Pascal (unit)

pascal
A pressure gauge reading in psi (red scale) and kPa (black scale)
Unit information
Unit system:	SI derived unit
Unit of...	Pressure / Stress
Symbol:	Pa
Named after:	Blaise Pascal
In SI base units:	1 Pa = 1 kg/(m·s2)

The pascal (symbol: Pa) is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre. In everyday life, the pascal is perhaps best known from meteorological barometric pressure reports, where it occurs in the form of hectopascals (1 hPa ≡ 100 Pa) or kilopascals (1 kPa ≡ 1000 Pa).^[1] In other contexts, the kilopascal is commonly used, for example on bicycle tire labels.^[2] One hectopascal corresponds to about 0.1% of atmospheric pressure slightly above sea level; one kilopascal is about 1% of atmospheric pressure. One hectopascal is equivalent to one millibar; one standard atmosphere is exactly equal to 101.325 kPa or 1013.25 hPa or 101325 Pa. The correspondent Imperial unit is pounds per square inch (psi).

Definition

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

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

Where N is the newton, m is the metre, kg is the kilogram, and s is the second.

Pressure units

v · d · e	Pascal	Bar	Technical atmosphere	Standard atmosphere	Torr	Pound per square inch
	Pa	bar	at	atm	torr	psi
1 Pa	≡ 1 N/m2	10−5	1.0197×10−5	9.8692×10−6	7.5006×10−3	145.04×10−6
1 bar	105	≡ 106 dyn/cm2	1.0197	0.98692	750.06	14.5037744
1 at	0.980665 ×105	0.980665	≡ 1 kp/cm2	0.96784	735.56	14.223
1 atm	1.01325 ×105	1.01325	1.0332	≡ p0	760	14.696
1 Torr	133.322	1.3332×10−3	1.3595×10−3	1.3158×10−3	= 1 mmHg	19.337×10−3
1 psi	6.895×103	68.948×10−3	70.307×10−3	68.046×10−3	51.715	≡ 1 lbF/in2

Origin

The unit is named after Blaise Pascal, the eminent French mathematician, physicist, and philosopher noted for his experiments with a barometer, an instrument to measure air pressure. The name pascal was adopted for the SI unit newton per square metre by the 14th CGPM in 1971. [1]

This SI unit is named after Blaise Pascal. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (Pa). When an SI unit is spelled out in English, it should always begin with a lower case letter (pascal), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. —Based on The International System of Units, section 5.2.

Miscellaneous

Standard atmospheric pressure is 101,325 Pa = 101.325 kPa = 1013.25 hPa = 1013.25 mbar = 760 Torr^[4] = 14.696 psi. This definition is used for pneumatic fluid power (ISO R554), and in the aerospace (ISO 2533) and petroleum (ISO 5024) industries.

In 1985 the IUPAC recommended that the standard for atmospheric pressure should be harmonized to 100,000 Pa = 1 bar = 750 Torr. The same definition is used in the compressor and the pneumatic tool industries (ISO 2787).

The Unicode computer character set has dedicated symbols ㎩ (U+33A9) for Pa and ㎪ (U+33AA) for kPa, but these exist merely for backward-compatibility with some older ideographic character-sets and are therefore deprecated.

Uses

The pascal (Pa) or kilopascal (kPa) as a unit of pressure measurement is widely used throughout the world and largely replaces the pounds per square inch (psi) unit, except in some countries still using the Imperial measurement system.

Tectonophysicists use the gigapascal (GPa) in measuring or calculating tectonic forces within the earth.

In materials science, megapascals (MPa = N/mm²) or gigapascals (GPa = kN/mm²) are commonly used to measure stiffness or tensile strength of materials. Steel's tensile strength is approximately 200 GPa.

Other, older units of measure occasionally used for pressure are millimeters of mercury (Torr) and millimetres of water (1 mm H₂O = 9.80665 Pa).

In the cgs system, the unit of pressure is the barye (symbol ba), which is equal to one decipascal. The older kilogram-force per square centimetre corresponds to 98.0665 kPa,^[5] but is often rounded to 100 kPa in practice.

In the former mts system, the unit of pressure is the pièze (symbol pz), which is equal to one kilopascal.

Vehicle owners' guides now specify tire inflation in kilopascals.

Airtightness testing of buildings is measured at 50 Pa or 0.2 inches of water.^[6]

Hectopascal and millibar units

Main article: Bar (unit)

Meteorologists worldwide have for a long time measured atmospheric pressure in bars, which was originally equivalent to the average air pressure on Earth; the bar was divided into a thousand millibars to provide the precision meteorologists require. After the introduction of SI units, many preferred to preserve the customary pressure figures. Consequently, the bar was redefined as 100,000 pascals, which is only slightly lower than standard air pressure on Earth. Today many meteorologists prefer hectopascals (hPa) for air pressure, which are equivalent to millibars, while similar pressures are given in kilopascals in practically all other fields, since the hecto prefix is rarely used. Since official metrication, meteorologists in Canada use kilopascals (kPa),^[7][8] although in some other countries hectopascals are still in use.^{[9][10][11][12][13][14]}

1 hectopascal (hPa) ≡ 100 Pa ≡ 1 mbar.

1 kilopascal (kPa) ≡ 1000 Pa ≡ 10 hPa ≡ 10 mbar.

See also

• Pascal's law
• Orders of magnitude (pressure)

References

1. ^ U.S. Federal Meteorological Handbook
2. ^ ISO 5775: Bicycle tires and rims
3. ^ Table 3 (Section 2.2.2), SI Brochure, International Bureau of Weights and Measures
4. ^ "Resolution 4 of the 10th meeting of the CGPM". Conférence Générale des Poids et Mesures (CGPM). 1954. http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=10&RES=4. Retrieved 5 April 2010. 
5. ^ SensorsOne, Pressure unit conversion and converter. 2010.03.22.
6. ^ "Chapter 7 ResNet Standards: ResNet National Standard for Home Energy Audits". ResNet. 2010. http://www.resnet.us/hotnews/revised_chapter_seven_final_9-16-10.pdf. Retrieved 3 March 2011. 
7. ^ CTV News, weather; current conditions in Montreal
8. ^ Environment Canada weather, current conditions in Montreal
9. ^ KNMI
10. ^ KMI
11. ^ DWD
12. ^ JMA
13. ^ MDD
14. ^ NOAA