Unit of pressure equal to 100,000 Pa
The
bar
is a
metric unit
of
pressure
defined as 100,000
Pa
(100 kPa), though not part of the
International System of Units
(SI). A pressure of 1 bar is slightly less than the current average
atmospheric pressure
on Earth at
sea level
(approximately 1.013 bar).
[1]
[2]
By the
barometric formula
, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.
The bar and the millibar were introduced by the Norwegian meteorologist
Vilhelm Bjerknes
, who was a founder of the modern practice of
weather forecasting
, with the bar defined as one
mega
dyne
per square
centimeter
.
[3]
The
SI brochure
, despite previously mentioning the bar,
[
citation needed
]
now omits any mention of it.
[1]
The bar has been legally recognised in countries of the
European Union
since 2004.
[2]
The US
National Institute of Standards and Technology
(NIST)
deprecates
its use except for "limited use in
meteorology
" and lists it as one of several units that "must not be introduced in fields where they are not presently used".
[4]
The
International Astronomical Union
(IAU) also lists it under "Non-SI units and symbols whose continued use is deprecated".
[5]
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
[
edit
]
The bar is defined using the
SI derived unit
,
pascal
:
1 bar
≡ 100,000 Pa ≡ 100,000 N/m
2
.
Thus,
1 bar
is equal to:
and 1 bar is approximately equal to:
- 0.98692327
atm
- 14.503774
psi
- 29.529983
inHg
- 750.06158
mmHg
- 750.06168
Torr
- 1019.716 centimetres of water (cmH
2
O) (1 bar approximately corresponds to the gauge pressure of water (not absolute) at a depth of 10 meters).
Notes:
- 1
millibar
(
mbar
) = 1 one-thousandth bar, or
1
×
10
?3
bar
- 1 millibar = 1
hectopascal
(1 hPa = 100 Pa).
Origin
[
edit
]
The word
bar
has its origin in the
Ancient Greek
word
β?ρο?
(
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
[
edit
]
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 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 1.019716 m 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.
In the maritime ship industries, pressures in piping systems, such as cooling water systems, is often measured in bars.
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 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"). There exists no conversion between "BarG" and "BarA" for atmospheric pressure changes from time to time with a range of change about 50 mBar. Fuller descriptions such as "gauge pressure of 2 bars" or "2-bar gauge" are recommended.
[2]
[13]
See also
[
edit
]
References
[
edit
]
- This article incorporates material from the
Citizendium
article "
Bar (unit)
", which is licensed under the
Creative Commons Attribution-ShareAlike 3.0 Unported License
but not under the
GFDL
.
- ^
a
b
Le Systeme international d’unites
[
The International System of Units
]
(PDF)
(in French and English) (9th ed.), International Bureau of Weights and Measures, 2019,
ISBN
978-92-822-2272-0
.
- ^
a
b
c
British Standard
BS 350:2004
Conversion Factors for Units
.
- ^
"Nomenclature of the unit of absolute pressure, Charles F. Marvin, 1918"
(PDF)
.
noaa.gov
.
Archived
(PDF)
from the original on 29 April 2017
. Retrieved
6 May
2018
.
- ^
NIST Special Publication 1038
Archived
2016-03-19 at the
Wayback Machine
, Sec. 4.3.2;
NIST Special Publication 811, 2008 edition
Archived
2016-06-03 at the
Wayback Machine
, Sec. 5.2
- ^
International Astronomical Union Style Manual. Comm. 5 in IAU Transactions XXB, 1989, Table 6
- ^
Grave (unit)
- ^
Canada, Environment (2013-04-16).
"Canadian Weather at a Glance - Environment Canada"
.
www.weatheroffice.gc.ca
.
Archived
from the original on 2 January 2018
. Retrieved
6 May
2018
.
- ^
Canada, Environment (2013-04-16).
"Canadian Weather - Environment Canada"
.
www.weatheroffice.gc.ca
.
Archived
from the original on 2 January 2018
. Retrieved
6 May
2018
.
- ^
US government atmospheric pressure map
- ^
The Weather Channel
- ^
Scientific Committee on Oceanic Research (1983).
"Algorithms for computation of fundamental properties of seawater"
(PDF)
.
Archived
(PDF)
from the original on 2015-04-12
. Retrieved
2014-05-11
.
- ^
NFPA 68 Standard on Explosion Protection by Deflagration Venting
(2023 ed.).
- ^
"What do the letters 'g' and 'a' denote after a pressure unit? (FAQ - Pressure) : FAQs : Reference : National Physical Laboratory"
.
Archived
from the original on 7 April 2016
. Retrieved
7 February
2016
.
External links
[
edit
]
Pressure units
|
Pascal
|
Bar
|
Technical atmosphere
|
Standard atmosphere
|
Torr
|
Pound per square inch
|
(Pa)
|
(bar)
|
(at)
|
(atm)
|
(Torr)
|
(lbf/in
2
)
|
1 Pa
|
?
|
1 Pa =
10
?5
bar
|
1 Pa =
1.0197
×
10
?5
at
|
1 Pa =
9.8692
×
10
?6
atm
|
1 Pa =
7.5006
×
10
?3
Torr
|
1 Pa =
0.000
145
037
737
730
lbf/in
2
|
1 bar
|
10
5
|
?
|
=
1.0197
|
=
0.986
92
|
=
750.06
|
=
14.503
773
773
022
|
1 at
|
98
066
.5
|
0.980
665
|
?
|
0.967
841
105
3541
|
735.559
2401
|
14.223
343
307
1203
|
1 atm
|
≡
101
325
|
≡
1.013
25
|
1.0332
|
?
|
760
|
14.695
948
775
5142
|
1 Torr
|
133.322
368
421
|
0.001
333
224
|
0.001
359
51
|
1
/
760
?
0.001
315
789
|
?
|
0.019
336
775
|
1 lbf/in
2
|
6
894
.757
293
168
|
0.068
947
573
|
0.070
306
958
|
0.068
045
964
|
51.714
932
572
|
?
|