Cold Cathode (Penning) Vacuum Gauges



Gauge Heads & Cables Cold Cathode Gauges		Systems and Controllers Cold Cathode Gauges

Cold Cathode Ionization Gauge - Ionization gauges which operate with a cold cathode discharge are called cold cathode or Penning vacuum gauges. The discharge process in the gauge tube is in principle the same as in the electrode system of a sputter-ion pump.

A common feature of all types of cold-cathode ionization gauges is that they simply contain two unheated electrodes - a cathode and an anode - between which the so-called cold discharge is excited by means of a dc voltage (of about 2 kV) and maintained so that the discharge still continues at very low pressures. This is attained by the use of a magnetic field to make the paths of the electrons so long that their collision probability with gas molecules is sufficiently large to maintain the discharge on the formation of the required number of charge carriers. The magnetic field is so arranged that the magnetic field lines of force cross the electric field lines. In this way the electrons are confined to a spiral path and oscillate between the two cathode plates. The positive and negative charge carriers produced by collision move to the corresponding electrodes and form the pressure-dependent discharge current, which is indicated on the meter.

The reading is dependent on the nature of the gas. For further details see section 3.33. The upper limit of the measuring region is decided by the fact that slightly above 10^-2 mbar the cold cathode discharge changes to a glow discharge with intense light output in which the current - at constant voltage depends only to a small extent on the pressure and is, therefore, not suitable for measurement purposes.

In all Cold Cathode gauges there is considerable higher gas sorption than in the ionization vacuum gauge which operates with a hot cathode. A cold cathode gauge tube, there fore, pumps gases similarly to a sputter-ion pump (S = 10^-2 ltr s^-1). The ions produced in the discharage are accelerated to the cathode where they are partly retained and partly cause sputtering of the cathode material. The sputtered cathode material forms a gettering surface film at the walls of the gauge tube.

Despite these disadvantages, which bring about a relatively highly inaccurate pressure reading (up to +/-50%), the cold-cathode ionization gauge has two especially outstanding advantages. The first is that the gauge head is insensitive to the inrush of air and vibrations, and the second is that it is less susceptible to harm from contaminants. It is therefore recommended for dirty and industrial environments.