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Bar (unit)

January 04, 2023

For other uses, see Bar (disambiguation).

The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea level (approximately 1.013 bar).[1][2] By the barometric formula, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.

bar
A pressure of 700 bar flattened this length of aluminium tubing, which had a wall thickness of 5 millimetres (0.20 in).
General information
Unit systemMetric system
Unit ofpressure
Symbolbar
Conversions
1 bar in ...... is equal to ...
   SI units   100 kPa
   CGS units   106 Ba
   US customary units   14.50377 psi
   Atmospheres   0.986923 atm

The bar and the millibar were introduced by the Norwegian meteorologist Vilhelm Bjerknes, who was a founder of the modern practice of weather forecasting.[3]

The International System of Units, despite previously mentioning the bar, now omits any mention of it.[1] The bar has been legally recognised in countries of the European Union since 2004.[2] The US National Institute of Standards and Technology (NIST) deprecates its use except for "limited use in meteorology" and lists it as one of several units that "must not be introduced in fields where they are not presently used".[4] The International Astronomical Union (IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated".[5]

Units derived from the bar include the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar).

Definition and conversion

The bar is defined using the SI derived unit, pascal: 1 bar ≡ 100,000 Pa ≡ 100,000 N/m2.

Thus, 1 bar is equal to:

and 1 bar is approximately equal to:

  • 0.986923 atm
  • 14.50377 psi
  • 29.5300 inHg
  • 750.062 mmHg
  • 750.062 Torr
  • 1019.716 centimetres of water (cmH2O) (1 bar approximately corresponds to the pressure of water at a depth of 10 meters).

Notes:

  • 1 millibar (mbar) = 1 one-thousandth bar, or 1×10−3 bar
  • 1 millibar = 1 hectopascal (1 hPa = 100 Pa).

Origin

The word bar has its origin in the Ancient Greek word βάρος (baros), meaning weight. The unit's official symbol is bar; the earlier symbol b is now deprecated and conflicts with the use of b denoting the unit barn, but it is still encountered, especially as mb (rather than the proper mbar) to denote the millibar. Between 1793 and 1795, the word bar was used for a unit of weight in an early version of the metric system.[6]

Usage

 
Map showing atmospheric pressure in mbar or hPa
 
A tire-pressure gauge displaying bar (outside) and pounds per square inch (inside)

Atmospheric air pressure where standard atmospheric pressure is defined as 1013.25 mbar, 101.325 kPa, 1.01325 bar, which is about 14.7 pounds per square inch. Despite the millibar not being an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars as the values are convenient. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars; for the same reason, the hectopascal is now the standard unit used to express barometric pressures in aviation in most countries. For example, the weather office of Environment Canada uses kilopascals and hectopascals on their weather maps.[7][8] In contrast, Americans are familiar with the use of the millibar in US reports of hurricanes and other cyclonic storms.[9][10]

In fresh water, there is an approximate numerical equivalence between the change in pressure in decibars and the change in depth from the water surface in metres. Specifically, an increase of 1 decibar occurs for every 1.019716 m increase in depth. In sea water with respect to the gravity variation, the latitude and the geopotential anomaly the pressure can be converted into metres' depth according to an empirical formula (UNESCO Tech. Paper 44, p. 25).[11] As a result, decibars are commonly used in oceanography.

In scuba diving, bar is also the most widely used unit to express pressure, e.g. 200 bar being a full standard scuba tank, and depth increments of 10 metre of seawater being equivalent to 1 bar of pressure.

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. In measurement of vacuum and in vacuum engineering, residual pressures are typically given in millibar, although torr or millimeter of mercury (mmHg) were historically common.

Engineers that specialize in technical safety for offshore petrochemical facilities would be expected to exclusively refer to explosion loads in units of bars. A bar is a convenient unit of measure for pressures generated by low frequency vapor cloud explosions that are commonly considered as part of accidental loading risk studies.

