IrC
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Vacuum
Recently I was thinking about using a 3 neck flask I have for a plasma experiment. The only reasonably easy way to connect a vacuum tube and 2
electrodes is using large rubber stoppers. I have no doubt the flask is plenty thick enough to take 5 microns. The question I have is how much vacuum
is safe before the stoppers would suck in creating an implosion I would rather avoid. This question is so simple (possibly stupid?) I figured it
belonged in beginnings. I want to build the setup and try it but before I do I thought someone may have already had a similar experience and know
which may save me from a potential disaster. I thought about using epoxy on the stoppers but this may not work and I wish to be able to easily take
apart the experiment and make changes on the fly as it were. I also have a few very thick plasma experiment glass tubes which are open at each end
axially with similar ports to the 3 neck flask meaning would a stopper work there as well, assuming it will safely work at all. No need to point out
this may be a dumb question I already thought of that. Also, if someone knows from experience this could be a bad idea what method would be fairly
safe.
"Science is the belief in the ignorance of the experts" Richard Feynman
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turd
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| Quote: | | I have no doubt the flask is plenty thick enough to take 5 microns. |
At the risk of stating the obvious: What you care about is pressure difference, not absolute vacuum. Whether you have 10 mbar, 5 microns or 0.001
microns will have less of an impact than the weather it has outside. So either a flask is vacuum resistant or it is not. If they are sized
appropriately, the rubber stoppers will not be sucked in by a pressure difference of 1 bar and I'd be more scared about the sucking in of the
electrodes. But I have my doubts that they seal enough to reach 5 microns. Anyway, good luck.
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IrC
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I was just saying the glass appears to be thick enough to be of little concern by referencing 5 microns. The electrodes will be solid brass rods which
are a tight fit in the holes of the stoppers, with brass disks soldered on so that they cannot move inwards. What I was unsure of is the safety of the
stoppers themselves. Never before tried such a construction. What I wanted was a way to easily see inside with a device so constructed as to be
quickly taken apart and put back together. I had in mind experimenting with various fusor electrode configurations and also doing some experiments
with pulsed abnormal glow discharge. I was doing this with my bell jar but recently my 1 inch thick plexiglass bottom plate cracked where the
electrodes enter. This is the third time it has happened and I am burned out on buying new 12" square sheets to repair it. There is an incredible
amount of force on an 8 inch circle at high vacuum (the inner diameter of the bell jar). Eventually I will repair it but I want to find something else
besides plexiglass. It has to be a good enough insulator to stand 150 KV, be thick and extremely smooth. That will be later, right now I just wanted
to build something entirely different to do these experiments using items I have laying around.
"Science is the belief in the ignorance of the experts" Richard Feynman
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watson.fawkes
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Quote: Originally posted by IrC  | What I wanted was a way to easily see inside with a device so constructed as to be quickly taken apart and put back together.
[...]
It has to be a good enough insulator to stand 150 KV, be thick and extremely smooth. |
I'm not convinced that
putting rubber stoppers in a vacuum path is going to allow them to be easily removed. Do a test on a semi-expendable piece of glassware, probably one
with only a single joint. Instead of a solid rod, use a tube through the stopper and pull your vacuum through there. See how easy it actually is to
remove the stopper.
Do you need an entire plate to be insulating? Or is it good enough to have 150 kV of insulating feed-through?
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neptunium
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the force exerted by the atmosphere on any vacuum system is increasing up to a point and then stabilised and no matter how deep the vacuum gets the
force remain the same.
in other words ,if plotted on a graph with the force on y and vacuum onx axis, you'll get a steep slope first and then a flat line.
rubber stopper may also soften or melt if you messing with plasma, xrays, etc...
it could work for awhile but if you are done with repairs and quick fix i sudjest you'll find yourself a glass blower professional
who will custom build you a glass fusor with electrodes already in place...
thats what i did and i am very satisfy with those tubes...
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IrC
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@Watson I have feedthroughs and use color TV anode wire, on very high voltage experiments I have several feet of really expensive wire rated to over
200 KV without corona on the outside. Problem with many baseplate materials I tried was arcs inside the bell jar from electrodes right to various
points on the baseplate surface. I tried many materials. Must be very hard, smooth, damn good insulator. I know some materials were impregnated with
moisture from the air since it did not occur as often in winter months after a few weeks in very dry air, below 20 percent humidity. Usually it's 40
or higher here. Plexiglass has always worked best but over time cracks start which grow. Right where electrodes and vacuum port goes in. Even though I
made disks for both sides. Just from the upwards stress under vacuum, then releasing. So many repeat cycles you start seeing tiny cracks. Likely I'll
go back and fix it again but I am tired of it. I keep thinking it would be nice to find a foot square of marble or something like it. Maybe that would
work. I have done the hollow electrode thing before. Not a bad idea.
@Neptunium I actually have a couple like that plus a great one from university surplus with windows and gasketed (is that a word?) faces. Bunch of
bolts taking that apart. Annoying for quick on the fly experiments. I was sitting there looking at this 3 neck and how thick the glass was thinking
this might be a novel approach. Until Watson pointed out it may not like to come apart easily which was one of my reasons for desiring a new method.
If the stoppers will not come out easily it defeats the purpose. I was just worried about one suddenly blowing inwards. Now I have an added problem. I
need to think of a new way to go I guess.
"Science is the belief in the ignorance of the experts" Richard Feynman
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zed
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How big is the flask? Of course, the bigger it is...the weaker it is. Unlikely that you are using a really BIG flask, but just in case.
Atmospheric pressure is 15LB per square inch. An area 10 inches by 10 inches, is subjected to 1500 LBs of pressure, when its interior is evacuated.
At a certain point, bigger is not better.
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