The use of a quartz crystal microbalance for measuring vapour backstreaming from mechanical pumps

Abstract

The backstreaming of oil vapour from mechanical rotary pumps has been measured by condensing the vapours at 87°K on an hf crystal vibrator with a frequency of oscillation that depends on the condensate mass. Backstreaming rates were ontained for rotary pumps charged with lubricating oils with and without inhibitors. Experiments were made with both single and two-stage rotary pumps and the rate for a two-stage pump was half that of the equivalent single stage pump. A typical rate for a single stage pump at 15 millitorr gas pressure was 3ωmg cm −2 min −1 rising to 6ωmg cm −2 min −1 for an inhibited oil. Several methods of reducing backstreaming were assessed. These were as follows: enhancement of gas pressure to reduce oil vapour diffusion from the pump; condensation on a liquid nitrogen trap; adsorption on zeolites, alumina and charcoal; and the decomposition of the organic vapours in a glow discharge device. The greatest reduction in backstreaming with a moderate effect on the pumping speed was obtained using an adsorption trap charged with activated alumina. The trap was operated at room temperature and the backstreaming vapour penetrating the trap gave a mass flow rate of <1 per cent of the unrestricted value.