Pascal (unit)

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.

On Earth, standard atmospheric pressure is 101,325 Pa. 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 101,325 Pa. The corresponding 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:


 * $${\rm 1~Pa = 1~\frac{N}{m^2} = 1~\frac{kg}{m \cdot s^2}}$$

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 14th CGPM in 1971. 

Miscellaneous
Standard atmospheric pressure is 101325 Pa = 101.325 kPa = 1013.25 hPa = 1.01325 bar = 0.101325 MPa = 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 has largely replaced the pounds per square inch (psi) unit, except in some countries that still use the Imperial measurement system.

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

In materials science, megapascals (MPa = N/mm2) or gigapascals (GPa = kN/mm2) are commonly used to measure stiffness or tensile strength of materials. Examples of (approximate) tensile modulus for several common substances include nylon at 2-4; hemp (fibre) at 58, aluminium at 69; tooth enamel at 83, copper at 117, steel at approximately 200 GPa, silkworm silk at 500, and diamond at 1220.

The pascal is also equivalent to the SI unit of energy density, J/m3. This applies not only to the thermodynamics of pressurized gasses, but also to the energy density of electric, magnetic, and gravitational fields.

Other, older units of measure occasionally used for pressure are millimeters of mercury (Torr) and millimetres of water (1.0 mmH2O = 9.80665 Pa).

In the cgs system, the unit of pressure is the barye (symbol ba), which is equal to one decipascal. The older kilogram-force per square centimetre corresponds to 98.0665 kPa, but is it often rounded off 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.

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

Hectopascal and millibar units
Meteorologists worldwide have for a long time measured atmospheric pressure in bars, which was originally equivalent to the average air pressure on Earth; the bar was divided into a thousand millibars to provide the precision meteorologists require. After the introduction of SI units, many preferred to preserve the customary pressure figures. Consequently, the bar was redefined as 100,000 pascals, which is only slightly lower than standard air pressure on Earth. 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, since the hecto prefix is rarely used. Since official metrication, meteorologists in Canada use kilopascals (kPa), although in some other countries hectopascals are still in use.

As of 17 November 2011 the hectopascal is used in aviation as the altimeter setting.


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


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