Backstreaming from rotary pumps

Abstract

A differentially pumped 60° mass spectrometer has been used to examine the atmosphere above an untrapped rotary pump and also to examine the efficiency of various trapping arrangements. The use of a mass spectrometer for this work has the advantage that backstreamed contaminants from the rotary pump can be detected in the presence of large partial pressures of water vapour. Thus a sorbent trapping material can be examined when partly saturated with water vapour, a situation likely to arise during normal use on a vacuum system. Results showed that a 2 in. column of activated alumina balls trapped 99.7 per cent of the backstreamed vapours and reduced the pressure of backstreamed contaminants from the untrapped value of 3.8×10 −3 torr to about 2×10 −5 torr. The efficiency was unaffected by gross water contamination. 13X molecular sieve showed a similar efficiency but the performance deteriorated slightly after a large quantity of water had been sorbed. The backstreaming rate measured for a two stage rotary pump with an oil free low pressure stage was 8 per cent of that of an oil filled two-stage rotary pump. This rate for the dry stage pump was still sufficient to contaminate a vacuum system. A cheaper and more effective means of reducing the backstreaming was to fit an activated alumina trap to a rotary pump with normally lubricated stages.