# Pascal (unit)

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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

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

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

Template:SI unit lowercase Template:Pressure Units

### 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 (N/m^{2}) by the 14th CGPM in 1971.^{[4]}

## Miscellaneous

Standard atmospheric pressure is 101325 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

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.

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, megapascals (1 MPa = 1 N/mm^{2}) or gigapascals (1 GPa = 1 kN/mm^{2}) 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/m^{3}. 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 mmH_{2}O = 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 it is 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.

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 micropascals (20 millionths of a pascal).

Airtightness testing of buildings is measured at 50 Pa or Template:Convert of water.^{[7]}

### Hectopascal and millibar units

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Meteorologists worldwide have for a long time measured atmospheric pressure in bars, where one bar 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

## Notes and references

- ↑ Template:SIbrochure8th
- ↑ U.S. Federal Meteorological Handbook
- ↑ Table 3 (Section 2.2.2),
*SI Brochure*, International Bureau of Weights and Measures - ↑ bipm.fr
- ↑ Template:Cite web
- ↑ SensorsOne, Pressure unit conversion and converter. 2010.03.22.
- ↑ Template:Cite web
- ↑ CTV News, weather; current conditions in Montreal
- ↑ Environment Canada weather, current conditions in Montreal
- ↑ KNMI
- ↑ RMI
- ↑ DWD
- ↑ JMA
- ↑ MDD
- ↑ NOAA
- ↑ UK Met Office