|in|cm|spell=in|disp=sqbr}} long, ''b c, Fig.'' 37, fastened at one extremity to a screw, ''a'', and at the other passing over a pulley, ''c'', being strained tight by a silk thread and weight, ''d''.|John William Draper|''A Textbook on Chemistry'' (1861)}}

The pulley is connected to an index which moves over a graduated scale (e). The instrument can be made more sensitive by removing oils from the hair, such as by first soaking the hair in [[diethyl ether]].<ref>{{cite book |last=Draper |first=John William |title=A Textbook on Chemistry |publisher=Harper & Bros. |year=1861 |page=[https://archive.org/details/bub_gb_HKwS7QDh5eMC/page/n69 55] |url=https://archive.org/details/bub_gb_HKwS7QDh5eMC}}</ref>

{{anchor|Psychrometers}}

=== 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.

=== Chilled mirror dew point hygrometer ===
Dew point is the temperature at which a sample of moist air (or any other [[water vapor]]) at constant pressure reaches water vapor saturation. At this saturation temperature, further cooling results in condensation of water. Chilled mirror dewpoint hygrometers are some of the most precise instruments commonly available. They use a chilled mirror and optoelectronic mechanism to detect condensation on the mirror's surface. The temperature of the mirror is controlled by electronic feedback to maintain a dynamic equilibrium between evaporation and condensation, thus closely measuring the dew point temperature. An accuracy of 0.2&nbsp;°C is attainable with these devices, which correlates at typical office environments to a relative humidity accuracy of about ±1.2%. Older chilled-mirrors used a metallic mirror that needed cleaning and skilled labor.  Newer implementations of chilled-mirrors use highly polished surfaces that do not require routine cleaning.  

More recently, spectroscopic chilled-mirrors have been introduced. Using this method, the dew point is determined with spectroscopic light detection which ascertains the nature of the condensation. This method avoids many of the pitfalls of the previous chilled-mirrors and is capable of operating drift free.

Chilled-mirrors remain the reference measurement for calibration of other hygrometers.  This is due to their fundamental first-principle nature that refers to the core of condensation physics and measures temperature, which is one of the base quantities of the [[International System of Quantities]] (length, time, amount of substance, electric current, temperature, luminous intensity, mass).<ref>{{cite journal | url=https://www.nist.gov/pml/owm/metric-si/si-units | title=SI Units | journal=NIST | date=12 April 2010 }}</ref>