Bar (unit)

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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 system Metric system
Unit of Pressure
Symbolbar
Conversions
1 bar in ...... is equal to ...
    SI units    100  kPa
    CGS units    1.0×106  Ba
    US customary units    14.50377  psi

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.

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. [3]

The International Bureau of Weights and Measures (BIPM) lists the bar as one of the "non-SI units [that authors] should have the freedom to use", but has declined to include it among the "non-SI units accepted for use with the SI". [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). The notation bar(g), though deprecated by various bodies, [2] represents gauge pressure, i.e., pressure in bars above ambient or atmospheric pressure.

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:

Notes:

Origin

The word bar has its origin in the 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 Day5pressureforecast.png
Map showing atmospheric pressure in mbar or hPa
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 is often given in millibars, 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 meter of seawater being equivalent to 1 bar of pressure. (Americans alternatively measure pressure in psi.)

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 bar or 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.

Unicode has characters for "mb" (㏔, U+33D4) and "bar" (㍴, U+3374), 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 bar" or "2-bar gauge" are recommended. [2] [12]

See also

Related Research Articles

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Pressure measurement is the analysis of an applied force by a fluid on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure and display pressure in an integral unit are called pressure meters or pressure gauges or vacuum gauges. A manometer is a good example, as it uses the surface area and weight of a column of liquid to both measure and indicate pressure. Likewise the widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.

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The torr is a unit of pressure based on an absolute scale, defined as exactly 1/760 of a standard atmosphere. Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa).

Atmospheric pressure, also known as barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 101,325 Pa, which is equivalent to 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

Pascal (unit) SI derived unit of pressure

The pascal is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is defined as one newton per square metre and is equivalent to 10 barye (Ba) in the CGS system. The unit of measurement called standard atmosphere (atm) is defined as 101,325 Pa.

Hecto is a decimal unit prefix in the metric system denoting a factor of one hundred. It was adopted as a multiplier in 1795, and comes from the Greek ἑκατόν hekatón, meaning "hundred". In 19th century English it was sometimes spelled hecato, in line with a puristic opinion by Thomas Young. Its unit symbol as an SI prefix in the International System of Units (SI) is the lower case letter h.

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Pressure altitude is the altitude in the International Standard Atmosphere (ISA) with the same atmospheric pressure as that of the part of the atmosphere in question.

The standard atmosphere is a unit of pressure defined as 101325 Pa. It is sometimes used as a reference pressure or standard pressure. It is approximately equal to Earth's atmospheric pressure at sea level.

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Kilogram-force per square centimetre Unit of pressure

A kilogram-force per centimetre square (kgf/cm2), often just kilogram per square centimetre (kg/cm2), or kilopond per centimetre square is a deprecated unit of pressure using metric units. It is not a part of the International System of Units (SI), the modern metric system. 1 kgf/cm2 equals 98.0665 kPa (kilopascals). It is also known as a technical atmosphere.

Ambient pressure Pressure of the surrounding medium

The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object.

Pounds per square inch Unit of pressure or stress

The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 N/m2.

The angstrom or ångström is a metric unit of length equal to 10−10 m; that is, one ten-billionth of a metre, 0.1 nanometre, or 100 picometres. Its symbol is Å, a letter of the Swedish alphabet.

The December 2009 Midwest blizzard was a powerful extratropical cyclone which was of a category which meteorologists refer to as a cyclogenic bomb, a system which shows a drop in central pressure similar to the rapid intensification cycle of a tropical cyclone, more than 1 mbar per hour for 12 to 24 hours or more. A sustained drop averaging more than 2.5 mbar/h is termed explosive deepening/intensification, and this was the case with this rapidly deepening and intensifying storm as it traversed the Midwest and Ontario and on to Québec, Greenland and vicinity. In many locations wind, snowfall, and precipitation moisture content records dating back to the December 2, 1990 storm, the 1976-1978 period, the 1949 blizzard, or even further back were broken, with barometric pressure records falling as well. Both the central pressure (depth) and rate of change and differential over a given distance (intensity) were remarkable, and both caused hurricane-force winds in places.

Torricellis experiment

Torricelli's experiment was invented in Pisa in 1643 by the Italian scientist Evangelista Torricelli (1608-1647). The purpose of his experiment is to prove that the source of vacuum comes from atmospheric pressure.

Metre sea water Unit of pressure equal to one tenth of a bar

The metresea water (msw) is a metric unit of pressure used in underwater diving. It is defined as one tenth of a bar.

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 International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 127, ISBN   92-822-2213-6, archived (PDF) from the original on 2017-08-14.
  2. 1 2 3 4 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 (PDF) from the original on 29 April 2017. Retrieved 6 May 2018.
  4. 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
  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. Archived 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. Archived 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). Archived (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". Archived from the original on 7 April 2016. Retrieved 7 February 2016.
Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pound per square inch
(Pa)(bar)(at)(atm)(Torr)(lbf/in2)
1 Pa1 Pa ≡ 1 Pa1 Pa = 10−5 bar1 Pa = 1.0197×10−5 at1 Pa = 9.8692×10−6 atm1 Pa = 7.5006×10−3 Torr1 Pa = 0.000 145 037 737 730 lbf/in2
1 bar105≡ 100 kPa

 106  dyn/cm2

= 1.0197= 0.98692= 750.06= 14.503 773 773 022
1 at98066.50.980665≡ 1 kgf/cm20.967 841 105 354 1735.559 240 114.223 343 307 120 3
1 atm1013251.013251.0332176014.695 948 775 514 2
1 Torr133.322 368 4210.001 333 2240.001 359 511/760 ≈ 0.001 315 7891 Torr

≈ 1  mmHg

0.019 336 775
1 lbf/in26894.757 293 1680.068 947 5730.070 306 9580.068 045 96451.714 932 572≡ 1 lbf/in2