In the automotive field, turbocharger boost is often described in bars outside the United States. Tire pressure is often specified in bars. In hydraulic machinery components are rated to the maximum system oil pressure, which is typically in hundreds of bars, for example 300 bars is common for industrial fixed machinery.

Unicode has characters for "mb" (U+33D4 ㏔ SQUARE MB SMALL) and "bar" (U+3374 ㍴ SQUARE BAR), but they exist only for compatibility with legacy Asian encodings and are not intended to be used in new documents.

The kilobar, equivalent to 100 MPa, is commonly used in geological systems, particularly in experimental petrology.

"Bar(a)" and "bara" are sometimes used to indicate absolute pressures, and "bar(g)" and "barg" for gauge pressures. This usage is deprecated and fuller descriptions such as "gauge pressure of 2 bars" or "2-bar gauge" are recommended.[2][12]

See also

References

This article incorporates material from the Citizendium article "Bar (unit)", which is licensed under the Creative Commons Attribution-ShareAlike 3.0 Unported License but not under the GFDL.
  1. ^ a b Le Système international d’unités [The International System of Units] (PDF) (in French and English) (9th ed.), International Bureau of Weights and Measures, 2019, ISBN 978-92-822-2272-0.
  2. ^ a b c British Standard BS 350:2004 Conversion Factors for Units.
  3. ^ "Nomenclature of the unit of absolute pressure, Charles F. Marvin, 1918" (PDF). noaa.gov. (PDF) from the original on 29 April 2017. Retrieved 6 May 2018.
  4. ^ NIST Special Publication 1038 2016-03-19 at the Wayback Machine, Sec. 4.3.2; NIST Special Publication 811, 2008 edition 2016-06-03 at the Wayback Machine, Sec. 5.2
  5. ^ International Astronomical Union Style Manual. Comm. 5 in IAU Transactions XXB, 1989, Table 6
  6. ^ Grave (unit)
  7. ^ Canada, Environment (2013-04-16). "Canadian Weather at a Glance - Environment Canada". www.weatheroffice.gc.ca. from the original on 2 January 2018. Retrieved 6 May 2018.
  8. ^ Canada, Environment (2013-04-16). "Canadian Weather - Environment Canada". www.weatheroffice.gc.ca. from the original on 2 January 2018. Retrieved 6 May 2018.
  9. ^ US government atmospheric pressure map
  10. ^ The Weather Channel
  11. ^ Scientific Committee on Oceanic Research (1983). "Algorithms for computation of fundamental properties of seawater" (PDF). (PDF) from the original on 2015-04-12. Retrieved 2014-05-11.
  12. ^ "What do the letters 'g' and 'a' denote after a pressure unit? (FAQ - Pressure) : FAQs : Reference : National Physical Laboratory". from the original on 7 April 2016. Retrieved 7 February 2016.

External links

  • Official SI website: Table 8. Non-SI units accepted for use with the SI
  • US government atmospheric pressure map showing atmospheric pressure in mbar
Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pound per square inch
(Pa) (bar) (at) (atm) (Torr) (lbf/in2)
1 Pa — 1 Pa = 10−5 bar 1 Pa = 1.0197×10−5 at 1 Pa = 9.8692×10−6 atm 1 Pa = 7.5006×10−3 Torr 1 Pa = 0.000145037737730 lbf/in2
1 bar 105 — = 1.0197 = 0.98692 = 750.06 = 14.503773773022
1 at 98066.5 0.980665 — 0.9678411053541 735.5592401 14.2233433071203
1 atm ≡ 101325 ≡ 1.01325 1.0332 — 760 14.6959487755142
1 Torr 133.322368421 0.001333224 0.00135951 1/760 ≈ 0.001315789 — 0.019336775
1 lbf/in2 6894.757293168 0.068947573 0.070306958 0.068045964 51.714932572 —

