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 ultimate tensile strength, defined as one newton per square metre.[1] It is named after the French polymath 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.[2] Meteorological reports typically state atmospheric pressure in hectopascals.[3]

Etymology[edit]

The unit is named after Blaise Pascal, noted for his 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.[4]

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}}[5]

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

Miscellaneous[edit]

Standard atmospheric pressure is 101325 Pa (101.325 kPa).[6]
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 101325 Pa.[citation needed] 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.

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, the pascal measures the stiffness or tensile strength of materials.

approximate Young's modulus for common substances [7]
Material Young's modulus
nylon 6 2–4 GPa
hemp fibre 35 GPa
aluminium 69 GPa
tooth enamel 83 GPa
copper 117 GPa
structural steel 200 GPa
diamond 1220 GPa

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

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 µPa.

The airtightness of buildings is measured at 50 Pa.[8]

Hectopascal and millibar units[edit]

Main article: Bar (unit)

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),[9][10] although in some other countries hectopascals are still in use.[11][12][13][14][15][16][17]

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]