Primary vacuum standard for UHV range — Standing experience and present problems

Accurate calibrations of gauges to ever lower pressures (higher vacuum) are increasingly becoming necessary. The calibrations over the entire ultra high vacuum (UHV) range i.e. up to 10⁻¹⁰ Pa are already sometimes demanded by the users of vacuum measuring devices. Standards covering part of this range are available in some national metrological laboratories while primary standards covering the entire UHV range are under development. However, the most suitable principle of the primary standard for generating such a low pressure is still not clear. In spite of some attempts during the last century no sufficiently accurate absolute (“calibration-free”) gauge for high vacuum (HV) and UHV ranges has been developed to date. Apparatuses for generating gas pressure in a dynamic flow control method will continue to be used as primary standards for the UHV range, but, by virtue of technical difficulties, the equilibrium (Maxwellian velocity distribution) will be severely disturbed during the process of pressure generation that the pressure in the chamber will not be defined as a simple scalar quantity. This principal problem must be solved first by the development of a primary UHV standard. Another serious problem in the design of an apparatusfor generating controlled pressures on the order of 10⁻¹⁰ Pa requires the uncontrolled changes of gas pressure to 10⁻¹² Pa or lower. Hence an extremely high vacuum (XHV) apparatus must be used in which the requirements concerning the pumps and the outgassing rate from the materials are kept very strict.