Bar (unit of pressure)

The bar (symbol: bar) is a unit of pressure, but it is 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). By the barometric formula, 1 bar is strictly the atmospheric pressure on Earth at an altitude of 111 meters at 15 °C.

The bar and millibar were introduced by Norwegian meteorologist Vilhelm Bjerknes, who was a founder of the modern practice of weather prediction.

The International System of Units, while previously mentioning the bar, now omits any mention of it. The bar has been legally recognized in European Union countries since 2004. The "US National Institute of Standards and Technology (NIST)" obsoletes its use except for "limited use in meteorology" and lists it as one of several units that "should not be entered in fields in which they are not currently used." The "International Astronomical Union (IAU)" also, it lists it under "Non-SI units and symbols whose continued use is obsolete".

Units derived from the bar include:

Etymology

The word bar has its origins in the ancient Greek word βάρος (baros), which means weight. The official symbol for the unit is bar; the old symbol b is now deprecated and conflicts with the use of b denoting the unit barn, but is still found, especially as mb (more than the mbar itself) to denote the millibar. Between 1793 and 1795, the word bar was used for a unit of weight in an older version of the metric system.

Definition

The bar is defined using the SI, pascal: 1 bar ≡ 100 000 Pa ≡ 100 000 N/m².

In this way 1 bar is approximately equal to:

Notes:

• 1 millibar (mbar) = 1 thousandth of bar, or 1 x 10^-3 bar
• 1 millibar = 1 hectoPascal (1 hPa = 100 Pa).

Use

The atmospheric pressure of air 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. Although the millibar is not an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars because the values are convenient. After the advent of SI units, some meteorologists began to use hectopascals (symbol hPa), which are numerically equivalent to millibars; for the same reason, the hectopascal is now the standard unit used to express aviation barometric pressure in many countries. For example, the "weather office of Environment Canada" He uses kilopascals and hectopascals in his climate maps. In contrast, Americans are familiar with the use of millibars in "US reports" hurricanes and other cyclonic storms.

In fresh water, there is an equivalent numerical approximation between the change in pressure in decibars and the change in depth from surface water in meters. Specifically, a 1 decibar increase occurs for every 1.019716 m increase in depth. In seawater with respect to gravity variation, latitude and geopotential anomaly, pressure can be converted to depth in meters according to an empirical formula (UNESCO Tech. Paper 44, p. 25). As a result, decibars are commonly used in oceanography.

In diving, the bar is also the most used unit to express pressure, for example 200 bars being a full scuba tank, and pressure increments of 10 meters of sea water being equivalent to 1 bar of pressure.

Several engineers around the world use the bar as a unit of pressure since, in much of their work, using Pascals could involve using very large numbers. In vacuum measurements and vacuum engineering, residual pressures are typically given in millibars, although torr or millimeters of mercury (mmHg) were historically common.

Engineers who specialize in technical safety for offshore petrochemical facilities would be expecting exclusively to refer to burst charges in bar units. A bar is a convenient unit for measuring 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 of the United States. Pressure is often specified in bars. In hydraulic machinery components they are rated at the maximum system oil pressure, which is typically in the hundreds of bars, for example 300 bars is common for industrial stationary 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 they are not treated to be used in new documents.

The kilobar, equivalent to 100 MPa, is commonly used in geological systems, particularly in experimental petrology.

The "bar(a)" and "bara" are sometimes used to indicate absolute pressures, and "bar(g)" and "barg" for gauge pressures. This usage is obsolete and fuller descriptions such as "2 bar gauge pressures" or "2-bar gauge" are recommended.

Spelling Considerations

• Like substantive, the plural of unity follows the usual rules and is "bars", for example: "five thousandibares".
• Like symbol it may confuse, by matching the name in singular, but it must be written without plural termination, "bar", understanding a use of the symbol (the symbols do not admit plural termination).