Bar (unit)

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bar
Aluminium cylinder.jpg
An aluminium cylinder of wall thickness 5 millimetres (0.20 in) after an external pressure of 700 bar was applied to it
General information
Unit system metric system
Unit of pressure
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 is a metric unit of pressure defined as 100000  Pa (100 kPa or 1000 hPa), though not part of the International System of Units (SI). A pressure of 1 bar is 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.

Contents

The bar and the millibar were introduced by the Norwegian meteorologist Vilhelm Bjerknes, who was a founder of the modern practice of weather forecasting, with the bar defined as one mega dyne per square centimetre. [3]

The SI brochure, despite previously mentioning the bar, [4] 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". [5] The International Astronomical Union (IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated". [6]

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 ≡ 100000 Pa ≡ 100000 N/m2.

Thus, 1 bar is equal to:

1 millibar (mbar) is equal to:

Pressure units
 Pascals Bars Standard atmospheres Pounds per square inch Millimetres of mercury Inches of mercury Technical atmospheres Torrs
1  Pa ≡ 1  N ⁄ m 2= 1×10−5  bar ≈ 9.86923×10−6  atm ≈ 1.45038×10−4  psi ≈ 7.50062×10−3  mmHg ≈ 2.95300×10−4  inHg ≈ 1.01972×10−5  kgf/cm2 ≈ 7.50062×10−3  Torr
1  bar = 100000  Pa ≡ 100 000  N ⁄ m 2≈ 0.98692  atm ≈ 14.5038  psi ≈ 750.062  mmHg ≈ 29.5300  inHg ≈ 1.01972  kgf/cm2 ≈ 750.062  Torr
1  atm = 101325  Pa = 1.01325  bar ≡ 101 325  N ⁄ m 2≈ 14.6959  psi ≈ 760.000  mmHg ≈ 29.9213  inHg ≈ 1.03323  kgf/cm2 = 760  Torr
1  psi ≈ 6894.76  Pa ≈ 0.06895  bar ≈ 0.06805  atm ≡ 1  lb ⁄ in 2≈ 51.7149  mmHg ≈ 2.03602  inHg ≈ 0.07031  kgf/cm2 ≈ 51.7149  Torr
1  mmHg ≈ 133.322  Pa ≈ 1.33322×10−3  bar ≈ 1.31579×10−3  atm ≈ 0.01934  psi ≡  gn  × .001  m  × 13595.1  kg ⁄ m 3≈ 0.03937  inHg ≈ 1.35951×10−3  kgf/cm2 ≈ 1.00000  Torr
1  inHg ≈ 3386.39  Pa ≈ 0.03386  bar ≈ 0.03342  atm ≈ 0.49115  psi = 25.4  mmHg ≡  gn  × .254  m  × 13595.1  kg ⁄ m 3≈ 0.0345316  kgf/cm2 ≈ 25.4000  Torr
1  kgf⁄cm2 ≈ 98066.5  Pa ≈ 0.98066  bar ≈ 0.96784  atm ≈ 14.2233  psi ≈ 735.559  mmHg ≈ 28.9590  inHg ≡ 1  kgf ⁄ cm 2≈ 735.559  Torr
1  Torr ≈ 133.322  Pa ≈ 1.33322×10−3  bar ≈ 1.31579×10−3  atm ≈ 0.01934  psi ≈ 1.00000  mmHg ≈ 0.03937  inHg ≈ 1.35951×10−3  kgf/cm2 ≡ ⁠101 325/760⁠ = ⁠20 265/172⁠  N ⁄ m 2

Origin

The word bar has its origin in the Ancient Greek word βάρος (baros), meaning weight. The unit's official symbol is bar;[ citation needed ] the earlier symbol b is now deprecated and conflicts with the uses of b denoting the unit barn or bit, 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 mass (equal to the modern tonne) in an early version of the metric system. [9]

