Cold-cathode ionisation gauges are used widely to measure total pressures in the vacuum range, particularly in industrial plants and special plants for scientific purposes. But these seemingly indestructible devices have one major disadvantage - they cannot be used reliably in gas compositions with a high content of hydrocarbons during a long time, as the operation of the gauge in such an atmosphere contaminates the electrodes. By this a reliable pressure reading of the gauge becomes impossible. As part of a dissertation project the Otto von Guericke University in Magdeburg and the Steinbeis Transfer Centre for Vacuum Science and Technology have developed a cold-cathode ionisation gauge with a significantly increased lifetime.
The life time of a measuring device is defined as the operating time during which the magnitude of unavoidable measurement errors (caused e. g. by contamination) does not exceed a certain maximum value. Experiments with a lot of commercial cold cathode ionisation gauges have shown, that these gauges can run without interruption in an hydro carbon contaminated atmosphere only up to 1000 hours. After this time they are so contaminated, that a reliable pressure measurement is impossible.
A standard cold cathode gauge contains two electrodes - the cathode and the anode - with a high DC voltage between them. Under the effect of this voltage and a high magnetic field produced by a permanent magnet a gas discharge is ignited between the two electrodes. The discharge current is nearly proportional to the pressure and is used for the measurement of the pressure in the chamber with which the gauge is connected via the open end.
To avoid the disadvantages of the commercial gauges (short life time in hydro carbon containing atmospheres) an inverse double magnetron gauge was developed in co-operation of the Otto von Guericke University Magdeburg and the Steinbeis Transfer Centre for Vacuum Science and Technology by mounting two barrel shaped cathodes (K1, K2) one after the other in a cylindrical casing closed on one side. A concentric , rod-shaped anode (A) travels through the centre of the two cathodes. Thus two separate discharge cells are formed, which are penetrated by the magnetic field generated by a ring shaped permanent magnet (M) surrounding the casing (G). Two separate gas discharges are ignited between the two cathodes and the anode. But only the gas discharge between the cathode K1 and the anode is used for pressure measurement, while the other between cathode K2 and the anode merely cracks or polymerizes the hydrocarbons entering the gauge, because the cathode K2 is located closest to the vacuum chamber and the gas flow has to pass it before it enters cathode K1. The result: only a small percentage of hydro carbons vapours reaches the discharge space used for measurement - meaning the life time of the gauge is 3 to 4 times longer than with conventional gauges.
The electrical circuits used to generate the high voltage and to measure the discharge current are nearly identical to the circuits in a normal commercial cold-cathode ionisation manometer. The mechanical structure of the gauge is marginally more complex than for a normal cold cathode gauge. Cathode K2 which is not used for pressure measurement but only for trapping the hydro carbon vapours. It can easily be removed, cleaned and remounted, meaning the user can "regenerate" the gauge time and again. The internationally patented measuring device has a range of pressure measurement from ~ 10-10 mbar to about 10-3 mbar and is particularly suitable for use in industry and in big plants for nuclear research.
Prof. i. R. Dr. rer. nat. habil. Christian Edelmann
Steinbeis Transfer Center for Vacuum Physics and Technology (Freiberg)
Dr. rer. nat. Stefan Wilfert
GSI Helmholtzzentrum für Schwerionenforschung GmbH (Darmstadt)