Low pressure limit of the bayard-alpert gauge

Abstract

For many years contradictory opinions have coexisted in vacuum technology on the low pressure limitation of the Bayard-Alpert gauge. On the one hand, theory predicted that a reduction of the ion collector thickness below 0.1 mm should decrease the sensitivity such that the gain obtained through the subsequent decrease of residual current is, in the end, lost: hence, a constant measuring limit in the low 10 −11 torr range. On the other hand, sporadic tests on gauges with collectors only a few microns thick inside closed grids indicated that a limit one order of magnitude lower might nevertheless be achieved. Results are presented here which show that by pursuing the latter while simultaneously enhancing the gauge sensitivity (larger grid diameter and deeper penetration of electrons in the grid volume), modulatedB.A. gauges with a measuring limitation in the 10 −13 torr (10 −11Pa) range can be built. An interesting result of this approach is a modulation factor k (Redhead's definition) above 0.8. This important improvement in the low pressure performance of the B.A. gauge has been obtained in spite of filament induced residual currents (space charge degassing and tungsten evaporation) which can appear in the 10 −12 torr (10 −10Pa) range, independently of the gauge type. As a result, a modulated B.A. gauge, characterised by a low pressure measuring uncertainly lower than 2×10 −13 torr (2.6×10 −11Pa), has beenselected for replacing about 400 less performing gauges of this type which are currently in use on the ISR at CERN. These existing gauges have, in fact, become obsolete due to the ISR pressure reduction into the 10 −12torr (10 −10Pa) range.