Leak Troubles

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.
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Leak Troubles

Post by Dennis P Brown »

No knowledge of that specific gauge head but have cleaned an Edwards. That had a filament that one could not clean since it had a coating (the rest could be cleaned and that fixed the gauges issues nicely.) Bottom-line: call the company and confirm that cleaning is allowed and what solvent is acceptable before trying to clean it. Destroying a gauge isn't fun.

Is there a way to cross reference the gauge head's performance? I used someone else's gauge system to cross calibrate mine when I was unsure if it was any good. Also, cheap TC gauges are available on ebay that can partly serve that purpose; I later bought a cheap ionization gauge/head to backup and cross reference my existing High Vac gauge.
Peter Schmelcher
Posts: 228
Joined: Sat Nov 13, 2010 1:56 am
Real name: Peter Schmelcher

Re: Leak Troubles

Post by Peter Schmelcher »

Tom have you tested your penning gauge electrical feedthroughs as they can leak, mine did and on a nearby part I had a cracked weld.

Wrap nearby or all fittings with plastic wrap and/or tape. This helps reduce dust off overspray when hunting for multiple leaks and if possible bag your gauge in sections.

Unfortunately some leaks can have time constants measured in hours.

The joys of using recycled parts.

Good hunting
-Peter
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

Thanks for the help guys.

I'm a bit stuck at the minute on any testing, as I can't get a gasket to fit the internal CF fitting in the gauge. It doesn't seem to fit any of the standardised fittings. The gaskets outside diameter is 29.4mm (1.157") and this is an exact fit for the port, I tried DN25 gaskets, 32.79mm (1.291") OD and they won't fit, too big. ID is 21.7mm (0.853") and thickness is 3.8mm (0.15"). Does anybody have any idea of what sort of size fitting this is? The port itself is a bit strange too, compared to normal conflat. It only has a single recess, so the edge of the fitting itself acts as the knife edge, as far as I can tell - instead of having two recesses, with the middle one being the knife edge. See attached picture.

I tried running with the old gasket, with some hope that it might hold, but no luck. When I find replacements I can try and follow the advice given. I'm also in a hard spot regarding technical support on the gauge - LEISK closed down a number of years ago, and they've left absolutely no information behind. Whoops.

Tom
Attachments
See single recess
See single recess
gauge port.JPG (27.26 KiB) Viewed 6185 times
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Leak Troubles

Post by Dennis P Brown »

That normally uses a copper gasket and that is often best (consider getting some); however, special "o-rings" can be purchased that fit those types of flanges and you need to get the correct sized one from the company's that sell those types of fittings - that is, as you have found out, those are not a standard size because they are not o-rings but square (flat sided) gaskets that look very much like O-rings. Failing that, cutting your own flat "washer" shape gasket is not exactly difficult and will work until you can buy the correct gasket.

A semi-safe cleaning fluid is absolute ethanol alcohol. Can't say it wouldn't harm any coatings but it is the best compromise choice. A dip in a warm bath of the Alcohol might do the trick - do not use compressed air to dry since that might damage filaments.
Peter Schmelcher
Posts: 228
Joined: Sat Nov 13, 2010 1:56 am
Real name: Peter Schmelcher

Re: Leak Troubles

Post by Peter Schmelcher »

If your gauge works fine at higher pressures and then stops as the pressure drops you might have a magnetic field strength problem. If you adjust the magnet position and center it over the cell it might operate to a lower pressure. It is also possible heat has demagnetized the magnet.

The voltage between your electrodes could also be low. You might read a few other penning manuals for typical voltages.
Penning manual TINA14E1_Web.pdf
(752.87 KiB) Downloaded 861 times
BTW there is nothing magical about conflat seals, they worked well enough, others used them and it became one of the survivors. Wheeler also designed a wire sealed flange, just a squished copper wire between two flat surfaces.

If you can’t buy a replacement seal, repair the old one with a little solder in the groove or make your own by filing a CF gasket to make it fit.



I meet a retired engineer enjoying life as a part time docent in the Santa Fe technology museum. A piece of trivia is vacuum chambers for the Manhattan project were made of plywood with big diffusion pumps. I asked how thick the metal liner was and he responded just painted on the outside.

The physics of some high vacuum gauges
Ionazation CERN p145.pdf
(1.3 MiB) Downloaded 1740 times
-Peter
Jerry Biehler
Posts: 975
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Leak Troubles

Post by Jerry Biehler »

Look up copper crush gaskets, they are used on all sorts of things like hydraulics. Aluminum ones too. Either will work and there is a huge selection in size.

Heck, you could put a nitrile o-ring in there and it would be fine. You would be limited on bake out temps but I would not worry about it for what you are doing.
Tom McCarthy
Posts: 405
Joined: Wed May 08, 2013 3:36 pm
Real name: Tom McCarthy
Location: Ireland
Contact:

Re: Leak Troubles

Post by Tom McCarthy »

Whoa, thanks for the tip Jerry, copper crush turned up a treat, and affordable too. Ordered a pack of 1.5mm thick ones, they should do, old ones were 2mm. Arriving Monday.

Thanks for the gauge info Peter, the chamber actually seems to be holding up at 20 microns with the old gasket in the gauge, I've yet to check readings with the turbopump running though. Reading through those docs.

The more you get into it, the less mystifying the nuts and bolts of science hardware seems to get. From the outset it seems incomprehensible, but things simplify as you work on them. Point being it seemed to me at the start as if conflat etc had to be rigorously justified by in depth models and so on. In reality, the only model that looks like it matters is experience.
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Leak Troubles

Post by Richard Hull »

That is what the hands-on imperative is all about.....Do and learn. It is why "labs" are used in college. Labs in college are only for exposure. Living of life must forever be a constant "lab" experience with tremendous and continuous hands-on events, going far beyond mere exposure to border on or achieve expertise.

When no working with the hands you should reading with the eyes.

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
Post Reply

Return to “Vacuum Technology (& FAQs)”