January 04, 2023
unit, other, uses, disambiguation, metric, unit, pressure, part, international, system, units, defined, exactly, equal, slightly, less, than, current, average, atmospheric, pressure, earth, level, approximately, barometric, formula, roughly, atmospheric, press. For other uses see Bar disambiguation The bar is a metric unit of pressure but not part of the International System of Units SI It is defined as exactly equal to 100 000 Pa 100 kPa or slightly less than the current average atmospheric pressure on Earth at sea level approximately 1 013 bar 1 2 By the barometric formula 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 C barA pressure of 700 bar flattened this length of aluminium tubing which had a wall thickness of 5 millimetres 0 20 in General informationUnit systemMetric systemUnit ofpressureSymbolbarConversions1 bar in is equal to SI units 100 kPa CGS units 106 Ba US customary units 14 50377 psi Atmospheres 0 986923 atmThe bar and the millibar were introduced by the Norwegian meteorologist Vilhelm Bjerknes who was a founder of the modern practice of weather forecasting 3 The International System of Units despite previously mentioning the bar now omits any mention of it 1 The bar has been legally recognised in countries of the European Union since 2004 2 The US National Institute of Standards and Technology NIST deprecates its use except for limited use in meteorology and lists it as one of several units that must not be introduced in fields where they are not presently used 4 The International Astronomical Union IAU also lists it under Non SI units and symbols whose continued use is deprecated 5 Units derived from the bar include the megabar symbol Mbar kilobar symbol kbar decibar symbol dbar centibar symbol cbar and millibar symbol mbar Contents 1 Definition and conversion 2 Origin 3 Usage 4 See also 5 References 6 External linksDefinition and conversion EditThe bar is defined using the SI derived unit pascal 1 bar 100 000 Pa 100 000 N m2 Thus 1 bar is equal to 1 000 000 Ba barye in cgs units and 1 bar is approximately equal to 0 986923 atm 14 50377 psi 29 5300 inHg 750 062 mmHg 750 062 Torr 1019 716 centimetres of water cmH2O 1 bar approximately corresponds to the pressure of water at a depth of 10 meters Notes 1 millibar mbar 1 one thousandth bar or 1 10 3 bar 1 millibar 1 hectopascal 1 hPa 100 Pa Origin EditThe word bar has its origin in the Ancient Greek word baros baros meaning weight The unit s official symbol is bar the earlier symbol b is now deprecated and conflicts with the use of b denoting the unit barn but it is still encountered especially as mb rather than the proper mbar to denote the millibar Between 1793 and 1795 the word bar was used for a unit of weight in an early version of the metric system 6 Usage Edit Map showing atmospheric pressure in mbar or hPa A tire pressure gauge displaying bar outside and pounds per square inch inside Atmospheric air pressure where standard atmospheric pressure is defined as 1013 25 mbar 101 325 kPa 1 01325 bar which is about 14 7 pounds per square inch Despite the millibar not being an SI unit meteorologists and weather reporters worldwide have long measured air pressure in millibars as the values are convenient After the advent of SI units some meteorologists began using hectopascals symbol hPa which are numerically equivalent to millibars for the same reason the hectopascal is now the standard unit used to express barometric pressures in aviation in most countries For example the weather office of Environment Canada uses kilopascals and hectopascals on their weather maps 7 8 In contrast Americans are familiar with the use of the millibar in US reports of hurricanes and other cyclonic storms 9 10 In fresh water there is an approximate numerical equivalence between the change in pressure in decibars and the change in depth from the water surface in metres Specifically an increase of 1 decibar occurs for every 1 019716 m increase in depth In sea water with respect to the gravity variation the latitude and the geopotential anomaly the pressure can be converted into metres depth according to an empirical formula UNESCO Tech Paper 44 p 25 11 As a result decibars are commonly used in oceanography In scuba diving bar