Leak in vacuum system (diffusion pump?)

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
Thomas Sheeleigh
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Leak in vacuum system (diffusion pump?)

Post by Thomas Sheeleigh » Mon Mar 30, 2020 3:07 am

I am finishing up the assembly of my vacuum system and am currently testing all the equipment. Using just the roughing pump I was able to pull a minimum of 250 microns measured in my chamber. My roughing pump is a used Edwards e2m12 I got off of Ebay, which after cleaning was able to pull around 18 microns at the head (took a few seconds). The pump is rated for 10cfm so I dont think my problem lies there. In trying to find the leak I heard a faint but recurring, metallic popping noise coming from my diffusion pump that would slowly die away as the pressure climbed back up to ATM (everything was turned off). I am now worried that there may be a leak in the body of the diffusion pump and that noise is the sound of air coming up through the oil in its base. The diff pump was also an Ebay purchase and it was a bit beat up when I received it. Some of the cooling lines around its body were bent or slightly crushed but no obvious dents or cracks in the pumps body. Another bit of info that may help is that when I turn off my pump and the system slowly leaks back to ATM pressure, my chamber (which is at this point isolated from the diffusion/roughing pump) leaks at a rate of around 100 microns/second. As this is my first time building and operating such a system I don't know if that is reasonable or if the source of the leak could be closer to the chamber. I am now asking the more seasoned vacuum chasers out there if there is something here that I may be missing or if indeed there is a high possibility of a diff leaking pump body. Any info/comments are useful to me. I have attached a photo of the vacuum system below so you could get a better understanding of what I am working with.
Attachments
20200329_224734.jpg
Main chamber with pressure sensor and vacuum connection on the far side T, view port in center and Granville-Phillips gauge on close side
20200329_224727.jpg
That tube is coming up is from the vacuum pump system below and connects to a bellows valve and then a long (unfortunately) SS hose to the chamber.
20200329_221426.jpg
roughing pump in foreground with diffusion pump stack in the back. The diffusion pump has a cryo-trap and gate valve on top (came with the pump). The roughing pump has a T splitter which connects to the outlet stem on the diff pump and a KF connection at the top of the diff pump stack. (The image appears to be rotated 90 deg CC)

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Maciek Szymanski
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Re: Leak in vacuum system (diffusion pump?)

Post by Maciek Szymanski » Mon Mar 30, 2020 6:47 am

I’m afraid that with such long, small cross section hose, elbow and two 90 degree valves (and the one at the chamber looks really tiny) there is no way to pump anything with a diff pump. The diffusion pumps do not “pull” the gas down the line but the particles must “fall” into the pump by the brownian motion to be pumped out. For this to happen you need a big diameter straight and shortest possible connection between the chamber and the pump. Your setup looks rather a like a particle trap - your effective pumping speed may be less than the outgassing rate of the system. The best solution is to bolt the chamber directly to the pump or, as it is done with most process vessels with a short elbow of diameter equal to the pump flange.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Maciek Szymanski
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Re: Leak in vacuum system (diffusion pump?)

Post by Maciek Szymanski » Mon Mar 30, 2020 7:15 am

The best test is to fix your ion gauge to the flange at the end of the elbow and test the pump system alone.
“Begin at the beginning," the King said, very gravely, "and go on till you come to the end: then stop.” ― Lewis Carroll, Alice in Wonderland

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Richard Hull
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Re: Leak in vacuum system (diffusion pump?)

Post by Richard Hull » Mon Mar 30, 2020 8:39 am

What was your base pressure with the mechanical pump for the whole system before you started the Diff pump?? You should never start a diff pump until your mechanical pump has ideally pulled the entire system and chamber down below 50 microns!! That popping you heard might have been bad news for you pump oil.

If your mechanical pump can't pull the entire system to 50 microns you have big sealing problems. Based on all that volume I see in your system, I feel sorry for your poor mechanical pump. Did you test seal your system as I am doing in baby steps? You have no idea where you are now.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Leak in vacuum system (diffusion pump?)

Post by John Futter » Mon Mar 30, 2020 9:09 am

you need to divide and conquer
ie you need to shift your conflat 2.75 blank to the end of your flexible hose that is just before your pirani
you need to shift the pirani to the other side of this
is this alright then reconnect
if not suspect all joints beforehand
and so the story unfolds

far too many joints /bits and pieces to tell where problem is!!!!

Thomas Sheeleigh
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Re: Leak in vacuum system (diffusion pump?)

Post by Thomas Sheeleigh » Mon Mar 30, 2020 4:08 pm

Sorry for the confusion, my diffusion pump was not running. It was only my roughing pump. As Richard said, my vacuum wasn't strong enough for me to feel comfortable starting my diffusion pump. This is why I was confused to hear any noise coming from the diff pump body. Thanks for the advice so far, this should save me some troubleshooting time.

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Mark Rowley
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Re: Leak in vacuum system (diffusion pump?)

Post by Mark Rowley » Mon Mar 30, 2020 5:30 pm

Hi Thomas,

I have to reiterate on what Richard said. You have way too much plumbing between your diffusion pump and Fusor chamber. I’d venture a guess that your plumbing has close to 3 times the total volume of the Fusor! That elbow coming through the desk is easily large enough to suffice for a Fusor itself.

Here’s my recommendation...get rid of everything I marked in red. Keep the distance and volume between your diff pump and Fusor as short as possible.

Get an 18” bellows line to go between the diff pump and Fusor. And of course, your valve should be placed in the midst but without unnecessary voluminous plumbing connectors.

Keep it simple!

Mark Rowley
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Thomas Sheeleigh
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Re: Leak in vacuum system (diffusion pump?)

Post by Thomas Sheeleigh » Mon Mar 30, 2020 7:08 pm

Thanks for all the advice, I really do appreciate it. I'm going to work on reducing the amount of excess volume.

One last question, when I bought the diffusion pump it came with everything you see in the picture from the diff pump to that large elbow piece coming through the desktop. Presumably, whoever was using the pump before I got it was using the whole stack with all that excess volume. Now, we don't know what the rest of their system looked like but they would have had to pump down to single/low double-digit micron pressures before using the diffusion pump. Does the cryo-trap have a big enough impact on pumping to justify all that excess space? It seems to me like it would work in a similar manner to the diffusion pump in that it just condenses the air that happens to venture off in that direction (through Brownian motion). Or maybe they just had a larger mechanical pump?

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Re: Leak in vacuum system (diffusion pump?)

Post by John Futter » Mon Mar 30, 2020 9:16 pm

could have been off a SEM
As you are not going to use the cold trap (the bit directly above your diff stack) take it off
they are notorious for springing leaks due to the excessive thermal cycling they undergo

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Richard Hull
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Re: Leak in vacuum system (diffusion pump?)

Post by Richard Hull » Mon Mar 30, 2020 9:43 pm

It looks like you spent some money buying adapters to work off the huge volumes of what you received. Smart money would have been to get rid of 80% of the big stuff from the throat of the diff pump and immediately make or pay a machine shop to make a single adaptation from the mouth of the Diff pump to a standard 2.75 conflat flange or a KF25 stub fitting, and then go from there.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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