Pascal (unit)#Hectopascal and millibar units

{{Infobox Unit

| bgcolour =

| name = pascal

| image = 250px

| caption = A pressure gauge reading in psi (red scale) and kPa (black scale)

| standard = SI

| quantity = pressure or stress

| symbol = Pa

| namedafter = Blaise Pascal

| units1 = SI base units:

| inunits1 = kg⋅m⁻¹⋅s⁻²

| units2 = US customary units:

| inunits2 = {{cvt|1e0|Pa|psi|sigfig=6|disp=out|lk=out|comma=gaps}}

| units3 = atmosphere:

| inunits3 = {{convert|1|Pa|atm|sigfig=6|disp=out|comma=gaps}}

| units4 = bar:

| inunits4 = 10⁻⁵ bar

| units5 = barye (CGS unit)

| inunits5 = 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 an SI coherent derived unit defined as one newton per square metre (N/m²).{{SIbrochure8th|page=118}} 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.

The unit of measurement called standard atmosphere (atm) is defined as {{val|101,325|u=Pa}}.{{cite web|url=http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=10&RES=4 |publisher=BIPM |title=Definition of the standard atmosphere|access-date=2015-02-16}}

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{{Cite web|url=https://w1.weather.gov/glossary/index.php?word=inches+of+mercury|title=National Weather Service glossary page on inches of mercury}} or millibars (hectopascals).{{Cite web|url=https://www.wpc.ncep.noaa.gov/sfc/90fwbg.gif|title=US government atmospheric pressure map}}{{Cite web|url=https://s.w-x.co/util/image/map/WEB_Current_Weather_Map_1280x720.jpg?v=ap&w=1280&h=720&api=7db9fe61-7414-47b5-9871-e17d87b8b6a0|title=The Weather Channel}} In Canada, these reports are given in kilopascals.{{Cite web|url=https://weather.gc.ca/canada_e.html|title=Canadian Weather – Environment Canada|first=Environment|last=Canada|date=16 April 2013|website=weather.gc.ca}}

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.[http://www.bipm.fr/en/convention/cgpm/14/pascal-siemens.html bipm.fr]. {{webarchive|url=https://web.archive.org/web/20070630020548/http://www.bipm.fr/en/convention/cgpm/14/pascal-siemens.html |date=30 June 2007 }}.[http://www.bipm.org/utils/common/pdf/CGPM/CGPM14.pdf#page=78 Minutes of the 14. General Conference on Weights and Measures], 1971, p. 78.

Definition

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

: ${\rm 1~Pa = 1~N/m^2 = 1~kg/(m {\cdot} s^2) = 1~J/m^3 }$

where N is the newton, m is the metre, kg is the kilogram, s is the second, and J is the joule.[http://www.bipm.org/en/si/si_brochure/chapter2/2-2/table3.html Table 3 (Section 2.2.2)]. {{webarchive|url=https://web.archive.org/web/20070618123613/http://www.bipm.org/en/si/si_brochure/chapter2/2-2/table3.html |date=18 June 2007 }}. SI Brochure. International Bureau of Weights and Measures.

One pascal is the pressure exerted by a force of 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 {{convert|101325|Pa|kPa|abbr=on|comma=gaps}}.{{cite web |url=http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=10&RES=4 |title = Resolution 4 of the 10th meeting of the CGPM |publisher=Conférence Générale des Poids et Mesures |year = 1954 |access-date = 2010-04-05 |archive-url=https://web.archive.org/web/20210330142301/http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=10&RES=4 |archive-date=30 March 2021 |url-status=dead }} 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.IUPAC.org, Gold Book, [http://goldbook.iupac.org/S05921.html Standard Pressure]

Unicode has dedicated code-points {{Unichar|33a9|Square Pa}} and {{Unichar|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.{{cite web|url=https://www.unicode.org/charts/PDF/U3300.pdf|title=CJK Compatibility|access-date=2016-02-21|date=2015}}{{cite book|publisher=The Unicode Consortium|title=The Unicode Standard, Version 8.0.0|location=Mountain View, CA|date=2015|isbn=978-1-936213-10-8|url=https://www.unicode.org/versions/Unicode8.0.0|access-date=2016-02-21}}

