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Bar (unit)
Bar (unit)
Bar (unit)
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bar
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 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 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

[edit]

The bar is defined using the SI derived unit, pascal: 1 bar ≡ 100000 Pa ≡ 100000 N/m2.

Thus, 1 bar is equal to:

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 at a depth of 10 metres).

1 millibar (mbar) is equal to:

  • 0.001 bar
  • 100 Pa.
Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pound per square inch
(Pa) (bar) (at) (atm) (Torr) (psi)
1 Pa — 10−5 bar 1.0197×10−5 at 9.8692×10−6 atm 7.5006×10−3 Torr 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 psi 6894.757293168 0.068947573 0.070306958 0.068045964 51.714932572 —

Origin

[edit]

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

[edit]
Map showing atmospheric pressure in millibar, or hectopascals
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

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Bar (unit)

View on Grokipedia
from Grokipedia
The bar is a metric unit of defined as exactly 100,000 pascals (Pa), equivalent to 100 kilopascals (kPa) or 10510^5 Pa, where 1 Pa equals 1 newton per square meter (N/m²). Although not part of the (SI), the bar is accepted for use with the SI and is widely employed in , , and scientific applications due to its convenient decimal relation to the pascal. It is approximately equal to standard at , which measures 1.01325 bar or 101,325 Pa. In physical chemistry, 1 bar serves as the recommended standard pressure for defining equilibrium constants and standard states since 1982, replacing the former 1 atm value for greater alignment with SI conventions. The bar's practical value lies in its near-equivalence to atmospheric pressure.

Definition and Properties

Definition

The bar is a metric unit of pressure defined as exactly 100,000 pascals (Pa), or 10510^5 Pa. Although not part of the International System of Units (SI), the bar is a non-SI unit accepted for use with the SI and is derived from the pascal, the SI unit of pressure. Pressure itself is defined as force applied perpendicular to a surface, divided by the area of that surface, with the pascal representing one newton of force per square metre (N/m²). The symbol for the bar is "bar", written in lowercase roman (upright) type and not italicized, consistent with SI style conventions for unit symbols. This distinguishes it from related units such as the technical atmosphere (at), defined as exactly 98,066.5 Pa, and the standard atmosphere (), defined as exactly 101,325 Pa; unlike these, the bar provides an exact, round value of 100,000 Pa without approximation to natural atmospheric conditions.

Physical Significance

The bar unit derives its physical significance from its close approximation to the standard at Earth's , making it a practical reference for environmental pressures. Precisely, 1 bar equals approximately 0.986923 ospheres (atm), while the mean sea-level is about 1.01325 bar. This near-equivalence positions the bar as an intuitive benchmark for typical ambient conditions, where exact adherence to SI units like the pascal is secondary to relatable scales in fields such as and daily monitoring. Submultiples of the bar extend its applicability to finer pressure measurements. The millibar (mbar), defined as 100 pascals, is widely used for quantifying smaller-scale s, including variations in atmospheric conditions. Similarly, the microbar (μbar), or 0.1 pascals, serves in specialized domains like acoustics for sound pressure levels and for detecting subtle ground vibrations. In , isobaric surfaces—regions where remains constant—play a key role in modeling flows, with 1 bar levels often referenced as they align closely with near-surface atmospheric layers in both air and systems.

Conversions and Equivalents

Relation to SI Units

The bar is a metric unit of defined exactly as 100 kilopascals (kPa), which corresponds to 100,000 pascals (Pa). Although the bar itself is not an SI unit, this exact equivalence facilitates its integration into SI-based calculations and measurements. The pascal is the coherent derived SI unit for , defined as the pressure resulting from a force of one newton acting uniformly over an area of one , expressed as 1 Pa=1 N/m21 \, \text{Pa} = 1 \, \text{N/m}^2. The inverse relationship follows directly from this definition, such that 1 Pa=10−5 bar1 \, \text{Pa} = 10^{-5} \, \text{bar}. The bar also derives a practical connection to the hectopascal (hPa), an SI prefix multiple of the pascal widely used in atmospheric sciences, where 1 hPa=100 Pa1 \, \text{hPa} = 100 \, \text{Pa} and is exactly equivalent to one millibar (mbar). Consequently, 1 bar=1000 hPa1 \, \text{bar} = 1000 \, \text{hPa}. In SI contexts, pressure conversions between the bar and pascal are straightforward using the defining relationship: P(bar)=P(Pa)105P_{\text{(bar)}} = \frac{P_{\text{(Pa)}}}{10^5} This equation ensures precise interoperability when expressing pressures in either unit.