Usage

Map showing atmospheric pressure in millibar, or hectopascals Day5pressureforecast.png
Map showing atmospheric pressure in millibar, or hectopascals
A tire-pressure gauge displaying bar (outside) and pounds per square inch (inside) ReifendruckPruefen.jpg
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 millibar as the values are convenient. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibar; for the same reason, the hectopascal is now the standard unit used to express barometric pressures in aviation in most countries. For example, the Meteorological Service of Canada uses kilopascals and hectopascals on their weather maps. [10] [11] In contrast, Americans are familiar with the use of the millibar in US reports of hurricanes and other cyclonic storms. [12] [13]

In fresh water, there is an approximate numerical equivalence between the change in pressure in decibar and the change in depth from the water surface in metres. Specifically, an increase of 1 decibar occurs for an increase in depth of 1.019716 m. 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). [14] As a result, decibar is 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 millimetre of mercury (mmHg) were historically common.

Pressures resulting from deflagrations are often expressed in units of bar. [15]

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

In the maritime ship industries, pressures in piping systems, such as cooling water systems, is often measured in bar.

Unicode has characters for "mb" (U+33D4㏔SQUARE MB SMALL), "bar" (U+3374㍴SQUARE BAR) and ミリバール (U+334A㍊SQUARE MIRIBAARU; "millibar" spelt in katakana), 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.

The abbreviations "bar(a)" and "bara" are sometimes used to indicate absolute pressures, and "bar(g)" and "barg" for gauge pressures. The usage is deprecated but still prevails in the oil industry (often by capitalized "BarG" and "BarA"). As gauge pressure is relative to the current ambient pressure, which may vary in absolute terms by about 50 mbar, "BarG" and "BarA" are not interconvertible. Fuller descriptions such as "gauge pressure of 2 bars" or "2-bar gauge" are recommended. [2] [16]

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. 1 2 The International System of Units (PDF), V3.01 (9th ed.), International Bureau of Weights and Measures, Aug 2024, ISBN   978-92-822-2272-0 .
  2. 1 2 3 British Standard BS 350:2004 Conversion Factors for Units.
  3. ↑ "Nomenclature of the unit of absolute pressure, Charles F. Marvin, 1918" (PDF). noaa.gov. Archived from the original (PDF) on 29 April 2017. Retrieved 6 May 2018.
  4. ↑ Resolution 6 of the 9th CGPM, 1948, lists the bar (symbol bar) in a table illustrating writing and printing of unit symbols
  5. ↑ 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
  6. ↑ International Astronomical Union Style Manual. Comm. 5 in IAU Transactions XXB, 1989, Table 6
  7. ↑ Allen, William H. (1965-01-01). "Dictionary of Technical Terms for Aerospace Use". NASA Special Publication. bar.... Some writers have used bar as equivalent to barye (1 dyne per square centimeter). ... barye.... Sometimes called bar or microbar.... microbar (abbr μb).... In British literature the term barye has been used.... Unfortunately, the bar was once used in acoustics to mean 1 dyne per square centimeter, but this is no longer correct.
  8. ↑ Marvin, Charles F. (1918-03-30). "Nomenclature of the Unit of Absolute Pressure" (PDF). Monthly Weather Review. 46 (2). Washington, D.C.: 73–75. Bibcode:1918MWRv...46...73M. doi:10.1175/1520-0493(1918)46<73:NOTUOA>2.0.CO;2. Archived from the original (PDF) on 2017-04-29. T. W. Richards and A. E. Kennelly employed the term 'bar' to signify a pressure of 1 dyne per square centimeter
  9. ↑ "Instructions abrégée sur les mesures déduites de la grandeur de la terre et sur les calculs relatifs à leur division décimale, 1793: gravet, bar". 1793. Archived from the original on 2023-01-15. Retrieved 2016-05-06.
  10. ↑ 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.
  11. ↑ 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.
  12. ↑ US government atmospheric pressure map
  13. ↑ The Weather Channel
  14. ↑ 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.
  15. ↑ NFPA 68 Standard on Explosion Protection by Deflagration Venting (2023 ed.).
  16. ↑ "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.
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