Bar (unit): Difference between revisions
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{{dambigbox|text=This article is about Bar, a unit of [[pressure]]. For other usages, see [[Bar (disambiguation)]].}} | {{dambigbox|text=This article is about Bar, a unit of [[pressure]]. For other usages, see [[Bar (disambiguation)]].}} | ||
The '''[[International System of Units|bar]]''' (symbol '''bar''') is widely used in many countries as a unit of pressure. It is not an [[International System of Units|SI]] unit, nor is it a [[Centimeter gram second system of units|cgs]] unit, but it is accepted for use with SI units by [[National Institute of Standards and Technology|NIST]].<ref | The '''[[International System of Units|bar]]''' (symbol '''bar''') is widely used in many countries as a unit of pressure. It is not an [[International System of Units|SI]] unit, nor is it a [[Centimeter gram second system of units|cgs]] unit, but it is accepted for use with SI units by [[National Institute of Standards and Technology|NIST]].<ref name=NIST/> | ||
Except for the power of ten, the definition of bar fits in the sequence of SI pressure units ([[Pascal (unit)|Pa]], kPa, MPa), namely, 1 bar ≡ 100,000 Pa = 100 kPa = 0.1 MPa. This is in contrast to the well-known unit of pressure, [[atmosphere (unit)|atmosphere]], which now is ''defined'' to be 1.01325 bar exactly. As a rule of thumb, a bar is (almost) equal to an atmosphere. | Except for the power of ten, the definition of bar fits in the sequence of SI pressure units ([[Pascal (unit)|Pa]], kPa, MPa), namely, 1 bar ≡ 100,000 Pa = 100 kPa = 0.1 MPa. This is in contrast to the well-known unit of pressure, [[atmosphere (unit)|atmosphere]], which now is ''defined'' to be 1.01325 bar exactly. As a rule of thumb, a bar is (almost) equal to an atmosphere. | ||
The bar and the millibar were introduced by the British [[meteorologist]] William Napier Shaw in 1909. William Napier Shaw was the director of the [[Meteorological Office]] in [[London]] from 1907 to 1920.<ref | The bar and the millibar were introduced by the British [[meteorologist]] William Napier Shaw in 1909. William Napier Shaw was the director of the [[Meteorological Office]] in [[London]] from 1907 to 1920.<ref name=jrank/> | ||
==Definition== | ==Definition== | ||
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==Usage== | ==Usage== | ||
Although millibars are not an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars. For example, the weather office of [[Environment Canada]] uses hectopascals on their weather maps.<ref | Although millibars are not an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars. For example, the weather office of [[Environment Canada]] uses hectopascals on their weather maps.<ref name=EnvCan/> | ||
[[Atmospheric pressure|Atmospheric air pressure]] is often expressed in millibars and sea level atmospheric air pressure is defined as 1013.25 mbar which is equivalent to 1 atm. | [[Atmospheric pressure|Atmospheric air pressure]] is often expressed in millibars and sea level atmospheric air pressure is defined as 1013.25 mbar which is equivalent to 1 atm. | ||
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In the [[United States]], where pressures are still often expressed in pounds per square inch (symbol ''psi''), gauge pressures are referred to as ''psig'' and absolute pressures are referred to as ''psia''. Gauge pressure is also sometimes spelled as as ''gage pressure''. | In the [[United States]], where pressures are still often expressed in pounds per square inch (symbol ''psi''), gauge pressures are referred to as ''psig'' and absolute pressures are referred to as ''psia''. Gauge pressure is also sometimes spelled as as ''gage pressure''. | ||
Sometimes, the context in which the word pressure is used helps to identify it as meaning either the absolute or gauge pressure. However, in truth, whenever a pressure is expressed in any units (bar, Pa, psi, atm, etc.), it should be denoted in some manner as being either absolute or gauge pressure to avoid any possible misunderstanding. One recommended way of doing so is to spell out what is meant, for example as ''bar gauge'' or ''kPa absolute''.<ref | Sometimes, the context in which the word pressure is used helps to identify it as meaning either the absolute or gauge pressure. However, in truth, whenever a pressure is expressed in any units (bar, Pa, psi, atm, etc.), it should be denoted in some manner as being either absolute or gauge pressure to avoid any possible misunderstanding. One recommended way of doing so is to spell out what is meant, for example as ''bar gauge'' or ''kPa absolute''.<ref name=NPL/><ref name=Jones/> | ||
==Other pressure units== | ==Other pressure units== | ||
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==References== | ==References== | ||
{{reflist}} | {{reflist| refs= | ||
<ref name=NIST>[http://physics.nist.gov/cuu/Units/outside.html Units Outside of the SI, Table 7] (from the NIST website)</ref> | |||
<ref name=jrank>[http://encyclopedia.jrank.org/Cambridge/entries/061/Sir-William-Napier-Shaw.html Sir William Napier Shaw]</ref> | |||
<ref name=EnvCan>[http://www.weatheroffice.gc.ca/jet_stream/index_e.html Environment Canada Weather Map]</ref> | |||
<ref name=NPL>[http://www.npl.co.uk/server.php?controller=search&action=index&q=absolute%20and%20gauge%20pressure&resultsPerPage=&restrict=&restrictToCategory=&resultsPerPage=20 Search Results 1 and 2] (from the website of the National Physics Laboratory, United Kingdom)</ref> | |||
<ref name=Jones>{{cite book|author=Arnold Ivan Jones and Cornelius Wandmacher|title=Metric Units in Engineering:Going SI|edition=Revised Edition|publisher=American Society of Civil Engineers|year=2007|pages=page 147|id=ISBN 0-7844-0070-9}}</ref> | |||
}} | |||
Revision as of 14:01, 2 July 2011
The bar (symbol bar) is widely used in many countries as a unit of pressure. It is not an SI unit, nor is it a cgs unit, but it is accepted for use with SI units by NIST.[1]
Except for the power of ten, the definition of bar fits in the sequence of SI pressure units (Pa, kPa, MPa), namely, 1 bar ≡ 100,000 Pa = 100 kPa = 0.1 MPa. This is in contrast to the well-known unit of pressure, atmosphere, which now is defined to be 1.01325 bar exactly. As a rule of thumb, a bar is (almost) equal to an atmosphere.