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.

class="wikitable" \|+ Approximate Young's modulus for common substances{{cite web\|url=http://www.engineeringtoolbox.com/young-modulus-d_417.html\|title=Tensile Modulus – Modulus of Elasticity or Young's Modulus – for some common Materials\|access-date=2015-02-16}} ! Material !! Young's modulus {{br}}(GPa)
Nylon 6	align="right"\| 2–4
Hemp fibre	align="right"\| 35
Aluminium	align="right"\| 69
Tooth enamel	align="right"\| 83
Copper	align="right"\| 117
Structural steel	align="right"\| 200
Diamond	align="right"\| 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.

{{cite web

|url = http://www.resnet.us/hotnews/revised_chapter_seven_final_9-16-10.pdf

|title = Chapter 7 ResNet Standards: ResNet National Standard for Home Energy Audits

|publisher = ResNet

|year = 2010

|access-date = 2011-03-03

|url-status = dead

|archive-url = https://web.archive.org/web/20110726045004/http://www.resnet.us/hotnews/revised_chapter_seven_final_9-16-10.pdf

|archive-date = 26 July 2011

}}

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).{{cite web |title=BP Guideline {{!}} Target:BP |url=https://targetbp.org/guidelines17/ |website=American Heart Association |access-date=18 May 2020}} Convert mmHg to SI units as follows: {{nowrap|1= 1 mmHg = {{val|0.13332|u=kPa}}}}. Hence the 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 ({{val|100,000|u=Pa}}), which is close to the average air pressure on Earth, and the millibar. Since the introduction of SI units, meteorologists generally measure atmospheric pressure in hectopascals (hPa), equal to 100 pascals or 1 millibar.{{cite web|url=http://www.knmi.nl/actueel/|title=KNMI – Weer – Waarnemingen|access-date=4 December 2016}}{{cite web|url=http://www.meteo.be/meteo/view/fr/68771-FAQ%20sur%20le%20temps.html?view=195695|title=Comment convertir la pression? – IRM|access-date=4 December 2016}}{{Cite web|url=http://www.dwd.de/de/WundK/W_aktuell/Beobachtungen.htm|title=DWD|access-date=20 December 2006|archive-url=https://web.archive.org/web/20080210040845/http://www.dwd.de/de/WundK/W_aktuell/Beobachtungen.htm|archive-date=10 February 2008|url-status=dead}}{{cite web|url=http://www.jma.go.jp/en/g3/index.html|title=Japan Meteorological Agency – Weather Maps|access-date=4 December 2016}}{{Cite web|url=http://www.meteoam.it/modules.php?name=analisiPrevisioniSuolo|archive-url=https://web.archive.org/web/20060506192242/http://www.meteoam.it/modules.php?name=analisiPrevisioniSuolo|url-status=dead|title=MDD|archive-date=6 May 2006}}[http://weather.noaa.gov/weather/current/KDCA.html NOAA]{{cite web|url=http://www.metoffice.gov.uk/guide/weather/symbols#units|title=Key to symbols and terms|first=Met Office|last=United Kingdom|access-date=4 December 2016}} Exceptions include Canada, which uses kilopascals (kPa). In many other fields of science, prefixes that are a power of 1000 are preferred, which theoretically excludes hectopascal from use.{{Cite web|url=http://www.ctv.ca/servlet/HTMLTemplate/!ctvDynNews/Weather/Weather?City=Montreal|archive-url=https://web.archive.org/web/20110604131321/http://www.ctv.ca/servlet/HTMLTemplate/!ctvDynNews/Weather/Weather?City=Montreal|url-status=dead|title=CTV News, weather; current conditions in Montreal|archive-date=4 June 2011}}{{cite web|url=http://www.weatheroffice.gc.ca/city/pages/qc-147_metric_e.html|title=Montréal, QC – 7 Day Forecast – Environment Canada|first=Environment|last=Canada|access-date=4 December 2016|archive-date=30 November 2017|archive-url=https://web.archive.org/web/20171130163853/http://www.weatheroffice.gc.ca/city/pages/qc-147_metric_e.html|url-status=dead}}

