User:Milton Beychok/Sandbox: Difference between revisions

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The '''bar''' (symbol '''bar''') is widely used in many countries as a unit of pressure. It is  not an [[International System of Units|SI]] unit, nor is it a [[Centimeter gram second system of units|cgs]] unit, but it is accepted for use with SI units by [[National Institute of Standards and Technology|NIST]].<ref>[http://physics.nist.gov/cuu/Units/outside.html Units Outside of the SI, Table 7]</ref>


==Definition==
The bar, decibar (symbol ''dbar''), centibar (symbol ''cbar'') and millibar (symbol ''mbar'' or ''mb'')  are defined as:
* 1 bar = 10<sup>5</sup> Pa ([[Pascal (unit)|Pascals]]) = 10<sup>6</sup> [[dyne]]s per square [[metre|centimetre]] = 0.987 atm ([[Atmosphere (unit)|atmospheres]])
* 1 dbar = 0.1 bar = 10<sup>4</sup> Pa = 10 kPa
* 1 cbar = 0.01 bar = 10 <sup>3</sup> Pa = 1 kPa
* 1 mbar = 0.001 bar = 10<sup>2</sup> Pa = 1 hPa (hectopascal)
The bar and the millibar were introduced by Sir Napier Shaw in 1909.
== Usage ==
Although millibars are not an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars. For example, the weather office of [[Environment Canada]] uses hectopascals on their weather maps.<ref>[http://www.weatheroffice.gc.ca/jet_stream/index_e.html Environment Canada Weather Map]</ref>
[[Atmospheric pressure|Atmospheric air pressure]] is often expressed in millibars and sea level atmospheric air pressure is defined as 1013.25 mbar which is equivalent to 1 atm.
In water, there is an approximate numerical equivalence between the change in pressure in decibars and the change in depth from the sea surface in metres.  Specifically, an increase of 1 decibar occurs for every 1.019716 metre increase in depth close to the surface. As a result, decibars are commonly used in [[oceanography]].
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.
== Absolute pressure and gauge pressure ==
[[Bourdon tube]] [[pressure gauges]], vehicle tire gauges and many other types of pressure gauges are zero referenced to atmospheric pressure, which means that they measure the pressure above atmospheric pressure. However, absolute pressures are zero referenced to a complete vacuum. Thus, the absolute pressure of any system is the gauge pressure of the system plus atmospheric pressure.
Absolute pressures expressed in bar are often referred to as ''bara'', whereas gauge pressures expressed in bar are often referred to as ''barg''.
In the [[United States]], where pressures are still often expressed in pounds per square inch (symbol ''psi''), gauge pressures are referred to as ''psig'' and absolute pressures are referred to as ''psia''.
Sometimes, the context in which the word pressure is used helps to identify it as meaning either the absolute or gauge pressure. However, in truth, whenever a pressure is expressed in any units (bar, Pa, psi, atm, etc.), it should be denoted in some manner as being either absolute or gauge pressure to avoid any possible misunderstanding. For example, the well-known [[ideal gas law]] of '''''pV''''' = '''''nRT''''' is only valid when the pressure '''''p''''' is the absolute pressure.
== References ==
{{reflist}}

Revision as of 19:08, 10 May 2008

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