Editing Pressure (section)

===Units===
[[File:Barometer mercury column hg.jpg|thumb|right|Mercury column]]
The [[SI]] unit for pressure is the [[Pascal (unit)|pascal]] (Pa), equal to one [[newton (unit)|newton]] per [[square metre]] (N/m<sup>2</sup>, or kg·m<sup>−1</sup>·s<sup>−2</sup>). This name for the unit was added in 1971;<ref>{{cite web |url=http://www.bipm.fr/en/convention/cgpm/14/pascal-siemens.html |title=14th Conference of the International Bureau of Weights and Measures |publisher=Bipm.fr |access-date=2012-03-27 |url-status=dead |archive-url=https://web.archive.org/web/20070630020548/http://www.bipm.fr/en/convention/cgpm/14/pascal-siemens.html |archive-date=2007-06-30 }}</ref> before that, pressure in SI was expressed in newtons per square metre.

Other units of pressure, such as [[pound-force per square inch|pounds per square inch]] (lbf/in<sup>2</sup>) and [[bar (unit)|bar]], are also in common use. The [[Centimetre–gram–second system of units|CGS]] unit of pressure is the [[barye]] (Ba), equal to 1&nbsp;dyn·cm<sup>−2</sup>, or 0.1&nbsp;Pa. Pressure is sometimes expressed in grams-force or kilograms-force per square centimetre ("g/cm<sup>2</sup>" or "kg/cm<sup>2</sup>"<!--don't add an f to kg, this is making the point about usage without it-->) and the like without properly identifying the force units. But using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as units of force is deprecated in SI. The [[technical atmosphere]] (symbol: at) is 1&nbsp;kgf/cm<sup>2</sup> (98.0665&nbsp;kPa, or 14.223&nbsp;psi).

Pressure is related to [[energy density]] and may be expressed in units such as [[joule]]s per cubic metre (J/m<sup>3</sup>, which is equal to Pa).
Mathematically:
<math display="block">p = \frac{F \cdot \text{distance}}{A \cdot \text{distance}} = \frac{\text{work}}{\text{volume}} = \frac{\text{energy (J)}}{\text{volume }(\text{m}^3)}.</math>

Some [[meteorologist]]s prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit [[millibar]] (mbar). Similar pressures are given in kilopascals (kPa) in most other fields, except aviation where the hecto- prefix is commonly used. The inch of mercury is still used in the United States. Oceanographers usually measure underwater pressure in [[decibar]]s (dbar) because pressure in the ocean increases by approximately one decibar per metre depth.

The [[Atmosphere (unit)|standard atmosphere]] (atm) is an established constant. It is approximately equal to typical air pressure at Earth [[mean sea level]] and is defined as {{val|101325|u=Pa}} (IUPAC recommends the value {{val|100000|u=Pa}}, but prior to 1982 the value {{val|101325|u=Pa}} (= 1 atm) was usually used).<ref>{{cite web |title=IUPAC Quantities, Units and Symbols in Physical Chemistry |url=https://goldbook.iupac.org/terms/view/S05921 |website=IUPAC Gold Book |access-date=29 January 2025}}</ref>

Because pressure is commonly measured by its ability to displace a column of liquid in a [[manometer]], pressures are often expressed as a depth of a particular fluid (e.g., [[centimetres of water]], [[millimetres of mercury]] or [[inches of mercury]]). The most common choices are [[Mercury (element)|mercury]] (Hg) and water; water is nontoxic and readily available, while mercury's high density allows a shorter column (and so a smaller manometer) to be used to measure a given pressure. The pressure exerted by a column of liquid of height ''h'' and density ''ρ'' is given by the hydrostatic pressure equation {{nowrap|1=''p'' = ''ρgh''}}, where ''g'' is the [[gravitational acceleration]]. Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely.

