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The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Young's modulus and tensile strength). It is a measure of perpendicular force per unit area i.e. equivalent to one newton per square meter or one Joule per cubic meter. In everyday life, the pascal is perhaps best known from meteorological air-pressure reports, where it occurs in the form of kilopascal (1 kPa = 1000 Pa). In other contexts, the kilopascal is more commonly used, for example on bicycle tire labels. One hectopascal corresponds to about 0.1% and one kilopascal to about 1% of atmospheric pressure (near sea level): one hectopascal is thus equivalent to a millibar.
Definition
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 |
1.0197×10−5 |
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 |
0.96784 |
735.56 |
14.223 |
| 1 atm |
101,325 |
1.01325 |
1.0332 |
≡ 1 atm |
760 |
14.696 |
| 1 torr |
133.322 |
1.3332×10−3 |
1.3595×10−3 |
1.3158×10−3 |
≡ 1 Torr; = 1 mmHg |
19.337×10−3 |
| 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 = 10.197×10−6 at = 9.8692×10−6 atm, etc.
Note: mmHg is an abbreviation for millimetres of mercury.
Origin
The unit is named after Blaise Pascal, the eminent French mathematician, physicist, and philosopher noted for his experiments with a barometer, an instrument to measure air pressure. The name pascal was adopted for the SI unit newton per square metre by the 14th CGPM in 1971.
| This SI unit is named after Blaise Pascal. As with all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (Pa). When an SI unit is spelled out in English, it should always begin with a lowercase letter (pascal), except for at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because of the "d".
— Based on The International System of Units, section 5.2.
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Miscellaneous
Standard atmospheric pressure is 101,325 Pa = 101.325 kPa = 1013.25 hPa = 1013.25 mbar = 760 Torr. This definition is used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.
In 1985, IUPAC recommended that standard atmospheric pressure should be harmonized to 100,000 Pa = 1 bar = 750 Torr. The same definition is used in the compressor and the pneumatic tool industries (ISO 2787).
The Unicode computer character set has dedicated symbols ㎩ (U+33A9) for Pa and ㎪ (U+33AA) for kPa, but these exist merely for backward-compatibility with some older ideographic character-sets and are therefore deprecated.
Uses
Meteorologists worldwide have for a long time measured atmospheric pressure in millibars. After the introduction of SI units, many preferred to preserve the customary pressure figures. Therefore, meteorologists use hectopascals (hPa) today for air pressure, which are equivalent to millibars, while similar pressures are given in kilopascals in practically all other fields, where the hecto prefix is hardly ever used. Since official metrication, meteorologists in Canada use kilopascals (kPa), see for example CTV News, weather; current conditions in Montréal and CBC weather, current conditions in Montréal, although in some other countries hectopascals are still in use, see for example KNMI, KMI, DWD, JMA, MDD and NOAA.
In the former mts system, the unit of pressure is the pièze (symbol pz), which is equal to one kilopascal.
In the former cgs system, the unit of pressure is the barye (symbol ba), which is equal to one decipascal.
Vehicle owners' guides now specify tire inflation in kilopascals.
See also
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