Equivalents to Non-SI Units

The bar unit, defined as exactly 100,000 pascals, has several equivalents in traditional non-SI pressure units commonly used in engineering, meteorology, and industry. One bar is approximately equal to 14.5038 pounds per square inch (psi), a unit prevalent in the United States for measuring tire pressure, hydraulic systems, and mechanical specifications. In vacuum technology and older scientific contexts, the bar relates to the millimeter of mercury (mmHg), also known as the , where 1 bar ≈ 750.06168 mmHg under standard conditions of 0°C and gravity of 9.80665 m/s². This equivalence stems from the historical definition tying mmHg to fractions. The standard atmosphere (), defined exactly as 101,325 Pa, provides another key non-SI benchmark; thus, 1 bar ≈ 0.986923 , or conversely, 1 atm ≈ 1.01325 bar. This slight difference highlights the bar's role as a rounded metric approximation to sea-level . For quick reference, the following table summarizes these conversions with approximate values rounded to six decimal places for common precision in applications; exact values derive from the bar's SI definition and require the intermediary pascal for computation.
From BarTo psiTo mmHg (torr)To atm
114.503774750.0616830.986923

History and Etymology

Etymology

The term "bar" as a unit of pressure derives from the word βάρος (báros), meaning "" or "," reflecting the unit's conceptual link to the exerted by weight per unit area. This linguistic root underscores the historical association of with gravitational in scientific . The term was proposed in 1903 by American chemist Theodore W. Richards in his publication on the compressibility of gases, where he suggested "bar" for a pressure of 1 per square centimeter and "megabar" for 10^6 per square centimeter within the centimeter-gram-second (CGS) . This introduction occurred amid late 19th- and early 20th-century efforts to expand the with coherent units beyond the strict SI framework, facilitating precise work in physics and chemistry. The unit gained traction among scientists and was later standardized for broader use, including in by figures like William Napier Shaw around 1909. In scientific contexts, "bar" specifically refers to this pressure unit, distinguishing it from homonymous terms in other domains, such as a legal obstruction or a public establishment serving drinks.

Historical Development

The bar unit of pressure was first proposed in 1903 by American chemist Theodore W. Richards, who suggested the term "megabar" for a pressure equivalent to one million barye (10^6 barye, or 10^6 dynes per square centimeter), in his publication on standardization of units. This proposal aimed to provide a convenient metric for expressing atmospheric and engineering pressures in the emerging CGS system. The term "bar" was used for the base unit of 1 dyne per square centimeter, but the modern bar corresponds to the old megabar. The proposal gained traction through subsequent publications and was independently reinforced by physicist Arthur Edwin Kennelly in related work on pressure measurements. The bar and millibar were introduced for meteorological use by British meteorologist William Napier Shaw in 1909. In the early , the bar saw initial adoption in European and engineering contexts, particularly in Britain and , where it offered a practical alternative to traditional units like millimeters of mercury for barometric readings and hydraulic calculations. British meteorologist William Napier Shaw further popularized the bar and millibar in 1909 while directing the Meteorological Office, integrating it into weather charting and forecasting practices that influenced continental European standards before the full embrace of SI units in the late . The unit received official international recognition at the 9th General Conference on Weights and Measures (CGPM) in 1948, where it was formally defined as 10^6 dynes per square centimeter and its symbol "bar" was standardized in the context of practical units compatible with the meter-kilogram-second (MKS) system. This endorsement facilitated its integration into global scientific literature amid post-war efforts to unify measurement practices. As the (SI) was established in the 1960s, with the pascal adopted as the coherent unit of pressure in 1960 (and named in 1971), the bar was aligned to exactly 100,000 pascals (10^5 Pa), ensuring consistency while retaining its non-SI status for specialized applications. By 1982, the 16th CGPM and concurrent recommendations from the International Union of Pure and Applied Chemistry (IUPAC) confirmed the bar's exact equivalence to 100 kilopascals (kPa), decoupling it from earlier dependencies on mercury column standards (such as the standard atmosphere of 760 mmHg at 0°C, which equals approximately 1.01325 bar). This precise definition, rooted in the pascal's SI foundation, solidified the bar's role in precise thermodynamic and engineering contexts without reliance on physical artifacts like mercury barometers.

