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The bar (symbol bar), decibar (symbol dbar) and the millibar (symbol mbar, also mb) are units of pressure. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface They are not SI units, nor are they cgs units, but they are accepted for use with the SI. The centimetre-gram-second system ( CGS) is a system of physical units. The bar is still widely used in descriptions of pressure because it is about the same as atmospheric pressure.

Contents

Definition

The bar, decibar and millibar are defined as:

(A pascal is one newton per square meter. The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical )

Origin

The word bar has its origin in the Greek word βάρος (baros), meaning weight. Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly In the Physical sciences weight is a Measurement of the gravitational Force acting on an object Its official symbol is "bar"; the earlier "b" is now deprecated, but still often seen especially in "mb" rather than the proper "mbar" for millibars.

The bar and millibar were introduced by Sir Napier Shaw in 1909 and internationally adopted in 1929. Sir William Napier Shaw FRS, ( March 4, 1854 - March 23, 1945) was a British Meteorologist.

Discussion

Atmospheric air pressure is often given in millibars where "standard" sea level pressure is defined as 1013. Mean sea level (MSL is the average (mean height of the Sea, with reference to a suitable reference surface 25 mbar (hPa), equal to 1. 01325 bar. Despite millibars not being an SI unit, they are still used locally in meteorology in some countries to describe atmospheric pressure. The SI unit is the pascal (Pa), with 1 mbar = 100 Pa = 1 hPa = 0. 1 kPa. Meteorologists worldwide have long measured air pressure in millibars. After the introduction of SI units, others use hectopascals (which are equivalent to millibars) so they could stick to the same numeric scale. Similar pressures are given in kilopascals in practically all other fields, where the hecto prefix is hardly ever used. In particular, Canadian weather reports use kilopascals (which could also be called centibars).

Americans are familiar with the millibar in US reports of hurricanes and other cyclonic storms, where lower central pressure generally means higher winds and a stronger storm. A tropical cyclone is a storm system characterized by a low pressure center and numerous Thunderstorms that produce strong winds and Flooding

In some countries, pressure is measured with reference to atmospheric pressure. This is gauge pressure and denoted by barg, often written with no spaces, spoken "bar gauge", and sometimes using symbols such as 'bar(g)'. For example, if someone says that their car tyres are pressurised to 2. 3 bar they actually mean bar gauge: the pressure in the tyre is really 3. 3 bar, but only 2. 3 bar above atmospheric, which is the scale a tyre gauge would read. When absolute pressure is desired, it is sometimes denoted 'bara' or 'bar(a)' for "bar absolute". The alteration of units of measure for this purpose is now deprecated, with qualification of the physical property being preferred, e. g. , "The gauge pressure is 2. 3 bar; the absolute pressure is 3. 3 bar".

In water, there is an approximate numerical correspondence between the change in pressure in decibars and the change in depth from the sea surface in meters. Specifically, an increase of 1 decibar occurs for every 1. 019716 m increase in depth close to the surface. As a result, decibars are commonly used in oceanography. Oceanography (from the greek words Ωκεανός meaning Ocean and γράφω meaning to write also called oceanology or

Unicode has a character for "mb": (㏔), but exists only for compatibility with legacy Asian encodings. In Computing, Unicode is an Industry standard allowing Computers to consistently represent and manipulate text expressed in most of the world's There is also a character "bar": ㍴.

Pressure Units
 
pascal
(Pa)
bar
(bar)		 technical atmosphere
(at)
atmosphere
(atm)
torr
(Torr)		 pound-force per
square inch
(psi)
1 Pa ≡ 1 N/m2 10−5 1. A technical atmosphere (symbol at is a non- SI unit of Pressure equal to one Kilogram-force per square centimeter The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. The pound per square inch or more accurately pound-force per square inch (symbol psi or lbf/in² or lbf/in²) is a unit of The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical 0197×10−5 9. 8692×10−6 7. 5006×10−3 145. 04×10−6
1 bar 100,000 ≡ 106 dyn/cm2 1. 0197 0. 98692 750. 06 14. 504
1 at 98,066. 5 0. 980665 ≡ 1 kgf/cm2 0. The unit kilogram-force ( kgf, often incorrectly just kg) or kilopond ( kp) is defined as the Force exerted by Earth's gravity 96784 735. 56 14. 223
1 atm 101,325 1. 01325 1. 0332 ≡ 1 atm 760 14. The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm 696
1 torr 133. 322 1. 3332×10−3 1. 3595×10−3 1. 3158×10−3 ≡ 1 Torr; ≈ 1 mmHg 19. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. 337×10−3
1 psi 6,894. 76 68. 948×10−3 70. 307×10−3 68. 046×10−3 51. 715 ≡ 1 lbf/in2

Example reading:  1 Pa = 1 N/m2  = 10−5 bar  = 10. This article deals with the unit of force For the unit of mass see Pound (mass. 197×10−6 at  = 9. 8692×10−6 atm, etc.
Note:  mmHg is an abbreviation for millimetres of mercury.


See also

External links

Conversion of units refers to conversion factors between different Units of measurement for the same Quantity.
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