Bar (unit)

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Short description: Unit of pressure equal to 100,000 Pa
bar
Aluminium cylinder.jpg
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 is a metric unit of pressure defined as 100,000 Pa (100 kPa), 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.

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 megadyne per square centimeter.[3]

The SI brochure, despite previously mentioning the bar,[citation needed] 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:

  • 1,000,000 Ba (barye) (in cgs units);

and 1 bar is approximately equal to:

  • 0.98692327 atm
  • 14.503774 psi
  • 29.529983 inHg
  • 750.06158 mmHg
  • 750.06168 Torr
  • 1019.716 centimetres of water (cmH2O) (1 bar approximately corresponds to the gauge pressure of water (not absolute) 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;[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 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.

Pressures resulting from deflagrations are often expressed in units of bars.[12]

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.

The abbreviations "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][13]

See also

References

  1. ↑ 1.0 1.1 International Bureau of Weights and Measures (2019-05-20), SI Brochure: The International System of Units (SI) (9th ed.), ISBN 978-92-822-2272-0, https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf .
  2. ↑ 2.0 2.1 2.2 British Standard BS 350:2004 Conversion Factors for Units.
  3. ↑ "Nomenclature of the unit of absolute pressure, Charles F. Marvin, 1918". http://docs.lib.noaa.gov/rescue/mwr/046/mwr-046-02-0073.pdf. 
  4. ↑ NIST Special Publication 1038 , Sec. 4.3.2; NIST Special Publication 811, 2008 edition , 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". http://www.weatheroffice.gc.ca/jet_stream/index_e.html. 
  8. ↑ Canada, Environment (2013-04-16). "Canadian Weather - Environment Canada". http://www.weatheroffice.gc.ca/canada_e.html. 
  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". http://unesdoc.unesco.org/images/0005/000598/059832eb.pdf. 
  12. ↑ NFPA 68 Standard on Explosion Protection by Deflagration Venting (2023 ed.). 
  13. ↑ "What do the letters 'g' and 'a' denote after a pressure unit? (FAQ - Pressure) : FAQs : Reference : National Physical Laboratory". http://www.npl.co.uk/reference/faqs/what-do-the-letters-'g'-and-'a'-denote-after-a-pressure-unit-(faq-pressure). 

External links

Pressure units
v · d · e Pascal Bar Technical atmosphere Standard atmosphere Torr Pounds per square inch
(Pa) (bar) (at) (atm) (Torr) (lbf/in2)
1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 0.000 145 037 737 730
1 bar 105 ≡ 100 kPa

≡ 106 dyn/cm2

1.0197 0.98692 750.06 14.503 773 773 022
1 at 98066.5 0.980665 ≡ 1 kgf/cm2 0.967 841 105 354 1 735.559 240 1 14.223 343 307 120 3
1 atm ≡ 101325 ≡ 1.01325 1.0332 1 760 14.695 948 775 514 2
1 Torr 133.322 368 421 0.001 333 224 0.001 359 51 1/760 ≈ 0.001 315 789 1 Torr

≈ 1 mmHg

0.019 336 775
1 lbf/in2 6894.757 293 168 0.068 947 573 0.070 306 958 0.068 045 964 51.714 932 572 ≡ 1 lbf/in2



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