Measurement Methods for Gross and Fine Vacuum

Gases exert forces on the walls of a closed volume. These result from collisions of the moving molecules with the walls. Pressure is defined as the ratio of normal force f on a plane surface portion A. The resulting unit according to the SI system is  

The sum of all molecular collision forces related to the unit area is called total pressure, the proportional pressure of a gas species is named partial pressure . The sum of the partial pressures of all gas species present in the closed volume yields the total pressure (Dalton s law). 

According to the SI system the unit of pressure is Pascal . Additionally the units bar (=10 Pa) respectively mbar (=100 Pa) are in use and accepted. 

For a direct pressure measurement, the difference between the pressure to be measured and a known reference pressure is measured. Because of this measurement method engineers specify three different kinds of pressure (see DIN 1314)  

In vacuum technology normally the absolute pressure is stated. To generate Zero pressure, a reference vacuum has to be generated with a pressure small compared to the pressure to be measured (there is no empty space in practice). 

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