Hydrogen leak detection

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.
Ross Moffett
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Hydrogen leak detection

Post by Ross Moffett » Tue Sep 10, 2013 5:08 am

I wonder if hydrogen might be useful for leak detection as helium is?

Hydrogen detector with 100 ppm+ sensitivity at $8 is hard to compete with, looking at pricing for used leak detectors.

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Carl Willis
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Re: Hydrogen leak detection

Post by Carl Willis » Tue Sep 10, 2013 6:31 am

Hydrogen is a major component of the residual gases in technical vacuum work, so wouldn't be high on the list as a leak detection tracer for vacuum systems. At the gross leak level, however, almost any common gas can be used as a leak tracer with an appropriate detector.

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Monroe Lee King Jr
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Re: Hydrogen leak detection

Post by Monroe Lee King Jr » Tue Sep 10, 2013 1:38 pm

I would think a Pirani gauge and Helium would be better. The gauge would change a detectable amount for a small leak. It would take a good deal of time to do it properly I would imagine but if you have ever searched for a tiny vacuum leak before you already know that.

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Re: Hydrogen leak detection

Post by Ross Moffett » Wed Sep 11, 2013 1:21 am

My objective is to have some way to evaluate my vacuum chamber welds, so per-contamination shouldn't be an issue. I was under the impression that helium would be used due to its better tendency to escape through small gaps (a "spongy" weld, for example). Would a refrigerant leak detector, or other leak detector, really work just as well for evaluating high-vacuum welds?

Maybe someone knows, I'm also concerned that the pre-existing weld on my stainless welded "garden sphere" may be leaky. Is there any experience with this, maybe the reason most people pursue hemispheres welded to conflat?

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Monroe Lee King Jr
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Re: Hydrogen leak detection

Post by Monroe Lee King Jr » Wed Sep 11, 2013 2:49 am

You might be able to Zyglo test for cracks it's not that expensive. You can make your own test with talcum powder and penetrating oil if you powder one side and spray penetrating oil on the other cracks show up. If you've made an X-Ray device this is a good place to use it. Brazing might be better than welding the thin stuff. Unless of course you are an experienced TIG welder. Magnaflux works on some stainless steels. Pressurizing a garden bowl with iffy welds could be dangerous indeed. You can use a Pirani gauge and test under vacuum. You just have to control your helium use, use a discarded ink pen attached to some surgical tube and dose a small area. Then wait for that to dissipate and give time for the Pirani gauge to register. Move on to the next little area pinch off the helium supply as fast as you can every time to keep the atmosphere as free of helium as you can. If you go releasing a large amount of helium you'll get a false indication from some other area that picks it up. Once you find a spot test it several times to make sure you found the spot. Freon would work with that gauge too, but don't inhale and gas from the chamber because freon makes phosgene gas if exposed to a hot filament (or cigarette) ect... a deadly gas.

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Carl Willis
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Re: Hydrogen leak detection

Post by Carl Willis » Wed Sep 11, 2013 8:38 am

Hi Ross,

Helium leak detection is the standard, and most sensitive, method used with vacuum systems. One of the advantages of buying commercial vacuum components is that all welds have been helium leak-checked. Any machine shop that specializes in vacuum technology will have a helium leak detector and trained operators. University and industry research labs typically have a helium leak detector. In home laboratories, they are uncommon due to their cost. You'll find no reference to any fusioneers owning these machines, although a few people do have RGAs that can be operated as helium (or other gas) leak detectors.

A frequently-documented method of hunting leaks in fusors is to start a discharge in the fusor and then spray or wipe suspect areas with alcohol or some other hydrocarbon. Leaks cause the discharge to change color from the pink hues typical of air or the brick red of deuterium, to a characteristic dark blue.

Vacuum gauges respond somewhat unpredictably to the ingress of tracers into a leak. A volatile liquid such as water or alcohol may initially cause a pressure surge when it enters a leak channel, but frequently these liquids actually plug the leak through surface tension and / or freezing.

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Re: Hydrogen leak detection

Post by Ross Moffett » Wed Sep 11, 2013 6:06 pm

Any comments about what I might expect to find with the pre-welded sphere?

I'm wondering if it will be a good idea to cut out the weld with a lathe and re-do it to CF flanges.. but it's not really necessary so far as fusing and vapor deposition (the other thing I will be doing in my chamber) are concerned, since I could insert the samples/fixtures through smaller flanges. I'd like to not cut and weld to large diameter conflat if previous fusioneers have been down this path and found the factory welds satisfactory. Mine is machine welded, there is no "stack of dimes" effect, and the weld appears to be very uniform and good.

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Re: Hydrogen leak detection

Post by Richard Hull » Wed Sep 11, 2013 6:24 pm

Nothing whatsoever will be known about the vacuum readiness of any weldment until a quality, calibrated vacuum gauge capable of reading to 10e-6 torr is in hand and the weldment is tested under a vacuum of at least 10e-5 torr.

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Re: Hydrogen leak detection

Post by Ross Moffett » Thu Sep 12, 2013 12:47 am

Then that is what I shall do. I'm going to weld a 2.75" Conflat to it and try both methods.. I figure I can stick this hydrogen detector in the exhaust of my roughing pump while I blow hydrogen over the welds. At $8 I can breadboard something to see if it works, will update when (if? :) ) such time comes.

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Re: Hydrogen leak detection

Post by John Futter » Thu Sep 12, 2013 1:46 am

Ross for your detector to work you would have to have a gross leak
A leak that could be found by pressurising the sphere and looking for bubbles with soapy water.
Helium leak detectors are 10,000 to 100,000 times more sensitive than that detector

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