Bell Jar for Demo Fusor

[Ed Meserve]

Hello, all. Our group is beginning to pull together the components for a demo fusor, and I have found two possible bell jar setups we might be able to use.

One for $70:
http://www.thesciencefair.com/Merchant2 ... gory_Code=

and one for $300:

http://www.a3bs.com/Vacuum/Vacuum-Exper ... _1469.html
http://www.a3bs.com/vacuum-bell-jar-u21 ... nReferer=1

The $70 dollar set is attractively priced, but will need to have feedthroughs tapped through the plate. What concerns me more is that on the website they say they don't know how much vacuum it would take.

So the question is this: do you folks think that the $70 bell jar setup will work with the vacuum required for a Demo fusor, or should we go with the 3B Scientific Setup?

Thanks in advance,
Ed

[SteveHansen]

The write up for the first unit is almost humorous: the description of the added piece of glass and "It can withstand small amounts of Vacuum. How much Vacuum, we do not know."

As to the second one, the description does not state the type of glass the bell jar is made from but I'd bet it's not borosilicate. I'm not sure how hot a demo fusor gets but that might be an issue.

I've used the Nalge plastic bell jars and they can be got to 100 milliTorr reliably so the glass ones should be better in that respect.

One question for all - how big does the chamber have to be for a demo fusor? (Put in a better way, how small in diameter can it be?)

Steve

[Brian_Gage]

Seriously Ed, the $70. bell jar sounds like junk. I tried to read the "manual" for the second one and it wouldn't come up in Adobe. I wasn't impressed by either, and I'm certain somebody here has leads on much better deals that won't implode when your group gets started. Remember to include a polycarbonate or safety glass enclosure to protect bystanders from accidents. Shards of glass from an implosion can easily take out eyes or slice a jugular vein.

Also, regarding the more expensive unit, the rather limited description of a feed-through for electricity doesn't sound like one capable of handling high voltage or high vacuum.

Safe experimentation. Brian

[Dan Tibbets]

For a demo fusor the vacuum obtainable generally is not a problem provided the pump is capable enough. I have used Mason 1 pint canning jars. The rubber on the lids make a good seal. Also have used an ~ 1.5 quart 'Pickle Jar' (or may be it started life as a mayonnaise jar?). And I have seen a demo fusor here that was made with ~ 2 inch diameter glass tube. The possibility of failure is real, but the price is cheap.

The recommendation of a shrapnel shield is very important with any glass or plastic container vacuum vessel. The localized heating from an electron beam or other localized plasma effect - or even tapping on the outside can cause failure when the general vacuum itself is easily resisted.

The question of the sealing of the feedthroughs, valves may be more significant than the bell jar- base sealing. Also note that the rubber (?) gasket on the base plate looks like one continuous sheet. Any plasma would cause a problem on this rubber. Careful surgery to trim away the rubber from everywhere except near the glass plate might help. Simply placing a metal plate (aluminum flashing, etc.) that covers the base out to the glass seal area would greatly reduce the out gassing issues from plasma directly impacting the rubber.

Dan Tibbets

[SteveHansen]

I've got a variety of pyrex "bell jar" type chambers with various feedthroughs, side and top - all have a 3.5" ID and heights are around 10-12" in most cases.

[jarrod1937]

Steve, are these for sale from you or somewhere else? I've been looking for bell jars that are adequate as well as having feed throughs.
And yes, the first bell jar is a little scary in my opinion, if they don't know how much of a vacuum it can take, run away fast, it's not worth the safety risk.

[Chris Bradley]

I think you'd have better luck with a vacuum dessicator as your starting point than these unspec'd cheapies. Usually more substantial thickness than lab vacuum bell-jars and cylinders too, plus common and cheap as used ebay items.

[jarrod1937]

Thank you for the advice, I'll research those and see what I come up with. Though initial research shows that vacuum desiccators are not rated for very high vacuums, compared to similarly priced bell jars.

http://www.amazon.com/Nalgene-Vacuum-Ch ... 703&sr=8-1

That seems to be rated for a full vacuum, although I am not sure of the validity of that statement. Have any links to vacuum desiccators that have a nice vacuum rating?

[Chris Bradley]

Mechanically speaking, a vacuum vessel is a vacuum vessel. The pressure on a vacuum vessel at 1000 microns is ~14.5psi. The pressure on a vacuum vessel at 10^-9 mbar is ~14.5psi.

What is of concern for high vacuum is the sealing, and glass dessicators come with wide, ground-flat glass flanges, which is as good as it gets for glass jars.

[SteveHansen]

Jarrod,

I have supplied quite a few custom and standard small chambers, all with feedthroughs.
My page at http://www.belljar.net/chambers.htm has some details but it needs updating (in progress) and design guidelines (tube sizes, etc.). My document on Ace Threds at http://www.belljar.net/vol11ace.pdf details the feedthrough that I use. I love Ace Threds and the ones I generally use will take 1/4, 3/8 or 1 inch tube or rod. I have supplied some chambers with 2 inch Ace Threds.

Basically, I have been supplying tubes that either go into standard compression fittings (the picture shows one at left that goes into a 1-3/8 tube to KF40 adapter) or on to a baseplate (the two other examples). The two larger tubes have inside diameters of about 3.5 inches. For scale, the height of the large diameter portions of these tubes is about 1 foot. These tubes go on a 5 x 7 x 1 inch base with fitting for 1/2 or 3/8" hose and buna gasket.

The tubes that go into compression fittings are generally good to 10-6 Torr range.

[jarrod1937]

I'm assuming you're quoting 14 psi due to that roughly being the amount of a single atmosphere of pressure. But doesn't the level of vacuum have an effect of stress on the vacuum container/vessel? If I do a vacuum at the 75 micron range, doesn't that put more stress on the container than 700,000 microns? Is the risk of implosion higher by using a container designed for a lesser vacuum than it's being used for?
Sorry for the questions, just trying to get my head around the details.

[Chris Bradley]

Jarrod Christman wrote:
> If I do a vacuum at the 75 micron range, doesn't that put more stress on the container than 700,000 microns?
Well, if you are *that* close to atmospheric (one bar is 760,000 micron) then of course that would make a significant difference.

Point I was making was that if you pull a chamber down to a coarse, 'low' vacuum, say, a Torr then that's already all but 0.1% of 1 atm pressure (and that's a real easy level of vac to pull, I'm confident an evacuated dessicator is typically pulled down that far). So therefore you can only ever make a further 0.1% difference to the differential pressure on the vessel, no matter how hard you pull the vacuum from then on, because you can't pull less than nothing.

I'll happily stand to be corrected, but as far as I understand it, a 'vacuum capable' vessel means it is *mechanically* capable at any vacuum level. Whether it is constructed in a manner fit to avoid leakage, out-gassing and a myriad of other 'high vacuum' issues is another thing altogether, but mechanically it either 'does' vacuum, or does not. (Of course, if you pull down something *not* rated for vacuum, that doesn't mean it is safe to do so, but nor does it mean it won't work. If you understand the risks, and consequences, of implosion and take whatever precautions you deem necessary, you can pump down drinks bottles and whatever, as many here have proved.)

[jarrod1937]

Thanks for the explanation, I believe I understand what you're saying now, and in doing so that drastically opens up the possible vacuum containers one can use.