# Pascal (unit)

The pascal (symbol: Pa) is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus and ultimate tensile strength. It is defined as one newton per square metre.[1] It is named after the French polymath Blaise Pascal.

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.

The unit of measurement called standard atmosphere (atm) is defined as 101325 Pa.[2] Meteorological reports in the United States typically state atmospheric pressure in millibars.[3][4] In Canada these reports are given in kilopascals.[5]

Pascal
A pressure gauge reading in psi (red scale) and kPa (black scale)
General information
Unit systemSI derived unit
Unit ofPressure or stress
SymbolPa
Named afterBlaise Pascal
Conversions
1 Pa in ...... is equal to ...
SI base units:   kgm−1s−2
US customary units:   1.450 × 10−4 psi
atmosphere:   9.869 × 10−6 atm
bar:   10−5 bar

## 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]

## Definition

The pascal can be expressed using SI derived units, or alternatively solely SI base units, as:

${\displaystyle {\rm {1~Pa=1~{\frac {N}{m^{2}}}=1~{\frac {kg}{m\cdot s^{2}}}=1~{\frac {J}{m^{3}}}}}}$

where N is the newton, m is the metre, kg is the kilogram, s is the second, and J is the joule.[7]

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).[8] 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.[9]

Unicode has dedicated code-points U+33A9 SQUARE PA and U+33AA SQUARE KPA in the CJK Compatibility block, but these exist only for backward-compatibility with some older ideographic character-sets and are therefore deprecated.[10][11]

## 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 [12]
Material Young's modulus
nylon 6 2–4 GPa
hemp fibre 35 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, J/m3. 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 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.[13]

### 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.[14][15][16][17][18][19][20] Exceptions include Canada, which use kilopascals (kPa). In many other fields of science, the SI is preferred, which means Pa with a prefix (in multiples of 1000) is preferred.[21][22]

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

## 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 2017-08-14
2. ^ "Definition of the standard atmosphere". BIPM. Retrieved 2015-02-16.
3. ^ US government atmospheric pressure map
4. ^ The Weather Channel
6. ^ bipm.fr Archived 30 June 2007 at the Wayback Machine
7. ^ Table 3 (Section 2.2.2) Archived 18 June 2007 at the Wayback Machine, SI Brochure, International Bureau of Weights and Measures
8. ^ "Resolution 4 of the 10th meeting of the CGPM". Conférence Générale des Poids et Mesures (CGPM). 1954. Retrieved 2010-04-05.
9. ^ IUPAC.org, Gold Book, Standard Pressure
10. ^ "CJK Compatibility" (PDF). 2015. Retrieved 2016-02-21.
11. ^ "The Unicode Standard, Version 8.0.0". Mountain View, CA: The Unicode Consortium. 2015. ISBN 978-1-936213-10-8. Retrieved 2016-02-21.
12. ^ "Tensile Modulus - Modulus of Elasticity or Young's Modulus - for some common Materials". Retrieved 2015-02-16.
13. ^ "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.
14. ^ "KNMI - Weer - Waarnemingen". Retrieved 4 December 2016.
15. ^ "Comment convertir la pression? - IRM". Retrieved 4 December 2016.
16. ^ DWD
17. ^ "Japan Meteorological Agency - Weather Maps". Retrieved 4 December 2016.
18. ^ MDD Archived 6 May 2006 at the Wayback Machine
19. ^ NOAA
20. ^ United Kingdom, Met Office. "Key to symbols and terms". Retrieved 4 December 2016.
21. ^ CTV News, weather; current conditions in Montreal Archived 4 June 2011 at the Wayback Machine
22. ^ Canada, Environment. "Montréal, QC - 7 Day Forecast - Environment Canada". Retrieved 4 December 2016.

A dead weight tester apparatus uses known traceable weights to apply pressure to a fluid for checking the accuracy of readings from a pressure gauge. A dead weight tester (DWT) is a calibration standard method that uses a piston cylinder on which a load is placed to make an equilibrium with an applied pressure underneath the piston. Deadweight testers are so called primary standards which means that the pressure measured by a deadweight tester is defined through other quantities: length, mass and time.

Typically deadweight testers are used in calibration laboratories to calibrate pressure transfer standards like electronic pressure measuring devices.