Many countries still use millibars to measure atmospheric pressure. In practically all other fields, the kilopascal is used instead.Ambler Thompson (Editor) {{google books|pTw-SCI7EkoC|Guide for the Use of the International System of Units (SI) (rev. ): The ...|page=66}}

Multiples and submultiples

Decimal multiples and submultiples are formed using standard metric prefixes.

class="wikitable" \|+ ! colspan="3" \|Multiples ! colspan="3" \|Submultiples
Value !Name !Symbol !Value !Name !Symbol
10¹ Pa \|decapascal \|daPa \|10⁻¹ Pa \|decipascal \|dPa
10² Pa \|hectopascal \|hPa \|10⁻² Pa \|centipascal \|cPa
10³ Pa \|kilopascal \|kPa \|10⁻³ Pa \|millipascal \|mPa
10⁵ Pa \|bar (non-SI unit) \|bar \| \| \|
10⁶ Pa \|megapascal \|MPa \|10⁻⁶ Pa \|micropascal \|μPa
10⁹ Pa \|gigapascal \|GPa \|10⁻⁹ Pa \|nanopascal \|nPa
10¹² Pa \|terapascal \|TPa \|10⁻¹² Pa \|picopascal \|pPa
10¹⁵ Pa \|petapascal \|PPa \|10⁻¹⁵ Pa \|femtopascal \|fPa
10¹⁸ Pa \|exapascal \|EPa \|10⁻¹⁸ Pa \|attopascal \|aPa
10²¹ Pa \|zettapascal \|ZPa \|10⁻²¹ Pa \|zeptopascal \|zPa
10²⁴ Pa \|yottapascal \|YPa \|10⁻²⁴ Pa \|yoctopascal \|yPa
10²⁷ Pa \|ronnapascal \|RPa \|10⁻²⁷Pa \|rontopascal \|rPa
10³⁰ Pa \|quettapascal \|QPa \|10⁻³⁰ Pa \|quectopascal \|qPa

class="wikitable"

! colspan="3" |Multiples

! colspan="3" |Submultiples

Value

!Name

!Symbol

!Value

!Name

!Symbol

10¹ Pa

|decapascal

|daPa

|10⁻¹ Pa

|decipascal

|dPa

10² Pa

|hectopascal

|hPa

|10⁻² Pa

|centipascal

|cPa

10³ Pa

|kilopascal

|kPa

|10⁻³ Pa

|millipascal

|mPa

10⁵ Pa

|bar (non-SI unit)

|bar

10⁶ Pa

|megapascal

|MPa

|10⁻⁶ Pa

|micropascal

|μPa

10⁹ Pa

|gigapascal

|GPa

|10⁻⁹ Pa

|nanopascal

|nPa

10¹² Pa

|terapascal

|TPa

|10⁻¹² Pa

|picopascal

|pPa

10¹⁵ Pa

|petapascal

|PPa

|10⁻¹⁵ Pa

|femtopascal

|fPa

10¹⁸ Pa

|exapascal

|EPa

|10⁻¹⁸ Pa

|attopascal

|aPa

10²¹ Pa

|zettapascal

|ZPa

|10⁻²¹ Pa

|zeptopascal

|zPa

10²⁴ Pa

|yottapascal

|YPa

|10⁻²⁴ Pa

|yoctopascal

|yPa

10²⁷ Pa

|ronnapascal

|RPa

|10⁻²⁷Pa

|rontopascal

|rPa

10³⁰ Pa

|quettapascal

|QPa

|10⁻³⁰ Pa

|quectopascal

|qPa

References

Category:SI derived units

Category:Units of pressure

Category:Blaise Pascal

class="wikitable" \|+ Approximate Young's modulus for common substances{{cite web\|url=http://www.engineeringtoolbox.com/young-modulus-d_417.html\|title=Tensile Modulus – Modulus of Elasticity or Young's Modulus – for some common Materials\|access-date=2015-02-16}} ! Material !! Young's modulus {{br}}(GPa)
Nylon 6	align="right"\| 2–4
Hemp fibre	align="right"\| 35
Aluminium	align="right"\| 69
Tooth enamel	align="right"\| 83
Copper	align="right"\| 117
Structural steel	align="right"\| 200
Diamond	align="right"\| 1220