When millimetres of mercury (or inches of mercury) are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units.<ref>{{SIbrochure8th|page=127}}</ref> One millimetre of mercury is approximately equal to one [[torr]]. The water-based units still depend on the density of water, a measured, rather than defined, quantity. These ''manometric units'' are still encountered in many fields. [[Blood pressure]] is measured in millimetres (or centimetres) of mercury in most of the world, and lung pressures in centimetres of water are still common.{{citation needed|date=October 2021}}

[[Underwater diving|Underwater divers]] use the [[metre sea water]] (msw or MSW) and [[foot sea water]] (fsw or FSW) units of pressure, and these are the units for pressure gauges used to measure pressure exposure in [[diving chamber]]s and [[Dive computer|personal decompression computers]]. A msw is defined as 0.1&nbsp;bar (= 10,000&nbsp;Pa), is not the same as a linear metre of depth. 33.066&nbsp;fsw = 1&nbsp;atm{{citation needed|date=February 2023}} (1&nbsp;atm = 101,325&nbsp;Pa / 33.066 = 3,064.326 Pa).  The pressure conversion from msw to fsw is different from the length conversion: 10&nbsp;msw = 32.6336&nbsp;fsw, while 10&nbsp;m = 32.8083&nbsp;ft.{{citation needed|date=February 2023}}

Gauge<!--Editors are asked to PLEASE check the discussion page for this article before making changes regarding "gauge" vs. "gage" spelling issues. Much debate has transpired on this issue.--> pressure is often given in units with "g" appended, e.g. "kPag", "barg" or "psig", and units for measurements of absolute pressure are sometimes given a suffix of "a", to avoid confusion, for example "kPaa", "psia". However, the US [[National Institute of Standards and Technology]] recommends that, to avoid confusion, any modifiers be instead applied to the quantity being measured rather than the unit of measure.<ref name = pubs>{{cite journal |access-date= 2009-07-07 |journal=NIST |url=http://physics.nist.gov/Pubs/SP811/sec07.html#7.4 |title=Rules and Style Conventions for Expressing Values of Quantities |date=2 July 2009 |url-status=live |archive-url=https://web.archive.org/web/20090710192735/http://physics.nist.gov/Pubs/SP811/sec07.html#7.4 |archive-date=2009-07-10 }}</ref> For example, {{nowrap|1="''p''<sub>g</sub> = 100 psi"}} rather than {{nowrap|1="''p'' = 100 psig"}}.

Differential pressure is expressed in units with "d" appended; this type of measurement is useful when considering sealing performance or whether a valve will open or close.

Presently or formerly popular pressure units include the following:
*[[atmosphere (unit)|atmosphere]] (atm)
*manometric units:
**centimetre, inch, millimetre (torr) and micrometre (mTorr, micron) of mercury,
**{{anchor|H2O}}height of equivalent column of water, including [[Millimeters, water gauge|millimetre]] (mm {{chem|H|2|O}}), [[centimetre of water|centimetre]] (cm {{chem|H|2|O}}), metre, [[inch of water|inch]], and foot of water;
*imperial and customary units:
**[[kip (unit)|kip]], [[Ton-force#Short ton-force|short ton-force]], [[Ton-force#Long ton-force|long ton-force]], [[pound-force]], [[ounce-force]], and [[poundal]] per square inch,
**short ton-force and long ton-force per square inch,
**fsw (feet sea water) used in underwater diving, particularly in connection with diving pressure exposure and [[Decompression (diving)|decompression]];
*non-SI metric units:
**[[bar (unit)|bar]], decibar, [[millibar]],
***msw (metres sea water), used in underwater diving, particularly in connection with diving pressure exposure and [[Decompression (diving)|decompression]],
**kilogram-force, or kilopond, per square centimetre ([[technical atmosphere]]),
**gram-force and tonne-force (metric ton-force) per square centimetre,
**[[barye]] ([[dyne]] per square centimetre),
**kilogram-force and tonne-force per square metre,
**[[sthene]] per square metre ([[pieze]]).
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[[File:Pressure units.png|alt=A vizualization of pressure units as circles in log scale. A circle is to imagine a bubble, that such pressure would occupy.|thumb|Pressure units as circles in log scale. 4 graphs show scaled P units to unitary mTorr, Pa, mbar, bar. A circle is to imagine a bubble, that such pressure would occupy.]]
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