Applications and Usage

In Meteorology and Atmospheric Science

In meteorology, the millibar (mbar), a submultiple of the bar equal to 100 pascals, serves as a standard unit for depicting surface atmospheric pressure on weather charts, particularly through isobars—lines connecting points of equal pressure. These isobars are typically contoured at intervals of four millibars, starting from 1000 mbar, to illustrate pressure patterns and gradients that influence weather systems. This convention facilitates the visualization of high- and low-pressure areas, aiding forecasters in identifying fronts, cyclones, and anticyclones. Standard sea-level pressure is approximately 1013.2 millibars, with 1000 millibars often used as a reference for near-sea-level conditions in mapping and analysis. The millibar is integral to upper-air observations, such as those from —balloon-borne instruments that measure , , and humidity profiles through the atmosphere. Radiosonde data report in millibars or the equivalent hectopascals (hPa), providing vertical profiles essential for models. In barometric altimetry, readings in millibars help estimate altitude by relating local to a standard sea-level reference, with adjustments for non-standard conditions to ensure flight safety. Since January 1, 1982, the has recommended the hectopascal (hPa)—numerically identical to the millibar—as the preferred SI-derived unit for in its technical publications and observations, promoting standardization amid the global shift to SI units. Despite this, the millibar persists in many legacy meteorological systems, reports, and operational tools worldwide due to historical entrenchment and ease of use in decimal-based readings. In hurricane tracking, central pressure is routinely expressed in millibars by agencies like the to gauge storm intensity; for instance, pressures below 950 millibars indicate major hurricanes with potential for severe impacts. Pressure gradients, measured as millibar differences over distance, are critical for predictions, as steeper gradients (e.g., 4 millibars over 100 kilometers) correlate with stronger winds via the , balanced by Coriolis effects in geostrophic flow models.

In Engineering and Industry

In , the bar is widely used to specify tire pressures for , particularly in automotive applications where tires are typically inflated to 2 to 3 bar to ensure optimal handling, , and . This range balances load support and ride comfort, with manufacturers recommending adjustments based on load and conditions. In hydraulic systems, the bar serves as a standard unit for rating component capacities, such as pumps and cylinders, which often operate at pressures from 100 to 400 bar to transmit power efficiently in machinery like excavators and presses. Compressor ratings in industrial settings frequently employ the bar to denote maximum output pressure, with common models delivering 7 to 10 bar for general workshop tools and pneumatic systems, facilitating consistent performance in manufacturing processes. In scuba diving equipment, pressure gauges (SPGs) calibrated in bar monitor tank contents, where divers track remaining air in increments of 50 bar, aiding precise gas management during dives equivalent to depths up to 40 meters. For vacuum technology, absolute pressures around 0.1 bar are routine in rough vacuum applications, such as packaging and material handling, where this level achieves sufficient evacuation without requiring high-vacuum equipment. International standards from the ISO often reference the bar alongside MPa for pressure vessels and piping systems, as seen in ISO 23555-1, which covers gas control devices up to 10 MPa (100 bar) to ensure safety and interoperability in industrial installations. In metric-dominant regions like Europe, the bar offers advantages over psi by aligning with SI-derived units, simplifying calculations, specifications, and international trade in engineering projects while reducing conversion errors in cross-border collaborations. This unit's approximate equivalence to atmospheric pressure—about 1.013 bar at sea level—further aids intuitive assessments in system design.

Standards and Comparisons

Definition in International Standards

The bar is recognized by the International Bureau of Weights and Measures (BIPM) as a non-SI unit accepted for use alongside the (SI), but it is neither an nor a derived SI unit with a special name. This status allows its practical application in technical contexts without conflicting with SI coherence, though the pascal (Pa) remains the preferred SI unit for pressure. The symbol "bar" was formally included in the table of unit symbols by Resolution 7 of the 9th General Conference on Weights and Measures (CGPM) in 1948, establishing its notation within international . Subsequent CGPM actions, including Resolution 3 of the 14th CGPM in 1971, adopted the name "pascal" for the SI unit of (1 Pa = 1 /m²), thereby fixing the bar exactly at 100 000 Pa through its predefined relation of 10^5 Pa. In 1982, the International Union of Pure and Applied Chemistry (IUPAC) reinforced this equivalence by recommending 1 bar = 100 kPa as the standard reference in , aligning it precisely with SI definitions. European Union directives, such as the Pressure Equipment Directive 2014/68/EU, explicitly permit and utilize the bar for specifying maximum allowable pressures (e.g., equipment subject to PS > 0.5 bar), facilitating its role in regulatory and industrial pressure assessments across member states. Similarly, the (ISO) standards ISO 31-3 (1992) and its successor ISO 80000-4:2019 (Quantities and units — Part 4: ) authorize the bar in pressure expressions, defining it exactly as 1 bar = 10^5 Pa to ensure compatibility with SI while supporting its use in contexts. The 9th edition of the SI Brochure (2019) confirms the bar as a non-SI unit accepted for use with the SI, particularly in fields like and , while the pascal is the coherent SI unit for . The bar is a non-SI unit of accepted for use with the (SI), defined exactly as 100 000 pascals (Pa), making it 100 000 times larger than the pascal, the for equivalent to one newton per square meter. This scale difference renders the bar particularly convenient for expressing pressures on atmospheric and engineering scales, where values around 1 bar are common, avoiding the need for large numerical prefixes with pascals, such as the approximately 101 325 Pa of standard atmospheric . In contrast, the pascal is preferred in precision scientific work for its coherence within the SI system, enabling direct integration with other base units without additional conversion factors. Compared to the atmosphere (atm), another non-SI unit not accepted for general use with the SI, the bar is exactly metric and slightly lower in value, with 1 atm defined as precisely 101 325 Pa, or approximately 1.01325 bar. The atm originates from historical measurements using a mercury column of 760 mmHg at standard conditions, tying it to empirical observations rather than the bar's direct tie to the pascal. The bar typically denotes absolute pressure (bara), measured from , whereas the gauge bar (barg) measures pressure relative to the prevailing , such that barg approximates bar minus 1 atm under standard conditions. This distinction is critical in applications like systems, where gauge pressures ignore ambient variations for practical readings, but absolute pressures are essential for thermodynamic calculations. The bar offers simplicity in contexts through its alignment with typical ranges, reducing compared to the pascal's smaller scale, though the SI's universality favors the pascal to minimize unit proliferation and ensure global consistency. While the bar remains accepted alongside SI units, its non-fundamental status has led to recommendations for pascal use in formal , particularly following the 2019 SI redefinition that reinforced derived units like the pascal without altering standards.

