Pfeiffer Vacuum A Passion for Perfection

1.2.1 Pressure

Any gas enclosed within a volume is always uniformly distributed. The individual gas particles are constantly moving back and forth at high-speed within the volume; upon striking the vessel wall, they exert a force F on surface A due to pulse transmission. The pressure p that is exerted on the wall is defined as

Formula 1-3

Definition of pressure


Figure 1.2: Definition of total pressure

If the gas is made up of different types of gases, each of these gases will exert a pressure that corresponds to its concentration; this is called partial pressure. The sum of all partial pressures equals the total pressure. Air is a good example of this: In addition to its main constituents of nitrogen, oxygen and water vapor, air also contains many trace gases.


Figure 1.3: Definition of partial pressure

Table 1.1: Total pressure and composition of air at 20 °C and 50 % relative humidity

The pressure range of vacuum comprises the interval of 0 – 1 bar. A distinction is made between the following pressure ranges:

Table 1.2: Pressure ranges / Molecular number density

In accordance with Formula 1-3 for pressure definition, the SI unit Pa = N / m2 will be used for this purpose. Also customary in actual practice are the units of pressure shown in the conversion table below. It is very customary to use mbar as a unit of pressure.

Table 1.3:  Conversion table for units of pressurezoom table
Table 1.3: Conversion table for units of pressure

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