All upward, inverted-cone extension to all oil diffusion pump cold cap appears to provide a method for reducing amplitude and frequency of pressure bursts in a continuously pumped vacuum system. Indications are that the pressure fluctuations are a result of oil dripping from the cold cap and/or the liquid nitrogen trap directly over the diffusion pump, then reevaporating and backstreaming into the vacuum system. Oil drip wires are shown as necessary to guide the condensed oil to the pump inner walls rather than into the hot vapor stream. The erratic behavior of these bursts and the difficulties in pinpointing their behavior is given in considerable detail.
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Fluegge et al. (1971) studied this question.