Fundamentals of low-pressure measurement

A distinction must be made between the follow/ing vacuum gauges  

Instruments with indirect pressure measurement. In this case, the pressure is determined as a function of a pressure-dependent (or more accurately, density-dependent) property (thermal conductivity, ionization probability, electrical conductivity) of the gas. These properties are dependent on the molar mass as well as on the pressure. The pressure reading of the measuring instrument depends on the type of gas. 

Example If the pressure of a gas essentially consisting of pump fluid molecules is measured with an ionization vacuum gauge, then the pressure reading (applying to air or Nj), as shown in Table 3.2, is too high by a factor of about 10. 

Measurement of pressures in the rough vacuum range can be carried out relatively precisely by means of vacuum gauges with direct pressure measurement. Measurement of lower pressures, on the other hand, is almost always subject to a number of fundamental errors that limit the measuring accuracy right from the start so that it is not comparable at all to 

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