Note de Sécurité Lethal voltages from ion gauge/gas discharge interactions

2014 Human contact between ground and the metal parts of a vacuum chamber can be dangerous if the metal is not grounded, and if an ionization gauge is operating in the chamber at pressures of 0.1 pascal (7 x 10-4 torr) or higher. Lethal electrical shock can possibly occur if all of these conditions are present. Common grounding of the metal parts of the vacuum chamber and of the ionization gauge controller chassis removes this danger. Revue Phys. Appl. 20 (1985) 531-534 JUILLET 1985, An open ground circuit plus millitorr local pressure can expose personnel to lethal voltages during ionization gauge operation, especially during degassing. For the last 30 years 1 have worked with ion gauges and vacuum systems. 1 have looked for more ground circuits since recently making this discovery than in the previous 30 years ! 1 feel very fortunate to be alive, and able to tell you about this danger. It may eventually be possible to design gauges that are free of it, but presently there are hundreds of thousands of ionization gauge controllers and vacuum systems from many manufacturers around the world that could kill if not properly grounded. Prevent the dangerous path. Most of us have used our ionization gauges at pressures where some visual evidence of plasma-like formation was présent, i.e., there was a gentle purple glow in the tube. What interactions does this have ? Is it interfering with the gauge reading ? Is it dangerous ? If the purple glow occurs during the degas process, can this be dangerous in any way ? Are you willing to bet your life ? To answer these questions, a vacuum system was arranged as shown in figure 1. This permitted us to observe the behaviour of an ionization gauge, G1, while the pressure was raised into the 103 torr range, and higher, monitored by gauge G2. By closing valve Vl totally and opening valve V3 slightly, the pressure could be made to increase linearly with time at the