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Ionization Gauge – Cold Cathode Type

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The cold cathode type ionization gauge is used to replace the hot cathode type, as it produces errors at very high temperatures of the cathode. To overcome this problem, the electrodes must be roperly treated before use. All these problems can be nullified by vacuum measurement using cold cathode ionization gauge.

A Philips and Penning cold cathode gauge is shown below. The device consists of two cathodes and a hollow anode in between. An input voltage greater than 2 Kilovolt is applied between them. A strong magnetic field is produced due to the applied voltage and thus the electrons are ejected. This causes the gauge to operate. At pressures below 10-2 Torr, the mean free path of the gas is so large that a collision may not occur at all so that discharge is not sustained or ionization may not be initiated. This problem can be eliminated by a collimating magnetic field. This is shown in the figure below.

Ionization Gauge - Cold Cathode Type
                              Ionization Gauge – Cold Cathode Type

The collimating magnetic field increases the path length for the electrons, enabling discharges possible at pressures down to about 10-5 Torr. It is difficult to obtain linearity between the meter reading and pressure as there occurs interactions between the positive ions and electrons at high electric and magnetic fields.

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