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International pressure units

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If you load an area of 1 m² with 100 g weight force (F = 0.1 g × 10 m/s² = 1 N), this results in a pressure of 1 Pa.* The commonly used unit for higher pressuresin Europe is the Bar (bar). As can be seen in the calculation, the Pascal (Pa) is too small a classification for most technical applications.



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This approximately corresponds to the pressure exerted by a mass of 1 kg on 1 cm² or a mass of 10 t on 1 m².


*Comment: the exact value of the gravitational acceleration of g = 9.81 m/s² has not been used for the calculations on this page. Instead a rounded value of 10 m/s² has been used.


1 bar = 105 N/m² = 100 000 Pa

Fig. 1: Generation of 1 bar with different weight forces and surface areas

Values of the pressure units
Value
1 Pa 0,01
1 hPa 1
1 psi 68,97
1 mWs 98,07
1 mmHg 1,333
1 Torr 1,333
1 at 980,07
1 atm 1013,3

The legally admissible unit bar replaced the previously used pressure units at and atm in 1978. These units can no longer be used today. However, their story is interesting and reveals the connection with other pressure units that are sometimes
still used today: The technical atmosphere at was based on the hydrostatic pressure as a reference, which exerts a 10 meter high water column (see chapter 2.5). At a density of ρ = 1 kg/dm3 and with the gravitational acceleration of g = 9.80665 m/s2, the following applies:

  • bar: Bar
  • Pa: Pascal
  • hPa: Hectopascal
  • psi: Pound per square inch
  • mWS: Meter water column
  • mmHg: Millimeter of mercury
  • Torr: Millimeter of mercury
  • at: Technical atmosphere
  • atm: Standard atmosphere



1 atm=1.01325 bar=760 torr=760 mmHg

An index character was usually added to the technical atmosphere at depending on whether it was an absolute pressure (ata), a negative pressure (atu), or an overpressure (atü). This can be found, for example, on tire pressure devices at gas stations.


The reference for the standard atmosphere atm was the normal atmospheric pressure at sea level, which corresponds to the weight force of the earth’s atmosphere. The ambient pressure to which we are exposed pushed the mercury column in Torricelli’s barometer to a height of 760 mm:

From the pressure units listed above, only Bar and Pascal are listed in the International System of Units (SI). In the USA, units related to the metric system are used in economic and technical contexts. However, corresponding information in product specifications or data sheets is usually provided in psi (pound per square inch).


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