Vacuum System Just Stopped Working

[Joe Mayer]

Brief background. I am a member of a university research team working on pulsed plasma thrusters. We have a vacuum system consisting of a small vacuum chamber connected to an Edwards Diffstak 63/150P Diffusion pump along with an Edwards RV5 Rotary Vane pump. Our testing procedure consisted of plugging the Diffusion pump in to heat up while pulling a vacuum with the RV5 inside of the Diffusion pump. Once we bring the Diffusion pump down to at least 0.6 millibar we close off that valve and pull a vacuum in the chamber to at least 0.3 millibar before opening the Diffusion pump up to pull a lower vacuum.

Now on to the issues that we are having. Everything was working fine and then we had to take the chamber apart to add some parts and once we put everything back together we were no longer able to pull a vacuum below 3.0 millibar. We closed off each tube and were able to pull a vacuum (0.03 millibar) on the tubes and the chamber itself. When we connect to the Diffusion pump we are not able to get anything less than 3.0 millibar. The only changes to the Diffusion pump was that we changed the old oil and put fresh oil in.

My thoughts on what is going on:
1. There is a leak in the Diffusion pump and it is not allowing the RV5 to pull a vacuum.
2. There may be some water / oil / grease in the Diffusion pump that needs to "boil" away before we can pull lower.
3. The Diffusion pump oil needs to heat up more before optimal operation can occur.

Any ideas as to what could be wrong or what to check would be greatly appreciated. All the members of the team are really new to vacuum systems so any tips or tricks for future use would be greatly appreciated. Thanks in advance!

[Jeroen Vriesman]

If the pressure is constant (at some constant temperature), you might indeed have some vapor from the new oil.
What happens when you turn on the diff pump for a few moments (just to heat it a bit), does the pressure rise?

Water is usually 1.3mbar at room temperature, so it might be some light hydrocarbon.

I've seen vacuum oil in my turbo pump (tph330 where you can see the oil) giving bubbles for hours after changing the oil.
If the pressure does not rise with temperature, then it is a leak.

Can be hard sometimes, last time I was chasing something like that I put my whole system in separate pieces on different pumps, and put it back together again only to find out that the leak was in my pressure sensor, the sensor I tried to test everything with...

[John Futter]

Did you change the o-ring on the fill/ drain plug on the bottom of the diff pump.
not changing this o-ring after it has been hot for many months can lead to a massive leak at this point

[Joe Mayer]

Thank you for the responses. I will make sure that all of the o-rings are changed out with new ones and will run the RV5 as much as possible to make sure that any vapor is boiled out of the system. I'll have my team try to run the pumps at different temperatures to see if there is any pressure fluctuations.

Thanks again for all of the help!