Pascal (unit)

Pascal
A pressure gauge reading in psi (red scale) and kPa (black scale)
General information
Unit system	SI derived unit
Unit of	Pressure / Stress
Symbol	Pa
Named after	Blaise Pascal
In SI base units:	1 Pa = 1 kg/(m.s2)

The pascal (symbol: Pa) is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer and Catholic philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre. In everyday life, the pascal is perhaps best known from meteorological barometric pressure reports, where it occurs in the form of hectopascals (1 hPa ≡ 100 Pa) or kilopascals (1 kPa ≡ 1000 Pa). In other contexts, the kilopascal is 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 equivalent to one millibar; one standard atmosphere is exactly equal to 1013.25 hPa.

Definition

1 pascal (Pa) = 1 N/m² = 1 kg/(m·s²)

Pressure units

	Pascal	Bar	Technical atmosphere	Standard atmosphere	Torr	Pound per square inch
	(Pa)	(bar)	(at)	(atm)	(Torr)	(lbf/in2)
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 = 0.000145037737730 lbf/in2
1 bar	105	—	= 1.0197	= 0.98692	= 750.06	= 14.503773773022
1 at	98066.5	0.980665	—	0.9678411053541	735.5592401	14.2233433071203
1 atm	≡ 101325	≡ 1.01325	1.0332	—	760	14.6959487755142
1 Torr	133.322368421	0.001333224	0.00135951	⁠1/760⁠ ≈ 0.001315789	—	0.019336775
1 lbf/in2	6894.757293168	0.068947573	0.070306958	0.068045964	51.714932572	—

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]

The pascal is named after Blaise Pascal. As with every SI unit named for a person, its symbol starts with an upper case letter (Pa), but when written in full, it follows the rules for capitalisation of a common noun; i.e., pascal becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

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

The pascal (Pa) or kilopascal (kPa) as a unit of pressure measurement is widely used throughout the world and largely replaces the pounds per square inch (psi) unit, except in some countries still using the Imperial measurement system.

Tectonophysicists use the gigapascal (GPa) in measuring or calculating tectonic forces within the earth.

In Solid Mechanics, megapascals (MPa) or gigapascals (GPa) are commonly used to measure stiffness of materials (see Young's modulus).

Another unit for pressure measurement in common use today is millimetres of water (1 mm H₂O = 9.80665 Pa).

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 Montreal and Environtment Canada weather, current conditions in Montreal, 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 ≡ 10 mbar.

In the cgs system, the unit of pressure is the barye (symbol ba), which is equal to one decipascal. The older kg(force)/cm² corresponds to 98.0665 kPa, but is often rounded to 100 kPa in practice.

In the former mts system, the unit of pressure is the pièze (symbol pz), which is equal to one kilopascal.

Vehicle owners' guides now specify tire inflation in kilopascals.

Airtightness testing of buildings is measured at 50 Pa or 0.2 inches of water.

See also

• Pascal's law
• Orders of magnitude (pressure)