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: 1bar ≡ 100,000 Pa ≡ 100,000 N/m2.
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 weight in an early version of the metric system.[6]
Usage
Atmospheric air pressure where standard atmospheric pressure is defined as 1013.25 mbar, 101.325kPa, 1.01325bar, 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 1.019716m 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.[12]
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+33D4㏔SQUARE MB SMALL) and "bar" (U+3374㍴SQUARE 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 100MPa, 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][13]
This article uses material from the Wikipedia article Bar_(unit), and is written by contributors.
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