Bar (unit)

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 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.

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 millibar as the values are convenient. Since 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 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. In contrast, Americans are familiar with the use of the millibar in US reports of hurricanes and other cyclonic storms.

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. 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.

Engineers commonly use the bar as a unit of pressure because 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.

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.