Pascal (unit)

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Pascal
Psidial.jpg
A pressure gauge reading in psi (red scale) and kPa (black scale)
Unit information
Unit system SI derived unit
Unit of Pressure or 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, defined as one newton per square metre.[1] Pressure is a measure of force per unit area. It is named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal.

Common multiple units of the pascal are the hectopascal (1 hPa ≡ 100 Pa) which is equal to 1 mbar, the kilopascal (1 kPa ≡ 1000 Pa), the megapascal (1 MPa ≡ 1,000,000 Pa), and the gigapascal (1 GPa ≡ 1,000,000,000 Pa).

On Earth, standard atmospheric pressure is defined as 101.325 kPa. Meteorological barometric pressure reports typically report atmospheric pressure in hectopascals, [2] corresponding to about 0.1% of atmospheric pressure. The main corresponding imperial and US customary unit is the pound per square inch (psi); in the context of meteorology, the inch of mercury may also be encountered.

Definition[edit]

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.

This SI unit is named after Blaise Pascal. As with every International System of Units (SI) unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (Pa). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (pascal), except in a situation where any word in that position 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.

Pressure units
Pascal Bar Technical atmosphere Standard atmosphere Torr Pounds 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 1.450377×10−4
1 bar 105 ≡ 106 dyn/cm2 1.0197 0.98692 750.06 14.50377
1 at 0.980665 ×105 0.980665 ≡ 1 kp/cm2 0.9678411 735.5592 14.22334
1 atm 1.01325 ×105 1.01325 1.0332 p0 ≡ 760 14.69595
1 Torr 133.3224 1.333224×10−3 1.359551×10−3 1.315789×10−3 ≈ 1 mmHg 1.933678×10−2
1 psi 6.8948×103 6.8948×10−2 7.03069×10−2 6.8046×10−2 51.71493 ≡ 1 lbF/in2


Origin[edit]

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 (N/m2) by the 14th CGPM in 1971.[4]

Miscellaneous[edit]

Standard atmospheric pressure is 101 325 Pa
= 101.325 kPa
= 1013.25 hPa
= 1.01325 bar
= 1013.25 mbar
= 0.101325 MPa
= 760 Torr[5]
= 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 ≈ 750.06 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[edit]

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, including the United States.

Tectonophysicists use the gigapascal (GPa) in measuring or calculating tectonic forces 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, megapascals (MPa = N/mm2) or gigapascals (GPa = kN/mm2) are commonly used to measure stiffness or tensile strength of materials. Examples of (approximate) Young’s moduli for several common substances (in gigapascals) include nylon at 2–4; hemp (fibre) at 58, aluminium at 69; tooth enamel at 83, copper at 117, steel at approximately 200, and diamond at 1220.

The pascal is also equivalent to the SI unit of energy density, J/m3. This applies not only to the thermodynamics of pressurized gasses, but also to the energy density of electric, magnetic, and gravitational fields.

Other, older units of measure occasionally used for pressure are millimetres of mercury (Torr) and millimetres of water (1.0 mmH2O = 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 precisely to 98.0665 kPa,[6] but is it often rounded off 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.

Airtightness testing of buildings is measured at 50 Pa or 0.2 inches (5.1 mm) of water.[7]

Hectopascal and millibar units[edit]

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 granularity that 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),[8][9] although in some other countries hectopascals are still in use.[10][11][12][13][14][15][16]

As of 17 November 2011 the hectopascal is used in aviation as the altimeter setting.

1 hectopascal (hPa) ≡ 100 Pa ≡ 1 mbar.
1 kilopascal (kPa) ≡ 1000 Pa ≡ 10 hPa ≡ 10 mbar.

See also[edit]

Notes and references[edit]