Editing Pressure measurement (section)

==Units==
{{Pressure Units}}

[[File:Psidial.jpg|thumb|A [[pressure gauge]] reading in [[Pounds per square inch|psi]] (red scale) and [[kPa]] (black scale)]]

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 special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N·m<sup>−2</sup>. When indicated, the zero reference is stated in parentheses following the unit, for example 101 kPa (abs). The [[pounds per square inch|pound per square inch]] (psi) is still in widespread use in the US and Canada, for measuring, for instance, tire pressure. A letter is often appended to the psi unit to indicate the measurement's zero reference; psia for absolute, psig for gauge, psid for differential, although this practice is discouraged by the [[NIST]].<ref>[http://physics.nist.gov/Pubs/SP811/sec07.html#7.4 NIST]</ref>

Because pressure was once 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.,'' inches of water). Manometric measurement is the subject of [[pressure head]] calculations. The most common choices for a manometer's fluid are [[Mercury (element)|mercury]] (Hg) and water; water is nontoxic and readily available, while mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure. The abbreviation "W.C." or the words "water column" are often printed on gauges and measurements that use water for the manometer.
{{see also|Mercury pressure gauge}}

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. So measurements in "[[millimetres of mercury]]" or "[[inHg|inches of mercury]]" can be converted to SI units as long as attention is paid to the local factors of fluid density and [[Gravity of Earth|gravity]]. Temperature fluctuations change the value of fluid density, while location can affect gravity.

Although no longer preferred, these '''manometric units''' are still encountered in many fields. [[Blood pressure]] is measured in millimetres of mercury (see [[torr]]) in most of the world, [[central venous pressure]] and lung pressures in [[centimetre of water|centimeters of water]] are still common, as in settings for CPAP machines. Natural gas pipeline pressures are measured in [[inch of water|inches of water]], expressed as "inches W.C."

[[Underwater diving|Underwater divers]] use manometric units: the ambient pressure is measured in units of [[Metre sea water|metres sea water]] (msw) which is defined as equal to one tenth of a bar.{{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑10. Pressure Equivalents.}}<ref name="IMCA D022" /> The unit used in the US is the [[foot sea water]] (fsw), based on [[standard gravity]] and a sea-water density of 64&nbsp;lb/ft<sup>3</sup>. According to the US Navy Diving Manual, one fsw equals 0.30643&nbsp;msw, {{val|.030643|ul=bar}}, or {{val|0.44444|ul=psi}},{{sfn|''US Navy Diving Manual''|2016|loc=Table 2‑10. Pressure Equivalents.}}<ref name="IMCA D022" /> though elsewhere it states that 33&nbsp;fsw is {{val|14.7|u=psi}} (one atmosphere), which gives one fsw equal to about 0.445&nbsp;psi.<ref>Page 2-12.</ref> The msw and fsw are the conventional units for measurement of [[Underwater diving|diver]] pressure exposure used in [[decompression tables]] and the unit of calibration for [[pneumofathometer]]s and [[hyperbaric chamber]] [[pressure gauge]]s.{{sfn|''US Navy Diving Manual''|2016|loc=Section 18‑2.8.3}} Both msw and fsw are measured relative to normal atmospheric pressure.

In vacuum systems, the units [[torr]] (millimeter of mercury), [[Millimetre of mercury|micron]] (micrometer of mercury),<ref>{{Cite web|url=http://vacaero.com/information-resources/vacuum-pump-practice-with-howard-tring/1290-understanding-vacuum-measurement-units.html|title = Understanding Vacuum Measurement Units|date = 9 February 2013}}</ref> and inch of mercury ([[inHg]]) are most commonly used. Torr and micron usually indicates an absolute pressure, while inHg usually indicates a gauge pressure.

Atmospheric pressures are usually stated using hectopascal (hPa), kilopascal (kPa), millibar (mbar) or atmospheres ([[atmosphere (unit)|atm]]). In American and Canadian engineering, [[Stress (physics)|stress]] is often measured in [[kip (unit)|kip]]. Stress is not a true pressure since it is not [[scalar (physics)|scalar]]. In the [[cgs]] system the unit of pressure was the [[barye]] (ba), equal to 1 dyn·cm<sup>−2</sup>. In the [[Metre-tonne-second system of units|mts]] system, the unit of  pressure was the [[pieze]], equal to 1 [[sthene]] per square metre.

Many other hybrid units are used such as mmHg/cm<sup>2</sup> or grams-force/cm<sup>2</sup> (sometimes as [[Kilogram-force per square centimetre|kg/cm<sup>2</sup>]] without properly identifying the force units). Using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is prohibited in SI; the unit of force in SI is the newton (N).