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

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] The unit of measurement called standard atmosphere (atm) is defined as 101325 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 forecasts typically report atmospheric pressure in hectopascals per the recommendation of the World Meteorological Organization. Forecasts in the United States typically use millibars, [3] [4] in Canada these reports are given in kilopascals. [5]

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. [6] [7]

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. [8]

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). [9] 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. [10]

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. [11] [12]

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 use, because the pascal represents a very small quantity, the megapascal (MPa) is the preferred unit for these uses.

Approximate Young's modulus for common substances [13]
MaterialYoung's modulus
nylon 6 2–4 GPa
hemp fibre35 GPa
aluminium 69 GPa
tooth enamel 83 GPa
copper 117 GPa
structural steel 200 GPa
diamond 1220 GPa

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. [14]

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. [15] [16] [17] [18] [19] [20] [21] 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. [22] [23]

Many countries also use millibars. In practically all other fields, the kilopascal (1000 pascals) is used instead. [24]

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.

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

Atmospheric pressure, sometimes also called barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 101,325 Pa, 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.

Newton (unit) SI unit of force

The newton is the International System of Units (SI) derived unit of force. It is named after Isaac Newton in recognition of his work on classical mechanics, specifically Newton's second law of motion.

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 hekaton, 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.

Cloud base

A cloud base is the lowest altitude of the visible portion of a cloud. It is traditionally expressed either in metres or feet above mean sea level or above a planetary surface, or as the pressure level corresponding to this altitude in hectopascals.

A decametre or dekameter, symbol dam, is a unit of length in the metric system equal to ten metres; it is very rarely used.

Bar (unit) non-SI unit of pressure

The bar is a metric unit of pressure, but is not approved as part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), which is slightly less than the current average atmospheric pressure on Earth at sea level.

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

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.

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).

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).

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

Celsius Scale and unit of measurement for temperature

The Celsius scale, also known as the centigrade scale, is a temperature scale used by the International System of Units (SI). As an SI derived unit, it is used worldwide. In the United States, the Bahamas, Belize, the Cayman Islands and Liberia however, Fahrenheit remains the preferred scale for everyday temperature measurement. The degree Celsius can refer to a specific temperature on the Celsius scale or a unit to indicate a difference between two temperatures or an uncertainty. It is named after the Swedish astronomer Anders Celsius (1701–1744), who developed a similar temperature scale. Before being renamed to honor Anders Celsius in 1948, the unit was called centigrade, from the Latin centum, which means 100, and gradus, which means steps.

Pounds 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.

Introduction to the metric system

The metric system was developed during the French Revolution to replace the various measures previously used in France. The metre is the unit of length in the metric system and was originally based on the dimensions of the earth, as far as it could be measured at the time. The litre, is the unit of volume and was defined as one thousandth of a cubic metre. The metric unit of mass is the kilogram and it was defined as the mass of one litre of water. The metric system was, in the words of French philosopher Marquis de Condorcet, "for all people for all time".

Metre sea water Unit of pressure equal to one tenth of a bar

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