# Bar (unit)

Last updated
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 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, but not part of the International System of Units (SI); it is defined as 100,000  Pa (100 kPa). 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 centimeter. [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:

• 0.98692327  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; 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

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.

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+33D4SQUARE MB SMALL) and "bar" (U+3374SQUARE 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] [12]

## Related Research Articles

Pressure measurement is the measurement 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 mechanically are called pressure gauges,vacuum gauges or compound gauges. The widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.

Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure.

Standard temperature and pressure (STP) are various standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.

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).

United States customary units form a system of measurement units commonly used in the United States and most U.S. territories, since being standardized and adopted in 1832. The United States customary system developed from English units that were in use in the British Empire before the U.S. became an independent country. The United Kingdom's system of measures was overhauled in 1824 to create the imperial system, which was officially adopted in 1826, changing the definitions of some of its units. Consequently, while many U.S. units are essentially similar to their imperial counterparts, there are significant differences between the systems.

Atmospheric pressure, also known as air pressure or barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, 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.

The pascal is the unit of pressure in the International System of Units (SI). It is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is a SI coherent derived unit defined as one newton per square metre (N/m2). It is also equivalent to 10 barye in the CGS system. Common multiple units of the pascal are the hectopascal, which is equal to one millibar, and the kilopascal, which is equal to one centibar.

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.

The humidex is an index number used by Canadian meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and humidity. The term humidex was coined in 1965. The humidex is a nominally dimensionless quantity based on the dew point.

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 average atmospheric pressure at sea level.

The North American High is an impermanent high-pressure area or anticyclone created by a formative process that occurs when cool or cold dry air settles over North America. During summer, it is replaced with an Arctic Low, or a North American Low should it move over continental land.

Inch of mercury is a non-SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.

A kilogram-force per centimetre square (kgf/cm2), often just kilogram per square centimetre (kg/cm2), or kilopond per centimetre square (kp/cm2) 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.

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

The pound per square inch or, more accurately, pound-force per square inch, is a unit of measurement of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force with magnitude of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately 6,895 pascals.

The angstrom or ångström is a metric unit of length equal to 10−10 m; that is, one ten-billionth (US) of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. Its symbol is Å, a letter of the Swedish alphabet. The unit is named after the Swedish physicist Anders Jonas Ångström (1814–1874).

The November 2011 Bering Sea cyclone was one of the most powerful extratropical cyclones to affect Alaska on record. On November 8, the National Weather Service (NWS) began issuing severe weather warnings, saying that this was a near-record storm in the Bering Sea. It rapidly deepened from 973 mb (28.7 inHg) to 948 mb (28.0 inHg) in just 24 hours before bottoming out at 943 mbar, roughly comparable to a Category 3 or 4 hurricane. The storm had been deemed life-threatening by many people. The storm had a forward speed of at least 60 mph (97 km/h) before it had reached Alaska. The storm began affecting Alaska in the late hours of November 8, 2011. The highest gust recorded was 93 mph (150 km/h) on Little Diomede Island. One person was reported missing after being swept into the Bering Sea, and he was later pronounced dead.

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.

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

## References

1. 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. 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. 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.
7. 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.
8. 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.
9. "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 = 10−5 bar1 Pa = 1.0197×10−5 at1 Pa = 9.8692×10−6 atm1 Pa = 7.5006×10−3 Torr1 Pa = 0.000145037737730 lbf/in2
1 bar105= 1.0197= 0.98692= 750.06= 14.503773773022
1 at98066.50.9806650.9678411053541735.559240114.2233433071203
1 atm1013251.013251.033276014.6959487755142
1 Torr133.3223684210.0013332240.001359511/7600.0013157890.019336775
1 lbf/in26894.7572931680.0689475730.0703069580.06804596451.714932572