# Pascal (unit)

Physical unit
Unit name Pascal
Unit symbol ${\ displaystyle \ mathrm {Pa}}$
Physical quantity (s) pressure
Formula symbol ${\ displaystyle p}$
dimension ${\ displaystyle {\ mathsf {M \; L ^ {- 1} \; T ^ {- 2}}}}$
system International system of units
In SI units ${\ displaystyle \ mathrm {1 \, Pa = 1 \, {\ frac {N} {m ^ {2}}} = 1 \; {\ frac {kg} {m \ cdot s ^ {2}}}} }$
In CGS units ${\ displaystyle \ mathrm {1 \, Pa = 10 \; {\ frac {g} {cm \ cdot s ^ {2}}}}}$
Named after Blaise Pascal
Derived from Newtons , square meters

The pascal is a derived SI unit of pressure and mechanical stress . It is named after Blaise Pascal and is defined as follows:

1 Pa = 1  kg · m -1 · s -2 = 1  N · m -2

One pascal is the pressure that a force of one newton exerts on an area of ​​one square meter. Since this is a fairly small value, decimal multiples of the unit are often used, in addition to the hectopascal (1 hPa = 100 Pa) and the kilopascal (1 kPa = 1000 Pa) also the bar (1 bar = 10 5  Pa = 100 kPa) .

The mean air pressure of the atmosphere at sea level ( standard or norm pressure ) is

1 atm = 101325 Pa = 1013.25 hPa = 101.325 kPa = 1.01325 bar = 1013.25 mbar.

## history

Blaise Pascal (1623–1662) was a French philosopher and scientist who, among other things, dealt with the behavior of fluids and developed fundamentals such as the concept of pressure and vacuum. At the 5th  General Conference on Weights and Measures in 1913, the name “Pascal” was proposed for a unit of pressure that should have the value 10 N / cm 2 (that is the pressure that was later called 1 bar). However, no corresponding resolution was passed.

Even when the SI system was introduced in 1960, there was still no separate name for the unit of pressure; "Newtons per square meter" were used. However, this long name turned out to be unwieldy, especially since 1 N / m 2 is a very small pressure. In European industry the unit bar (= 10 5 N / m 2 ) was increasingly used, which corresponds almost exactly to the pressure of one atmosphere , and in meteorology the unit millibar was common. In order to avoid decimal multiples such as 10 5 as conversion factors in the SI system, the derived unit N / m 2 was given the name Pascal at the 14th General Conference on Weights and Measures in October 1971 .

The unit was established as a legal unit in Germany as early as 1969 . Pascal was introduced to meteorology on January 1st, 1984.

## Applications and typical sizes

Below are some size examples for different applications. SI prefixes are specified for the size range .

### Micropascals

The reference value for the sound pressure level L p  = 0 dB ( decibel ) is defined as 20 µPa sound pressure and is considered the hearing threshold. The loudness of 1  sone is defined at 1000 Hz sine tone and +40 dB, i.e. 2000 µPa. 1 Pa sound pressure corresponds to +94 dB and is therefore so loud that permanent exposure can damage hearing .

### Decapascal

The unit decapascal (1 daPa = 10 Pa) is often used in ventilation technology, whereby one decapascal corresponds to 0.1 mbar.

### Hectopascals

In meteorology , the air pressure of the atmosphere (at sea level on average 1013.25 hPa) is usually given in hectopascals (1 hPa = 100 Pa), because on the one hand the SI-compliant unit Pascal can be used and on the other hand you have a numerical value , which corresponds exactly to the millibars (mbar) used in the past .

In research, especially in vacuum technology , the millibar is often used as a unit.

### Kilopascals

The unit kilopascal (1 kPa = 1000 Pa = 0.1 N / cm 2 ) is used in automotive engineering, for example, for the SI-compliant specification of the tire inflation pressure. A pressure of 100 kPa corresponds to 1 bar. The test pressure is also usually given in kilopascals for sewer pipes.

### Megapascals

The unit megapascal (1 MPa = 1 million Pa = 1 N / mm 2 ) is used in technology e.g. B. used for the tensile strength of metals and in higher numbers also to describe explosions . The cold filling pressure of a halogen lamp with the noble gases neon and krypton at 22 ° C can e.g. B. 1.2 MPa (corresponds to 12 bar). A hydraulic line can be designed for an operating pressure of 400 bar = 40 MPa.

Megapascals are also z. B. used to describe the critical point in thermodynamics .

Yield strength , yield strength and yield stress in mechanical engineering are also usually given in megapascals. In civil engineering, too, the strength of concrete is given in megapascals.

The specific energy of an explosive indicates the pressure in megapascals that a kilogram of this explosive would generate in a closed volume of one liter in the event of an explosion.

### Gigapascals

The unit gigapascal (1 GPa = 1 billion Pa) describes the order of magnitude of pressures, e.g. B. Convert graphite into diamond : Graphite, pressed together in a hydraulic press at up to 6 GPa and temperatures above 1500 ° C, converts into diamond.

In the interior of the earth, the pressure at a depth of 410 km is 14 GPa (see 410 km discontinuity ). At temperatures of a few hundred degrees Celsius or at pressures of around 25 GPa, numerous rock-forming minerals are isochemically transformed into modifications that are more stable and crystallographically more densely packed under these conditions at a depth of around 700 km.

Modulus of elasticity and shear modulus , material constants that provide information about the linear elastic deformation of a component as a result of normal or shear force , are also given in gigapascals. Aluminum has z. B. a shear modulus of 25.5 GPa, steel of 79.3 GPa. The shear modulus of rocks is usually 30 GPa, see seismic moment .

## Conversion of pressure units

In addition to the Pascal, there are other units for pressure. A table can be found in the article pressure .

## Individual evidence

1. Minutes of the 5th General Conference on Weights and Measures , 1913, p. 56, Presentation of a project by the French government to define the base and derived units, accessed on Nov. 10, 2019
2. ^ A b Norman A. Anderson: Instrumentation for Process Measurement and Control . 3. Edition. CRC Press, 1997, ISBN 0-8493-9871-1 , pp. 37 ( limited preview in Google Book search).
3. Minutes of the 14th General Conference on Weights and Measures , 1971, p. 78, accessed Nov. 11, 2019
4. a b Pascal. (No longer available online.) In: Wetterlexikon. Die Welt, archived from the original on August 11, 2016 ; accessed on August 12, 2016 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
5. Example of this patent specification : DE2006000223 INCANDESCENT HALOGEN LAMP. Retrieved June 20, 2012 .
6. Jan Oliver Löfken: Almost as hard as diamond, but amorphous like glass. Wiley-VCH Verlag GmbH & Co. KGaA, October 26, 2011, accessed on June 20, 2012 .