Diffusion pumps

Diffusion pumps were invented around 1915.

In the article about the origins of Apiezon, it was mentioned that Bill Burch discovered that the low vapour pressure oils he was working with could be used as a replacement for Mercury. So what was so special and new about these diffusion pumps? Well they work with no moving parts at all! This means they are much more reliable, as there are no parts to break or wear out.

Before this invention, almost all pump types had been operated by mechanically driven pistons or vanes, which worked by sucking in and pushing out little pockets of air, and used valves to stop it returning. The size of the air pockets, the speed of the pistons and the leakiness of the valves were just some of the factors which, despite many improvements, really limited their abilities.

diagram of a vacuum pump









Diffusion pumps operate in an entirely different way; by heating the fluid (initially mercury and then later the low pressure oils) to a boil, and directing a high speed jet of vapours down onto a condensing surface, they are able to constantly push down air molecules into the bottom of the pump and thus maintain a pressure differential. A mechanical ‘backing pump’ can then remove the trapped air from bottom of the diffusion pump, while the fluid is re-boiled to keep the pump running.

Originally Mercury was the fluid of choice, but it had several disadvantages – aside from the toxic vapours it created, and its tendency to amalgamate with metals that could have otherwise been used for the construction of pumps, it was also rather prone to becoming fouled with traces of grease or oil that stopped it working all together.

Bill Burch’s discovery of using diffusion pumps and Apiezon oils overcame these problems as they were non-toxic, could be used with any type of pump material, and did not get easily fouled by contamination. Refinements were needed to both the oils and the pump designs in order to get the best performance, with some aiming for the lowest vacuum and others for the greatest pumping speed. The latter eventually led to Apiezon oil AP201 – specifically designed to operate well in the harsh environment of the very largest Edwards vapour booster diffusion pumps, which are capable of moving 15000 litres of gas a second!