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 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% of
atmospheric pressure slightly above sea level; one
kilopascal is about 1% of atmospheric pressure. One
hectopascal is equivalent to one
millibar; one standard
atmosphere is exactly equal to 101.325 kPa or 1013.25 hPa or 101325 Pa. The correspondent Imperial unit is
pounds per square inch (psi).
Definition
The pascal can be expressed using
SI derived units, or alternatively solely
SI base units, as:
:
Where
N is the
newton,
m is the
metre,
kg is the
kilogram, and
s is the
second.
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 14
th CGPM in 1971.
Miscellaneous
Standard atmospheric pressure is 101,325 Pa = 101.325 kPa = 1013.25 hPa = 1013.25
mbar = 760
Torr = 14.696
psi. This definition is used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.
In 1985 the IUPAC recommended that the standard for 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 () for Pa and () 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 = N/mm2) or gigapascals (GPa = kN/mm2) are commonly used to measure stiffness of materials (see Young's modulus).
Other, older units of measure occasionally used for pressure are millimeters of mercury (Torr) and millimetres of water (1 mm H2O = 9.80665 Pa).
Hectopascal and millibar units
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. Consequently, today many meteorologists prefer
hectopascals (hPa) 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 rarely used. Since official
metrication, meteorologists in Canada use
kilopascals (kPa), see for example
CTV News, weather; current conditions in Montreal and
Environment 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)
References
Category:SI derived units
Category:Units of pressure
Category:Blaise Pascal