References

  1. https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf
  2. https://www.noaa.gov/jetstream/atmosphere/air-pressure
  3. https://iupac.org/wp-content/uploads/2025/03/IUPAC-GB4Abridged.pdf
  4. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8
  5. https://www.nist.gov/pml/owm/metric-si/unit-conversion/pressure-and-gas-flow-unit-conversions
  6. https://www.wartsila.com/encyclopedia/term/standard-atmospheric-pressure
  7. https://www.sensorsone.com/bar-pressure-unit/
  8. https://www.merriam-webster.com/dictionary/millibar
  9. https://www.merriam-webster.com/dictionary/microbar
  10. https://physics.nist.gov/cgi-bin/cuu/Value?torrtorr
  11. https://physics.nist.gov/cgi-bin/cuu/Value?stdatm
  12. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors
  13. https://www.etymonline.com/word/bar
  14. https://wordpandit.com/word-root-baros/
  15. https://journals.ametsoc.org/view/journals/mwre/42/3/1520-0493_1914_42_141b_suia_2_0_co_2.pdf
  16. https://www.theochem.ru.nl/~pwormer/Knowino/knowino.org/wiki/Bar_%28unit%29.html
  17. https://www.bipm.org/en/committees/cg/cgpm/9-1948
  18. https://www.nist.gov/pml/special-publication-330/sp-330-appendix-1
  19. https://goldbook.iupac.org/terms/view/S05921
  20. https://www.noaa.gov/jetstream/analyze-appair-pressure-map
  21. https://www.weather.gov/source/zhu/ZHU_Training_Page/winds/pressure_winds/Pressure.htm
  22. https://www.webmet.com/met_monitoring/911.html
  23. https://asrs.arc.nasa.gov/publications/directline/dl9_low.htm
  24. http://www.dca.iag.usp.br/material/ritaynoue/aca-0523/referencias/WMO485.pdf
  25. https://www.nhc.noaa.gov/help/tcm.shtml?PRESS
  26. https://www.e-education.psu.edu/meteo3/l6_p4.html
  27. https://www.blackcircles.com/helpcentre/tyres/what-is-the-difference-between-psi-bar-kpa
  28. https://izoflexhose.com/about-us/blog/industrial-hoses/the-bar-as-a-unit-of-pressure-what-it-is-used-for-and-how-it-is-used/
  29. https://www.atlascopco.com/en-us/compressors/wiki/compressed-air-articles/sizing-an-air-compressor
  30. https://blog.padi.com/bar-or-psi/
  31. https://www.schmalz.com/en/support/know-how/vacuum-knowledge/basic-knowledge/measurement-units-for-vacuum-data/
  32. https://en.jumo.at/web/services/faq/pressure-measurement/international-pressure-units
  33. https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507
  34. https://www.bipm.org/en/committees/cg/cgpm/9-1948/resolution-7
  35. https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32014L0068
  36. https://www.iso.org/standard/64975.html
  37. https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-5-units-outside-si
  38. https://blog.beamex.com/pressure-units-and-pressure-unit-conversion
  39. https://www.nist.gov/system/files/documents/calibrations/pmc-2.pdf
  40. https://www.atlascopco.com/en-us/compressors/wiki/compressed-air-articles/difference-bar-gauge-bar-absolute
  41. https://control.com/technical-articles/absolute-vs-gauge-pressure-understanding-the-difference/
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