Issues diagnosing large leak in high vacuum system.

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Reed Upson
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Joined: Sat Dec 31, 2022 11:15 pm
Real name: Reed Upson

Issues diagnosing large leak in high vacuum system.

Post by Reed Upson »

Hi, I'm a little new here so I apologize if I can find information to solve my issue elsewhere! I have a high vacuum system with a two stage rotary vane mechanical pump for roughing and a cryosorption pump for finishing. The roughing pump isn't extraordinary, and is rated to pump down to 10^-3 torr. The issue is that I can't get it to pump below ~0.5 torr despite it running easily down to 10^-2 torr on previous tests. The only change to the vacuum system since those previous tests was the the addition of an O-ring sealed pneumatic isolation valve to isolate the roughing pump. That valve is rated down to 10^-9 torr with a leak rate far lower than what I am seeing. Although I purchased the valve used, it appears to be in very good condition.

The pressure curve is rather odd (image attached), and consists of a trough where the pressure hits its minimum (just after the roughing pump is switched on) followed by a rapid increase in pressure despite the running pump. The pressure slowly levels off to ~0.7torr, and when the pump is isolated the leak rate increases slightly (to ~3*10^-4 torr/sec).

Here are a few of my ideas as to what might be happening, please let me know whether these are credible explanations.
1) the valve is leaky (I doubt this).
2) the NPT fittings on the pump are leaky -- in fact I know this to be true. This doesn't explain the continued leak after the pump is isolated. I intend to seal these fittings in Faraday's wax as I have no better options at the moment (no high vacuum grease, only Teflon tape).
3) The O-rings in one or many of the KF fittings are leaking. It has been extremely cold recently (unheated lab) and these tests were run at ~5deg C. It likely gets down to -10deg C at some points, and the O-rings are fairly stiff. I am also wondering if the extreme cold could have formed ice inside the vacuum system which is now outgassing (other surfaces in the lab accumulated a very small amount of ice, although the vacuum system has been warmed and pumped down repeatedly and the same behavior persists).

Any insight into the issue would be helpful!

Regards,
Reed
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Richard Hull
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Re: Issues diagnosing large leak in high vacuum system.

Post by Richard Hull »

What can the mechanical pump do at the head? The isolation valve may have a leaky bellows (cracked at a fold).

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Sophie Yang
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Re: Issues diagnosing large leak in high vacuum system.

Post by Sophie Yang »

Since you have not provided all the information and pictures of your system, we may need to troubleshoot the problems one by one.

It can be seen from the pressure curve picture that your system has a large gas load, assuming that your system does not have any leakage, I think you should consider whether your cryopump is saturated and needs regeneration. In order to know whether the cryopump is outgassing, you can isolate the cryopump and pump the system again.

The other is to troubleshoot the rotary vane pump. You can independently test the limit pressure of the rotary vane pump with a vacuum gauge.

If you use the viton gasket, please note that its minimum operating temperature is - 15 ℃, and it cannot work at a lower temperature.

If there is no problem with your pumps, you need to consider the leakage. You'd better remove the valve for a separate test to determine whether it has leakage. Alternatively, you can spray some acetone on the valve and observe whether the pressure rises significantly.

Of course, the best solution is to use a helium leak detector or RGA, which can quickly tell you where the system leaks.

Sophie
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