Pascal (unit)

Last updated

pascal
Psidial.jpg
A pressure gauge reading in psi (red scale) and kPa (black scale)
General information
Unit system SI
Unit of pressure or stress
SymbolPa
Named after Blaise Pascal
Conversions
1 Pa in ...... is equal to ...
    SI base units:    kg m −1 s −2
    US customary units:   1.45038×10−4  psi
    atmosphere:   9.86923×10−6 atm
    bar:   10−5 bar
    barye (CGS unit)   10 Ba

The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI). It is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is a SI coherent derived unit defined as one newton per square metre (N/m2). [1] It is also equivalent to 10 barye (10 Ba) in the CGS system. Common multiple units of the pascal are the hectopascal (1 hPa = 100 Pa), which is equal to one millibar, and the kilopascal (1 kPa = 1000 Pa), which is equal to one centibar.

Contents

The unit of measurement called standard atmosphere (atm) is defined as 101,325 Pa. [2] Meteorological observations typically report atmospheric pressure in hectopascals per the recommendation of the World Meteorological Organization, thus a standard atmosphere (atm) or typical sea-level air pressure is about 1013 hPa. Reports in the United States typically use inches of mercury [3] or millibars (hectopascals). [4] [5] In Canada these reports are given in kilopascals. [6]

Etymology

The unit is named after Blaise Pascal, noted for his contributions to hydrodynamics and hydrostatics, and experiments with a barometer. The name pascal was adopted for the SI unit newton per square metre (N/m2) by the 14th General Conference on Weights and Measures in 1971. [7] [8]

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, s is the second, and J is the joule. [9]

One pascal is the pressure exerted by a force of magnitude one newton perpendicularly upon an area of one square metre.

Standard units

The unit of measurement called an atmosphere or a standard atmosphere (atm) is 101325 Pa (101.325 kPa). [10] This value is often used as a reference pressure and specified as such in some national and international standards, such as the International Organization for Standardization's ISO 2787 (pneumatic tools and compressors), ISO 2533 (aerospace) and ISO 5024 (petroleum). In contrast, International Union of Pure and Applied Chemistry (IUPAC) recommends the use of 100 kPa as a standard pressure when reporting the properties of substances. [11]

Unicode has dedicated code-points U+33A9SQUARE PA and U+33AASQUARE KPA in the CJK Compatibility block, but these exist only for backward-compatibility with some older ideographic character-sets and are therefore deprecated. [12] [13]

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 or the US customary system, including the United States.

Geophysicists use the gigapascal (GPa) in measuring or calculating tectonic stresses and pressures within the Earth.

Medical elastography measures tissue stiffness non-invasively with ultrasound or magnetic resonance imaging, and often displays the Young's modulus or shear modulus of tissue in kilopascals.

In materials science and engineering, the pascal measures the stiffness, tensile strength and compressive strength of materials. In engineering the megapascal (MPa) is the preferred unit for these uses, because the pascal represents a very small quantity.

Approximate Young's modulus for common substances [14]
MaterialYoung's modulus
(GPa)
Nylon 6 2–4
Hemp fibre35
Aluminium 69
Tooth enamel 83
Copper 117
Structural steel 200
Diamond 1220

The pascal is also equivalent to the SI unit of energy density, the joule per cubic metre. This applies not only to the thermodynamics of pressurised gases, but also to the energy density of electric, magnetic, and gravitational fields.

The pascal is used to measure sound pressure. Loudness is the subjective experience of sound pressure and is measured as a sound pressure level (SPL) on a logarithmic scale of the sound pressure relative to some reference pressure. For sound in air, a pressure of 20 μPa is considered to be at the threshold of hearing for humans and is a common reference pressure, so that its SPL is zero.

The airtightness of buildings is measured at 50 Pa. [15]

In medicine, blood pressure is measured in millimeters of mercury (mmHg, very close to one Torr). The normal adult blood pressure is less than 120 mmHg systolic BP (SBP) and less than 80 mmHg diastolic BP (DBP). [16] Convert mmHg to SI units as follows: 1 mmHg = 0.13332 kPa. Hence normal blood pressure in SI units is less than 16.0 kPa SBP and less than 10.7 kPa DBP. These values are similar to the pressure of water column of average human height; so pressure has to be measured on arm roughly at the level of the heart.

