Pascal (unit)

"Pascals" redirects here. For other uses, seePascal (disambiguation).

"Hectopascal" redirects here. For the song, seeHectopascal (song).

pascal
Apressure gaugereading inpsi(red scale) and kPa (black scale)
General information
Unit system	SI
Unit of	pressureorstress
Symbol	Pa
Named after	Blaise Pascal
Conversions
1 Pain...	... is equal to...
SI base units:	kg⋅m−1⋅s−2
US customary units:	1.45038×10−4psi
atmosphere:	9.86923×10−6atm
bar:	10−5bar
barye(CGS unit)	10 Ba

Thepascal(symbol:Pa) is the unit ofpressurein theInternational System of Units (SI).It is also used to quantifyinternal pressure,stress,Young's modulus,andultimate tensile strength.The unit, named afterBlaise Pascal,is anSI coherent derived unitdefined as onenewtonpersquare metre(N/m²).It is also equivalent to 10barye(10 Ba) in theCGSsystem. Common multiple units of the pascal are the hectopascal (1 hPa = 100 Pa), which is equal to onemillibar,and the kilopascal (1 kPa = 1000 Pa), which is equal to one centibar.

The unit of measurement calledstandard atmosphere (atm)is defined as 101,325 Pa. Meteorologicalobservations typically reportatmospheric pressurein hectopascals per the recommendation of theWorld Meteorological Organization,thus a standard atmosphere (atm) or typical sea-level air pressure is about 1013 hPa. Reports in the United States typically useinches of mercuryor millibars (hectopascals).In Canada these reports are given in kilopascals.

Etymology

The unit is named afterBlaise Pascal,noted for his contributions to hydrodynamics and hydrostatics, and experiments with abarometer.The namepascalwas adopted for the SI unit newton per square metre (N/m²) by the 14thGeneral Conference on Weights and Measuresin 1971.

Definition

The pascal can be expressed usingSI derived units,or alternatively solelySI base units,as:

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

where N is thenewton,m is themetre,kg is thekilogram,s is thesecond,and J is thejoule.

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 anatmosphere or a standard atmosphere(atm) is101325Pa (101.325 kPa).This value is often used as a reference pressure and specified as such in some national and international standards, such as theInternational 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.

Unicodehas dedicated code-pointsU+33A9㎩SQUARE PAandU+33AA㎪SQUARE KPAin theCJK Compatibilityblock, but these exist only for backward-compatibility with some older ideographic character-sets and are thereforedeprecated.

Uses

The pascal (Pa) or kilopascal (kPa) as a unit of pressure measurement is widely used throughout the world and has largely replaced thepounds per square inch(psi) unit, except in some countries that still use theimperial measurement systemor theUS customary system,including the United States.

Geophysicistsuse the gigapascal (GPa) in measuring or calculating tectonic stresses and pressures within theEarth.

Medicalelastographymeasures tissue stiffness non-invasively withultrasoundormagnetic resonance imaging,and often displays theYoung's modulusorshear modulusof tissue in kilopascals.

Inmaterials scienceandengineering,the pascal measures thestiffness,tensile strengthandcompressive strengthof 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

Material	Young's modulus (GPa)
Nylon 6	2–4
Hempfibre	35
Aluminium	69
Tooth enamel	83
Copper	117
Structuralsteel	200
Diamond	1220

The pascal is also equivalent to the SI unit ofenergy density,the joule per cubic metre. This applies not only to the thermodynamics of pressurised gases, but also to the energy density ofelectric,magnetic,andgravitationalfields.

The pascal is used to measuresound pressure.Loudnessis the subjective experience of sound pressure and is measured as asound 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 thethreshold of hearingfor humans and is a common reference pressure, so that its SPL is zero.

The airtightness of buildingsis measured at 50 Pa.

In medicine, blood pressure is measured inmillimeters of mercury(mmHg, very close to oneTorr). The normal adult blood pressure is less than 120 mmHg systolic BP (SBP) and less than 80 mmHg diastolic BP (DBP).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

Main article:Bar (unit)

The units of atmospheric pressure commonly used inmeteorologywere formerly thebar(100,000 Pa), which is close to the average air pressure on Earth, and the millibar. Since the introduction ofSI units,meteorologists generally measure pressures in hectopascals (hPa) unit, equal to 100 pascals or 1 millibar.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.

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

Multiples and submultiples

Decimal multiples and submultiples are formed using standardSI units.

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

See also

• Atmospheric pressurewhich gives the usage of the hbar and the mbar
• Centimetre of water
• Meteorology
• Metric prefix
• Orders of magnitude (pressure)
• Pascal's law
• Pressure measurement