High pressure ionization vacuum gauge

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. 

Hot-Cathode Ionization Gauges. For pressures below approximately lO " Pa, it is not possible, except under carehiUy controlled conditions, to detect the minute forces that result from the coUision of gas molecules with a soHd wall. The operation of the ion gauge is based on ionisa tion of gas molecules as a result of coUisions with electrons. These ions are then subsequendy collected by an ion collector. Ionisa tion gauges, used almost exclusively for pressure measurement in high, very high, ultrahigh, and extreme ultrahigh vacuums, measure molecular density or particle dux, not pressure itself. 

Fig. 7.10. Cutaway of a cold cathode (Penning) gauge. The wire anode (A) is connected through a ceramic insulator (i) to the high-voltage lead (H). Under the influence of the magnetic field created by M, the elcclrons travel a long spiral path between the body and the anode. The current, which arises from electrons and positive ions of the ionized gas molecules, is related to the pressure. The open end (C) is attached to the vacuum system.

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