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Vacuum system pressure rise rate

  1. Aug 13, 2012 #1
    Hello, I am currently working on a pulsed laser deposition (PLD) system whose volume is approximately 115 to 120 liters. As you can assume, there are a lot of ports in the chamber but a lot of them are windows or blanks (2.75, 4.5 and 6 inch). Significant ports (possible sources of leaks -virtual and real) include a bellows, sample and target holders and a couple other small volume devices. When pumping only with a turbo pump (backed by a scroll pump) the lowest pressure achieved is about 1.3*10^-7 Torr. Forget about ion pumps, they are not relevant to my question -I think- and so they are (running) isolated by a (leak tested) gate valve at ~10^-9 Torr. It is important to note that this system (main chamber and ion pump) have not been baked out in a long time after being vented several times in the last few months, so I don't find it surprising that the turbo hasn't pumped the main chamber down to 10^-8 Torr in 3-4 weeks or that the Ion pump is at ~high 10^-9 to low 10^-9 Torr (current in the mid 10^-5 A).

    Now, here's the thing. I do not have an RGA to leak check but it appears to me that a very simple test could somewhat give an idea of whether there is a leak or not. What I did was to close the (leak checked) gate valve to the turbo pump (remember ion pumps are isolated and their gate valve is leak free) so as to let the pressure rise slowly while monitoring it (see attached graph). Basically, the pressure rises ~2 orders of magnitude in ~35 minutes (from ~1.3*10^-7 to ~6.8*10^-5 Torr). To me this seems perfectly normal: unbaked (large volume) chamber walls outgas, air slowly creeps in, you name it. If there were a significant leak I would expect the pressure to rise much faster but then again I'm not sure. Any comments as to whether this looks like a leak or not? Thanks ahead!
     

    Attached Files:

  2. jcsd
  3. Aug 16, 2012 #2
    It's been decades since I did vacuum work, but you might try to pump it down several
    times in a row. Watch and record the time it takes the pressure to climb after each cycle.
    The out gassing will get less and less, so your pressure climb time should increase with each cycle. When you have somewhat of a stable time, you are just looking at your leak.
    Release some helium around each of the suspected leak areas and watch the pressure.
    The helium is so small it will more readily "fit" through the leak. I am hoping you will see a slight pressure spike when you pass the leak.
     
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