Pascal (unit)

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Pascal
A pressure gauge reading in psi (red scale) and kPa (black scale)
Unit information
Unit system SI derived unit
Unit of Pressure or 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,[1] 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 meter.

Common multiple units of the pascal are the hectopascal (1 hPa ≡ 100 Pa), kilopascal (1 kPa ≡ 1000 Pa), megapascal (1 MPa ≡ 1,000,000 Pa), and gigapascal (1 GPa ≡ 1,000,000,000 Pa).

On Earth, standard atmospheric pressure is 101.325 kPa. Meteorological barometric pressure reports typically report atmospheric pressure in hectopascals.[2] The kilopascal is used in other applications such as inflation guidance markings on bicycle tyres.[3] 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 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}}}}}}[4]

Where N is the newton, m is the metre, kg is the kilogram and s is the second.

This SI unit is named after Blaise Pascal. As with every International System of Units (SI) unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (Pa). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (pascal), except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. —Based on The International System of Units, section 5.2.

Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pounds per square inch
(Pa) (bar) (at) (atm) (Torr) (psi)
1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 1.450377×10−4
1 bar 105 ≡ 106 dyn/cm2 1.0197 0.98692 750.06 14.50377
1 at 0.980665 ×105 0.980665 ≡ 1 kp/cm2 0.9678411 735.5592 14.22334
1 atm 1.01325 ×105 1.01325 1.0332 ≡ p0 ≡ 760 14.69595
1 Torr 133.3224 1.333224×10−3 1.359551×10−3 1.315789×10−3 ≈ 1 mmHg 1.933678×10−2
1 psi 6.8948×103 6.8948×10−2 7.03069×10−2 6.8046×10−2 51.71493 ≡ 1 lbF/in2

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 (N/m2) by the 14th CGPM in 1971.[5]

Miscellaneous

Standard atmospheric pressure is 101 325 Pa
= 101.325 kPa
= 1013.25  hPa
= 1.01325 bar
= 1013.25 mbar
= 0.101325 MPa
= 760 Torr[6]
= 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 ≈ 750.06 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 has largely replaced the pounds per square inch (psi) unit, except in some countries that still use the Imperial measurement system, including the United States.

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

Transient elastography (FibroScan), a non-invasive method for estimating the extent of liver fibrosis, measures liver stiffness in kilopascals.

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) Young’s moduli for several common substances (in gigapascals) include nylon at 2–4; hemp (fibre) at 58, aluminium at 69; tooth enamel at 83, copper at 117, steel at approximately 200, 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,[7] 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.[8]

Hectopascal and millibar units

Main article: Bar (unit)

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),[9][10] although in some other countries hectopascals are still in use.[11][12][13][14][15][16][17]

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.

See also

Notes and references

  1. ↑ International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 118, ISBN 92-822-2213-6 
  2. ↑ U.S. Federal Meteorological Handbook
  3. ↑ ISO 5775: Bicycle tires and rims
  4. ↑ Table 3 (Section 2.2.2), SI Brochure, International Bureau of Weights and Measures
  5. ↑ bipm.fr
  6. ↑ "Resolution 4 of the 10th meeting of the CGPM". Conférence Générale des Poids et Mesures (CGPM). 1954. Retrieved 5 April 2010. 
  7. ↑ SensorsOne, Pressure unit conversion and converter. 2010.03.22.
  8. ↑ "Chapter 7 ResNet Standards: ResNet National Standard for Home Energy Audits". ResNet. 2010. Retrieved 3 March 2011. 
  9. ↑ CTV News, weather; current conditions in Montreal
  10. ↑ Environment Canada weather, current conditions in Montreal
  11. ↑ KNMI
  12. ↑ KMI
  13. ↑ DWD
  14. ↑ JMA
  15. ↑ MDD
  16. ↑ NOAA
  17. ↑ UK Met Office
Base units
Derived units
Accepted for use
with SI
See also
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