Know All About Diffusion Pump Part-2 Q.11 How to prevent back streaming of diffusion pump oil? Ans. To prevent back streaming of Diffusion Pump oil it must be ensured that1. System is “Roughed out” before Diffusion Pump is put in line. 2. Rotary vacuum pump is maintaining a vacuum of at least 10-2 Torr. 3. If process is causing release of a large amount of gas, then sometimes it may be required to slow down the process. 4. Cooling water output volume and temperature must be checked. If this is not as per the manufacturer’s recommended values then descaling and increasing cooling water circulation is required. 5. If problem of back streaming is frequent then increasing the capacity of backing pump is required to solve it. 6. Diffusion pump must have a cryotrap or at least baffles on top to trap gasses trying to flow out of pump into chamber. 7. All the valves should be checked from time to time. 8. Strange as it may sound, very low fore pressure can also cause back streaming. Q.12 Why is diffusion pump water cooled?
Cooling coils around diffusion pump Ans. A diffusion pump has no moving parts and therefore depends on diffusion pump oil for creation of vacuum by action of gas diffusion. For diffusion to occur the diffusion pump oil is heated and vaporized using heaters. It is then released through jets at high speed and in this way it traps air by diffusion process. Then it is cooled at walls of diffusion pump. This liquefies the oil vapors and these vapors again turn to liquid state. This completes the cycle and the process repeats continuously. Q.13 I have a Chinese diffusion pump (Not Varian) do I need to use some different kind of diffusion pump oil? Ans. Diffusion pumps as well as Diffusion pump oils are made as per a standard engineering practice of interchangeability. So a good Diffusion pump oil will be suitable for any Diffusion pump made anywhere in the world. Just like same petrol works for cars made by Honda, Suzuki, Audi, Toyota or any other brand.