The effect of magnetron discharge pressure measurement on the actual pressure in vacuum interrupters

Vacuum interrupters (VI) have proved extremely reliable over the past 60 years, but failure can occur, and is most often a consequence of loss of vacuum. If the vacuum pressure rises above a critical level, the dielectric strength of the vacuum collapses, resulting in functional failure of the vacuum interrupter. During manufacture the vacuum pressure of a VI is measured by stimulating an inverse magnetron discharge inside it. This has long been known to have the effect of pumping some or all of the gas away, but this effect has been believed to be temporary with the original pressure recovering within hours. We have found that with the majority of a sample of old VI, magnetron pumping can be very significant if the discharge is kept going for long enough, and the pressure can be reduced to a very low level. We also found when we tested the VI again two years later, that in the majority of cases the pumping appeared to have been permanent. There was only a slight pressure rise, which was assumed to be due to new gas slowly being desorbed from the solid materials of the VI.