Pascal (unit)

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pascal
Psidial.jpg
A pressure gauge reading in psi (red scale) and kPa (black scale)
General information
Unit system SI unit
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:   1.450×10−4  psi
    atmosphere:   9.869×10−6 atm
    bar:   10−5 bar
    barye (CGS unit)   10 Ba

The pascal (symbol: Pa) is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is defined as one newton per square metre [1] and is equivalent to 10 barye (Ba) in the CGS system. The unit of measurement called standard atmosphere (atm) is defined as 101,325 Pa. [2]

Contents

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. Meteorological observations typically report atmospheric pressure in hectopascals per the recommendation of the World Meteorological Organization. Reports in the United States typically use inches of mercury [3] or millibars, [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 101,325 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.

In measurements of sound pressure or loudness of sound, one pascal is equal to 94 decibels sound pressure level (SPL). The quietest sound a human can hear, known as the threshold of hearing, is 0 dB SPL, or 20 μPa.

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

In medicine, blood pressure is measured in millimeters of mercury (mm Hg). The normal adult blood pressure is less than 120 mm Hg systolic BP (SBP) and less than 80 mm Hg diastolic BP (DBP). [16] Convert mm  Hg to SI units as follows: 1 mm Hg = 0.13332 kPa. [17] Hence normal blood pressure in SI units is less than 16.0 kPa SBP and less than 10.7 kPa DBP.

Hectopascal and millibar units

The units of atmospheric pressure commonly used in meteorology were formerly the bar, 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. [18] [19] [20] [21] [22] [23] [24] 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. [25] [26]

Many countries also use millibars. In practically all other fields, the kilopascal (1000 pascals) is used instead. [27]

Multiples and submultiples

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

MultiplesSub-multiples
ValueNameSymbolValueNameSymbol
101 PadecapascaldaPa10−1 PadecipascaldPa
102 PahectopascalshPa10−2 PacentipascalcPa
103 PakilopascalkPa10−3 PamillipascalmPa
106 PamegapascalMPa10−6 PamicropascalμPa
109 PagigapascalGPa10−9 PananopascalnPa
1012 PaterapascalTPa10−12 PapicopascalpPa
1015 PapetapascalPPa10−15 PafemtopascalfPa
1018 PaexapascalEPa10−18 PaattopascalaPa
1021 PazettapascalZPa10−21 PazeptopascalzPa
1024 PaiottapascalYPa10−24 PaioctopascalyPa

See also

Related Research Articles

Pressure measurement Analysis of force applied by a fluid on a surface

Pressure measurement is the analysis 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 in an integral unit are called pressure meters or pressure gauges or vacuum gauges. A manometer is a good example, as it uses the surface area and weight of a column of liquid to both measure and indicate pressure. Likewise the widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.

Pressure Force distributed continuously 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 are 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 barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 101,325 Pa, which is equivalent to 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.

A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis to help find surface troughs, pressure systems and frontal boundaries.

The Dobson unit (DU) is a unit of measurement of the amount of a trace gas in a vertical column through the Earth's atmosphere. It originated, and continues to be primarily used in respect to, atmospheric ozone, whose total column amount, usually termed "total ozone", and sometimes "column abundance", is dominated by the high concentrations of ozone in the stratospheric ozone layer.

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.

Bar (unit) Unit of pressure equal to 100,000 Pa

The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or 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.

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 atmospheric pressure at sea level.

Millimetre of mercury 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. It is denoted mmHg or mm Hg.

Inch of mercury is a 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 used both in the natural gas industry to represent an amount of natural gas and in other industries where other gases are used. It is the unit commonly used when following the US customary system, a collection of standards set by the US National Institute of Standards and Technology. Another unit used for the same purpose is the standard cubic meter, customary when using SI units.

Kilogram-force per square centimetre Unit of pressure

A kilogram-force per centimetre square (kgf/cm2), often just kilogram per square centimetre (kg/cm2), or kilopond per centimetre square 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.

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The Armstrong limit or Armstrong's line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds. On Earth, the limit is around 18–19 km above sea level, above which atmospheric air pressure drops below 0.0618 atm. The U.S. Standard Atmospheric model sets the Armstrong pressure at an altitude of 63,000 feet (19,202 m).

Metric units are units based on the metre, gram or second and decimal multiples or sub-multiples of these. The most widely used examples are the units of the International System of Units (SI). By extension they include units of electromagnetism from the CGS and SI units systems, and other units for which use of SI prefixes has become the norm. Other unit systems using metric units include:

Airwatt or air watt is a measurement unit of the effectiveness of vacuum cleaners which refers to airflow and the amount of power (watts) a vacuum cleaner produces and uses. It can also be referred to as a measurement of the energy per unit time of the air flowing through an opening, which is related to the energy that electricity carries through the power cable (wattage).

Ambient pressure 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.

Pound per square inch Unit of pressure or stress

The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied to an area of one square inch. In SI units, 1 psi is approximately equal to 6895 N/m2.

Metre sea water 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

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  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. American Heart Association. Retrieved 18 May 2020.
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  18. "KNMI - Weer - Waarnemingen" . Retrieved 4 December 2016.
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  20. "DWD". Archived from the original on 10 February 2008. Retrieved 20 December 2006.
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  22. "MDD". Archived from the original on 6 May 2006.
  23. NOAA
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  27. Ambler Thompson (Editor) Guide for the Use of the International System of Units (SI) (rev. ): The ... , p. 66, at Google Books