Comparison of ionization vacuum gauges close to their low pressure limit

Parts of CERN’s accelerator complex and experiments, especially in the antimatter field, require a vacuum in the $10^{-12}$ mbar range or better. Thus gauges are needed to reliably measure XHV during experimental operation and in order to study the vacuum science needed for those experiments. We therefore built a setup to reach $1⋅10^{-13}$mbar in order to simultaneously compare different hot cathode ionization gauges with the ability to measure high UHV and XHV close to their lower pressure limit: Barion extended, Extractor IE514, a modulated Bayard–Alpert gauge and two Improved Helmer gauges. All gauges but the Extractor behave similarly with respect to small pressure variations around the limit pressure, while the Extractor seems to overestimate high UHV hydrogen pressure. We show how gauge operation determines our ultimate achievable pressure due to outgassing, which was comparable for all gauges and in the order of $Q∼10^{-10}$mbar l s$^{-1}$. Further we show the disturbances caused in the static system due to gauge pumping (visible only as electronic pumping), and report some of the possible difficulties and origins of noise when measuring pressures in the XHV range, including the thermoelectric effect.