is also the most widely used unit to express pressure e g 200 bar being a full standard scuba tank and depth increments of 10 metre of seawater being equivalent to 1 bar of pressure 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 In measurement of vacuum and in vacuum engineering residual pressures are typically given in millibar although torr or millimeter of mercury mmHg were historically common Engineers that specialize in technical safety for offshore petrochemical facilities would be expected to exclusively refer to explosion loads in units of bars A bar is a convenient unit of measure for pressures generated by low frequency vapor cloud explosions that are commonly considered as part of accidental loading risk studies In the automotive field turbocharger boost is often described in bars outside the United States Tire pressure is often specified in bars In hydraulic machinery components are rated to the maximum system oil pressure which is typically in hundreds of bars for example 300 bars is common for industrial fixed machinery Unicode has characters for mb U 33D4 SQUARE MB SMALL and bar U 3374 SQUARE BAR but they exist only for compatibility with legacy Asian encodings and are not intended to be used in new documents The kilobar equivalent to 100 MPa is commonly used in geological systems particularly in experimental petrology Bar a and bara are sometimes used to indicate absolute pressures and bar g and barg for gauge pressures This usage is deprecated and fuller descriptions such as gauge pressure of 2 bars or 2 bar gauge are recommended 2 12 See also EditAtmospheric pressure Centimetre of water Conversion of units Meteorology Metric prefix Metric units Orders of magnitude pressure Pressure measurementReferences EditThis article incorporates material from the Citizendium article Bar unit which is licensed under the Creative Commons Attribution ShareAlike 3 0 Unported License but not under the GFDL a b Le Systeme international d unites The International System of Units PDF in French and English 9th ed International Bureau of Weights and Measures 2019 ISBN 978 92 822 2272 0 a b c British Standard BS 350 2004 Conversion Factors for Units Nomenclature of the unit of absolute pressure Charles F Marvin 1918 PDF noaa gov Archived PDF from the original on 29 April 2017 Retrieved 6 May 2018 NIST Special Publication 1038 Archived 2016 03 19 at the Wayback Machine Sec 4 3 2 NIST Special Publication 811 2008 edition Archived 2016 06 03 at the Wayback Machine Sec 5 2 International Astronomical Union Style Manual Comm 5 in IAU Transactions XXB 1989 Table 6 Grave unit Canada Environment 2013 04 16 Canadian Weather at a Glance Environment Canada www weatheroffice gc ca Archived from the original on 2 January 2018 Retrieved 6 May 2018 Canada Environment 2013 04 16 Canadian Weather Environment Canada www weatheroffice gc ca Archived from the original on 2 January 2018 Retrieved 6 May 2018 US government atmospheric pressure map The Weather Channel Scientific Committee on Oceanic Research 1983 Algorithms for computation of fundamental properties of seawater PDF Archived PDF from the original on 2015 04 12 Retrieved 2014 05 11 What do the letters g and a denote after a pressure unit FAQ Pressure FAQs Reference National Physical Laboratory Archived from the original on 7 April 2016 Retrieved 7 February 2016 External links EditOfficial SI website Table 8 Non SI units accepted for use with the SI US government atmospheric pressure map showing atmospheric pressure in mbarPressure units vte Pascal Bar Technical atmosphere Standard atmosphere Torr Pound per square inch Pa bar at atm Torr lbf in2 1 Pa 1 Pa 10 5 bar 1 Pa 1 0197 10 5 at 1 Pa 9 8692 10 6 atm 1 Pa 7 5006 10 3 Torr 1 Pa 0 000145 037 737 730 lbf in21 bar 105 1 0197 0 98692 750 06 14 503773 773 0221 at 98066 5 0 980665 0 967841 105 3541 735 5592401 14 223343 307 12031 atm 101325 1 01325 1 0332 760 14 695948 775 51421 Torr 133 322368 421 0 001333 224 0 001359 51 1 760 0 001315 789 0 019336 7751 lbf in2 6894 757293 168 0 068947 573 0 070306 958 0 068045 964 51 714932 572 Retrieved from https en wikipedia org w index php title Bar unit amp oldid 1108530103, wikipedia, wiki, book, books, library,

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