Hectopascal and millibar units

The units of atmospheric pressure commonly used in meteorology were formerly the bar, (100,000 Pa) which was close to the average air pressure on Earth, and the millibar. Since the introduction of SI units, meteorologists generally measure pressures in hectopascals (hPa) unit, equal to 100 pascals or 1 millibar. [17] [18] [19] [20] [21] [22] [23] Exceptions include Canada, which uses kilopascals (kPa). In many other fields of science, prefixes that are a power of 1000 are preferred, which excludes the hectopascal from use. [24] [25]

Many countries also use millibars. In practically all other fields, the kilopascal (1000 pascals) is used instead. [26]

Multiples and submultiples

Decimal multiples and sub-multiples are formed using standard SI units.

MultiplesSub-multiples
ValueNameSymbolValueNameSymbol
101 PadecapascaldaPa10−1 PadecipascaldPa
102 PahectopascalhPa10−2 PacentipascalcPa
103 PakilopascalkPa10−3 PamillipascalmPa
105 Pabar (non-SI unit)bar10−6 PamicropascalμPa
106 PamegapascalMPa10−9 PananopascalnPa
109 PagigapascalGPa10−12 PapicopascalpPa
1012 PaterapascalTPa10−15 PafemtopascalfPa
1015 PapetapascalPPa10−18 PaattopascalaPa
1018 PaexapascalEPa10−21 PazeptopascalzPa
1021 PazettapascalZPa10−24 PayoctopascalyPa
1024 PayottapascalYPa10−27ParontopascalrPa
1027 ParonnapascalRPa10−30 PaquectopascalqPa
1030 PaquettapascalQPa

See also

Related Research Articles

<span class="mw-page-title-main">Pressure measurement</span> Analysis of force applied by a fluid on a surface

Pressure measurement is the measurement of an applied force by a fluid on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure and display pressure mechanically are called pressure gauges,vacuum gauges or compound gauges. The widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.

<span class="mw-page-title-main">Pressure</span> Force distributed over an area

Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure.

Standard temperature and pressure (STP) are various standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.

The torr is a unit of pressure based on an absolute scale, defined as exactly 1/760 of a standard atmosphere. Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa).

Atmospheric pressure, also known as air pressure or barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

Geopotential height or geopotential altitude is a vertical coordinate referenced to Earth's mean sea level that represents the work involved in lifting one unit of mass over one unit of length through a hypothetical space in which the acceleration of gravity is assumed constant. In SI units, a geopotential height difference of one meter implies the vertical transport of a parcel of one kilogram; adopting the standard gravity value, it corresponds to a constant work or potential energy difference of 9.80665 joules.

Hecto is a decimal unit prefix in the metric system denoting a factor of one hundred. It was adopted as a multiplier in 1795, and comes from the Greek ἑκατόν hekatón, meaning "hundred". In 19th century English it was sometimes spelled "hecato", in line with a puristic opinion by Thomas Young. Its unit symbol as an SI prefix in the International System of Units (SI) is the lower case letter h.

<span class="mw-page-title-main">Bar (unit)</span> Unit of pressure equal to 100,000 Pa

The bar is a metric unit of pressure defined as 100,000 Pa (100 kPa), though not part of the International System of Units (SI). A pressure of 1 bar is slightly less than the current average atmospheric pressure on Earth at sea level. By the barometric formula, 1 bar is roughly the atmospheric pressure on Earth at an altitude of 111 metres at 15 °C.

Given an atmospheric pressure measurement, the pressure altitude is the imputed altitude that the International Standard Atmosphere (ISA) model predicts to have the same pressure as the observed value.

The standard atmosphere is a unit of pressure defined as 101325 Pa. It is sometimes used as a reference pressure or standard pressure. It is approximately equal to Earth's average atmospheric pressure at sea level.

<span class="mw-page-title-main">Millimetre of mercury</span> Manometric unit of pressure

A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly 133.322387415 pascals or exactly 133.322 pascals. It is denoted mmHg or mm Hg.

The North American High is an impermanent high-pressure area or anticyclone created by a formative process that occurs when cool or cold dry air settles over North America. During summer, it is replaced with an Arctic Low, or a North American Low should it move over continental land.

Inch of mercury is a non-SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.

A standard cubic foot (scf) is a unit representing the amount of gas (such as natural gas) contained in a volume of one cubic foot at reference temperature and pressure conditions. It is the unit commonly used when following the customary system, a collection of standards set by the National Institute of Standards and Technology. Another unit used for the same purpose is the standard cubic metre (Sm3), derived from SI units, representing the amount of gas contained in a volume of one cubic meter at different reference conditions. The reference conditions depend on the type of gas and differ from other standard temperature and pressure conditions.

