Vacuum is Difficult

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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Jason C Wells
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Vacuum is Difficult

Post by Jason C Wells » Sun Sep 13, 2015 12:51 am

This note is for newbies that are newer than me.

You should know that high vacuum is hard to achieve. High vacuum requires a great deal of attention to detail. It saddens me to report that I am a mechanical engineer with 14 years of experience in diverse work. Vacuum _still_ manages to frustrate me. High pressure hydraulics is vastly more easy to do than this low pressure stuff. How so? You can see the leaks.

I am seriously thinking of building a vacuum test bench and "certifying" every tube, every part, every o-ring that goes into my apparatus. The cost would be worth the time saved.

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Jason C. Wells

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Richard Hull
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Re: Vacuum is Difficult

Post by Richard Hull » Sun Sep 13, 2015 7:08 am

Well spoken Jason. Vacuum is just one of the technical/scientific arts that must be mastered to do fusion. Newbies take note.

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.

prestonbarrows
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Re: Vacuum is Difficult

Post by prestonbarrows » Sun Sep 13, 2015 5:45 pm

Leaks in the torr range can be found with your ears and eyes. This is likely a sizable crack in a joint, a rotten o-ring, or untightened clamp/bolt.

Leaks in the 10-100 mtorr range can be found with a squirt bottle of methanol and a thermocouple gauge. Start from the bottom and work your way up (since the liquid will drip downwards) spritzing some methanol on all welds and seals. The pressure will spike when you hit the leak and you can narrow down the location with more careful applications.

Leaks in the 1E-4 and below range are best handled with an RGA and a tank of helium. Repeat the same process as above while looking at the partial pressure of helium. This time start from the top and work down since helium rises. RGA's are expensive but are the absolute best leak detection method. A poor man's version is to use a hot cathode gauge, magnetron gauge, or other composition-dependent gauge; when your test gas gets sucked into the leak it will alter the ionization current in the gauge and cause a blip in the pressure. This method can be tricky since blips in pressure are small and on the order of thse caused by normal outgassing and conditioning of a fresh system. Make sure the blips are repeatable at the same position. Using a digital o-scope is much nicer than a DMM for this method.

If you are using off the shelf vacuum hardware, the leak will essentially always be due to improperly installed o-rings/gaskets. So, there is not much benefit to a dedicated test bench unless you are making lots of custom vacuum hardware. When you remove the parts from the test bench and connect them to the full system, you just re-introduce the possibility of leaks again. You want to leak check everything in situ in it's final configuration in the end anyways.

And of course remember to only use UHV rated materials and dont leave enclosed volumes which will cause virtual leaks (think blind tapped holes). Any issues with these will lead to lots of outgassing and high pressures even if the vacuum chamber is perfectly sealed to the atmosphere. You will also never be able to find them with any external leak checker device.

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