pascal

 

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 hectopascal (1 hPa = 100 Pa).^[1] In other contexts, the kilopascal is more commonly used, for example on bicycle tire labels^[2]. One hectopascal corresponds to about 0.1% and one kilopascal to about 1% of atmospheric pressure (near sea level).

Definition

1 pascal (Pa) ≡ 1 N·m⁻² ≡ 1 J·m⁻³ ≡ 1 kg·m⁻¹·s⁻²

Pressure Units

	pascal (Pa)	bar (bar)	technical atmosphere (at)	atmosphere (atm)	torr (mmHg)	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 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. [1]

This SI unit is named after Blaise Pascal. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (Pa). But when an SI unit is spelled out, it should always be written in lowercase (pascal), unless it begins a sentence or is the name "degree Celsius". — Based on The International System of Units, section 5.2.

Miscellany

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).^[3]

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.

1 hectopascal (hPa) ≡ 100 Pa ≡ 1 mbar.

1 kilopascal (kPa) ≡ 1000 Pa ≡ 10 hPa.

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

• Orders of magnitude (pressure)

References

Further reading

• Gene Nygaard: Hectopascals? No! No! Never! – web page arguing against the use of hPa in metrology, in favour of kPa

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