Advice on locating reason for poor vacuum?

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.
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ian_krase
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Advice on locating reason for poor vacuum?

Post by ian_krase » Mon Nov 12, 2018 10:09 am

I've been trying to get to 10E-6 with my vacuum system, but having a hard time breaking 3E-5.

As far as I can tell (via leak testing with a trace gas) my system is airtight. However I suspect it's dirty (I'm a slob) and/or contaminated by my questionable vacuum grease decisions.

Does anybody have a way for identifying the problem (other than "be able to afford an RGA")? Warm up section-by-section with a heat gun?

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Dennis P Brown
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Re: Advice on locating reason for poor vacuum?

Post by Dennis P Brown » Mon Nov 12, 2018 11:26 am

There are a number of "tricks" to detect leaks depending on their type: real or virtual (i.e. out gassing.) For a virtual leak, the best method is time; that is, keep the system running for a day or two and see how much, if at all, it improves. A slow but steady improvement tends to indicate one is getting water and other contaminates off the inner surfaces and a virtual leak is the more likely source (or at least is dominating.) A simple heating of most parts of the vacuum system (maybe a hair dryer but heater tape is most often used - don't raise joints above 50 C unless they are copper) can also be used to see if an improvement can be obtained.

Relative to real (but slow) leaks, using a can of "Dust Off" or alcohol spray are good methods at detecting these types of leaks. Your high vac gauge will "jump" when a slow leak source is exposed to the spray. Eliminate the leaks and the system will improve.

If you used vacuum grease - your only real course of action is break down the system and clean off all contaminated parts that you used grease upon. High vacuum and grease are a very bad combination. Grease (even the best high vacuum type) tends to load up on water vapor and will continue to out gas for a very long time.

CaveJohnson
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Re: Advice on locating reason for poor vacuum?

Post by CaveJohnson » Mon Nov 12, 2018 12:25 pm

I have had very good success with brake cleaner, its much much more sensitive than lets say helium (without rga). My system was tight enough to go to -6 but i could still find hair cracks in an insulator with cleaner spray. Also yes, grease is very likely. One of my chambers made from glass had petroleoum jelly on the surface and it took a very very long time with lots of detergent to get rid of it and go to -6 again.

ian_krase
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Re: Advice on locating reason for poor vacuum?

Post by ian_krase » Tue Nov 13, 2018 2:10 am

To clarify a bit: this is about locating virtual leaks, slow permeation, and outgassing, not true leaks.

Detergent... why not toluene?

CaveJohnson
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Re: Advice on locating reason for poor vacuum?

Post by CaveJohnson » Tue Nov 13, 2018 8:56 am

Because in many countries outside the us toluene is very heavily regulated and all the good solvents are pretty much outlawed unless you have a businesses to buy them for. Also detergents are better for fine cleaning anyways as they can not only dissolve but deconstruct the grease into water soluble compounds.

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