The bar and the millibar were introduced by the British meteorologist William Napier Shaw in 1909. William Napier Shaw was the director of the Meteorological Office in London from 1907 to 1920.[2]
Definition
The bar, decibar (symbol dbar), centibar (symbol cbar) and millibar (symbol mbar or mb) are defined as:
- 1 bar = 105 Pa (Pascals) = 106 dyn/cm2 (dynes per square centimetre) = 0.987 atm (atmospheres)
- 1 dbar = 0.1 bar = 104 Pa = 10 kPa
- 1 cbar = 0.01 bar = 10 3 Pa = 1 kPa
- 1 mbar = 0.001 bar = 102 Pa = 1 hPa (hectopascal)
Usage
Although millibars are not an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars. For example, the weather office of Environment Canada uses hectopascals on their weather maps.[3]
Atmospheric air pressure is often expressed in millibars and sea level atmospheric air pressure is defined as 1013.25 mbar which is equivalent to 1 atm.
In water, there is an approximate numerical equivalence between the change in pressure in decibars and the change in depth from the sea surface in metres. Specifically, an increase of 1 decibar occurs for every 1.019716 metre increase in depth close to the surface. As a result, decibars are commonly used in oceanography.
Many engineers worldwide use the bar as a unit of pressure because, in much of their work, using pascals would involve using very large numbers.
Absolute pressure and gauge pressure
Bourdon tube pressure gauges, vehicle tire gauges and many other types of pressure gauges are zero referenced to atmospheric pressure, which means that they measure the pressure above atmospheric pressure. However, absolute pressures are zero referenced to a complete vacuum. Thus, the absolute pressure of any system is the gauge pressure of the system plus atmospheric pressure.
Absolute pressures expressed in bar are often referred to as bara, whereas gauge pressures expressed in bar are often referred to as barg.
In the United States, where pressures are still often expressed in pounds per square inch (symbol psi), gauge pressures are referred to as psig and absolute pressures are referred to as psia. Gauge pressure is also sometimes spelled as as gage pressure.
Sometimes, the context in which the word pressure is used helps to identify it as meaning either the absolute or gauge pressure. However, in truth, whenever a pressure is expressed in any units (bar, Pa, psi, atm, etc.), it should be denoted in some manner as being either absolute or gauge pressure to avoid any possible misunderstanding. One recommended way of doing so is to spell out what is meant, for example as bar gauge or kPa absolute.[4][5]
Other pressure units
| pascal (Pa) |
bar (bar) |
atmosphere (atm) |
torr (torr) |
pound-force per square inch (psi) |
kilogram-force per square centimeter (kgf/cm2) | |
|---|---|---|---|---|---|---|
| 1 Pa | ≡ 1 N/m2 | 10−5 | 9.8692×10−6 | 7.5006×10−3 | 145.04×10−6 | 1.01972×10−5 |
| 1 bar | 100,000 | ≡ 106 dyn/cm2 | 0.98692 | 750.06 | 14.504 | 1.01972 |
| 1 atm | 101,325 | 1.01325 | ≡ 1 atm | 760 | 14.696 | 1.03323 |
| 1 torr | 133.322 | 1.3332×10−3 | 1.3158×10−3 | ≡ 1 torr ≈ 1 mmHg |
19.337×10−3 | 1.35951×10−3 |
| 1 psi | 6,894.76 | 68.948×10−3 | 68.046×10−3 | 51.715 | ≡ 1 lbf/in2 | 7.03059×10−2 |
| 1 kgf/cm2 | 98,066.5 | 0.980665 | 0.967838 | 735.5576 | 14.22357 | ≡ 1 kgf/cm2 |
Example reading: 1 Pa = 1 N/m2 = 10−5 bar = 9.8692×10−6 atm = 7.5006×10−3 torr, etc.
Note: mmHg is an abbreviation for millimetre of mercury
References
- ↑ Units Outside of the SI, Table 7 (from the NIST website)
- ↑ Sir William Napier Shaw
- ↑ Environment Canada Weather Map
- ↑ Search Results 1 and 2 (from the website of the National Physics Laboratory, United Kingdom)
- ↑ Arnold Ivan Jones and Cornelius Wandmacher (2007). Metric Units in Engineering:Going SI, Revised Edition. American Society of Civil Engineers, page 147. ISBN 0-7844-0070-9.
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