About 25 years ago, a pressure gauge was designed and manufactured at CERN for measuring the extreme vacua produced in the experimental regions of the Intersecting Storage Ring (ISR). This gauge, based on the design of the Varian "Helmer" gauge, has since been used to measure pressures down to the 10^-14 Torr range inside vacuum systems pumped in different ways.

The recent development of non-evaporable getter thin films coatings, which allows achieving XHV routinely by transforming the vacuum system surfaces from a gas source into a pump, has revived the interest for this gauge, and more particularly for its low pressure limitations. Experimental evidence has been obtained, showing that the lowest pressure this gauge may measure at the extremity of a getter coated vacuum chamber is given by the ratio of the gauge degassing rate to the chamber pumping speed. The validity of this result has been extended down to 1 x 10^-13 Torr. These results, and the ongoing program to further reduce the lowest measurable pressure, are presented and discussed.