Total pressure measurements below 10 ⁻¹⁰ torr with non-magnetic ionization gauges

The modulation method has been applied to (i) the Bayard-Alpert gauge and (ii) the suppressor gauge to reduce their lower pressure limits and measurements made with these two gauges to pressures as low as 10 -14 torr. It is shown that the modulated Bayard-Alpert gauge can measure pressure to 3 × 10 -11 torr (10% error) without correction for the residual-current modulation effect, and measurements can be extended to 3 × 10 -13 torr if corrections for the residual-current modulation are made. The residual limit of the suppressor gauge has been measured (approximately 2 × 10 -13 torr). With modulation applied to the suppressor gauge the lowest measureable pressure, set by amplifier noise, is about 10 -13 torr for a 10: 1 signal-to-noise ratio. Electronic desorption of chemisorbed material from the grid of a gauge and/or a molecular flux in the gauge are shown to cause a limitation to the lowest measurable pressure; these limits may be higher than the x-ray limit under certain conditions.