Glossary of mechanical engineering

Most of the terms listed in Wikipedia glossaries are already defined and explained within Wikipedia itself. However, glossaries like this one are useful for looking up, comparing and reviewing large numbers of terms together. You can help enhance this page by adding new terms or writing definitions for existing ones.

This glossary of mechanical engineering terms pertains specifically to mechanical engineering and its sub-disciplines. For a broad overview of engineering, see glossary of engineering.

Index of chemistry articles

Chemistry (from Egyptian kēme (chem), meaning "earth") is the physical science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions.Below is a list of chemistry-related articles. Chemical compounds are listed separately at list of organic compounds, list of inorganic compounds or list of biomolecules.

Index of mechanical engineering articles

This is an alphabetical list of articles pertaining specifically to mechanical engineering. For a broad overview of engineering, please see List of engineering topics. For biographies please see List of engineers.

Index of physics articles (P)

The index of physics articles is split into multiple pages due to its size.

List of filename extensions (S–Z)

This alphabetical list of filename extensions contains standard extensions associated with computer files.

PL/SQL

PL/SQL (Procedural Language for SQL) is Oracle Corporation's procedural extension for SQL and the Oracle relational database. PL/SQL is available in Oracle Database (since version 6 - stored PL/SQL procedures/functions/packages/triggers since version 7), TimesTen in-memory database (since version 11.2.1), and IBM DB2 (since version 9.7). Oracle Corporation usually extends PL/SQL functionality with each successive release of the Oracle Database.

PL/SQL includes procedural language elements such as conditions and loops. It allows declaration of constants and variables, procedures and functions, types and variables of those types, and triggers. It can handle exceptions (runtime errors). Arrays are supported involving the use of PL/SQL collections. Implementations from version 8 of Oracle Database onwards have included features associated with object-orientation. One can create PL/SQL units such as procedures, functions, packages, types, and triggers, which are stored in the database for reuse by applications that use any of the Oracle Database programmatic interfaces.

PPU

PPU can refer to:

PPU, the IATA code for Papun Airport in Papun, Myanmar

Palestine Polytechnic University in Hebron, West Bank

Patliputra University in Patna, India

Peace Pledge Union, a British anti-war organisation

People's Protection Units, armed forces of the Kurdish Supreme Committee

Peoria and Pekin Union Railway, Illinois

Physics processing unit, a microprocessor typically used for video games

Picture Processing Unit, the component which generates a video signal in the Nintendo Entertainment System

Pirate Party of Ukraine, Ukrainian political party

The Plastic People of the Universe, a rock band from Prague

Point Park University in Pittsburgh, Pennsylvania

Power processing unit, a component responsible for regulating electrical power

Prvi Partizan, a Serbian ammunition producer often referred to as PPU for Prvi partizan Užice

Compiled Pascal Unit of Free Pascal similar to Delphi's DCU

Portable Pilot Unit

Price Per Unit

In fluid mechanics, pressure head is the height of a liquid column that corresponds to a particular pressure exerted by the liquid column on the base of its container. It may also be called static pressure head or simply static head (but not static head pressure). It is mathematically expressed as:

${\displaystyle \psi ={\frac {p}{\gamma }}={\frac {p}{\rho \,g}}}$

where

${\displaystyle \psi }$ is pressure head (which is actually a length, typically in units of meters),
${\displaystyle p}$ is fluid pressure (i.e. force per unit area, typically expressed in pascals),
${\displaystyle \gamma }$ is the specific weight (i.e. force per unit volume, typically expressed in N/m3 units),
${\displaystyle \rho }$ is the density of the fluid (i.e. mass per unit volume, typically expressed in kg/m3), and
${\displaystyle g}$ is acceleration due to gravity (i.e. rate of change of velocity, expressed in m/s2).

Note that in this equation, the pressure term may be gauge pressure or absolute pressure, depending on the design of the container and whether it is open to the ambient air or sealed without air.

Torr

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

Historically, one torr was intended to be the same as one "millimeter of mercury". However, subsequent redefinitions of the two units made them slightly different (by less than 0.000015%). The torr is not part of the International System of Units (SI), but it is often combined with the metric prefix milli to name one millitorr (mTorr) or 0.001 Torr.

The unit was named after Evangelista Torricelli, an Italian physicist and mathematician who discovered the principle of the barometer in 1644.

Base units
Derived units
with special names
Other accepted units