<span class="mw-page-title-main">Kilogram-force per square centimetre</span> Unit of pressure

A kilogram-force per centimetre square (kgf/cm2), often just kilogram per square centimetre (kg/cm2), or kilopond per centimetre square (kp/cm2) is a deprecated unit of pressure using metric units. It is not a part of the International System of Units (SI), the modern metric system. 1 kgf/cm2 equals 98.0665 kPa (kilopascals). It is also known as a technical atmosphere.

<span class="mw-page-title-main">Ambient pressure</span> Pressure of the surrounding medium

The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, in contact with the object.

<span class="mw-page-title-main">Celsius</span> Scale and unit of measurement for temperature

The degree Celsius is the unit of temperature on the Celsius scale, one of two temperature scales used in the International System of Units (SI), the other being the closely related Kelvin scale. The degree Celsius can refer to a specific temperature on the Celsius scale or to a difference or range between two temperatures. It is named after the Swedish astronomer Anders Celsius (1701–1744), who proposed the first version of it in 1742. The unit was called centigrade in several languages for many years. In 1948, the International Committee for Weights and Measures renamed it to honor Celsius and also to remove confusion with the term for one hundredth of a gradian in some languages. Most countries use this scale.

<span class="mw-page-title-main">Pound per square inch</span> Unit of pressure or stress

The pound per square inch or, more accurately, pound-force per square inch, is a unit of measurement of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force with magnitude of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately 6,895 pascals.

<span class="mw-page-title-main">Torricelli's experiment</span>

Torricelli's experiment was invented in Pisa in 1643 by the Italian scientist Evangelista Torricelli (1608-1647). The purpose of his experiment is to prove that the source of vacuum comes from atmospheric pressure.

<span class="mw-page-title-main">Metre sea water</span> Unit of pressure equal to one tenth of a bar

The metresea water (msw) is a metric unit of pressure used in underwater diving. It is defined as one tenth of a bar.

References

  1. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 118, ISBN   92-822-2213-6, archived (PDF) from the original on 4 June 2021, retrieved 16 December 2021
  2. "Definition of the standard atmosphere". BIPM . Retrieved 16 February 2015.
  3. "National Weather Service glossary page on inches of mercury".
  4. "US government atmospheric pressure map".
  5. "The Weather Channel".
  6. Canada, Environment (16 April 2013). "Canadian Weather - Environment Canada". weather.gc.ca.
  7. bipm.fr. Archived 30 June 2007 at the Wayback Machine .
  8. Minutes of the 14. General Conference on Weights and Measures, 1971, p. 78.
  9. Table 3 (Section 2.2.2). Archived 18 June 2007 at the Wayback Machine . SI Brochure. International Bureau of Weights and Measures.
  10. "Resolution 4 of the 10th meeting of the CGPM". Conférence Générale des Poids et Mesures. 1954. Archived from the original on 30 March 2021. Retrieved 5 April 2010.
  11. IUPAC.org, Gold Book, Standard Pressure
  12. "CJK Compatibility" (PDF). 2015. Retrieved 21 February 2016.
  13. The Unicode Standard, Version 8.0.0. Mountain View, CA: The Unicode Consortium. 2015. ISBN   978-1-936213-10-8 . Retrieved 21 February 2016.
  14. "Tensile Modulus – Modulus of Elasticity or Young's Modulus – for some common Materials" . Retrieved 16 February 2015.
  15. "Chapter 7 ResNet Standards: ResNet National Standard for Home Energy Audits" (PDF). ResNet. 2010. Archived from the original (PDF) on 26 July 2011. Retrieved 3 March 2011.
  16. "BP Guideline | Target:BP". American Heart Association. Retrieved 18 May 2020.
  17. "KNMI – Weer – Waarnemingen" . Retrieved 4 December 2016.
  18. "Comment convertir la pression? – IRM" . Retrieved 4 December 2016.
  19. "DWD". Archived from the original on 10 February 2008. Retrieved 20 December 2006.
  20. "Japan Meteorological Agency – Weather Maps" . Retrieved 4 December 2016.
  21. "MDD". Archived from the original on 6 May 2006.
  22. NOAA
  23. United Kingdom, Met Office. "Key to symbols and terms" . Retrieved 4 December 2016.
  24. "CTV News, weather; current conditions in Montreal". Archived from the original on 4 June 2011.
  25. Canada, Environment. "Montréal, QC – 7 Day Forecast – Environment Canada". Archived from the original on 30 November 2017. Retrieved 4 December 2016.
  26. Ambler Thompson (Editor) Guide for the Use of the International System of Units (SI) (rev. ): The ... , p. 66, at Google Books