Calibration of Total Pressure Gauges in the UHV and XHV Regions.

Abstract

When total pressures decrease to the XHV region, measurements with ionization gauges become increasingly sensitive to disturbances such as electron- or photonstimulated desorption, outgassing of the gauges, and production of secondary electrons by X-rays. Therefore, it is very important to have a reliable pressure scale also in the XHV region. The laboratory for vacuum physics at the Physikalisch-Tech-nische Bundesanstalt (PTB) in Germany provides a calibration system with molecular beam expansion for pressures down to 10-10 Pa and is building a new calibration system based on continuous expansion for a lower calibration limit in the 10-11 Pa decade. The advantages and disadvantages of the two systems will be discussed and calibrations with the molecular beam system will be presented. For determining the outgassing rate of the vacuum chamber of the new calibration system, a new method was applied and compared to the classical methods, the throughput method and the pressure rise method. In particular it was checked whether the outgassing rate depends on the pumping speed applied to the chamber as proposed in a theory by Malev.