Fusor Forums

Hi,

I found a large glass bell to use as a chamber for a demonstration fusor. I had issues with the gasket's air-tightness (My gasket comes from a crappy rubber sheet cut by hand).

Can play dough be used as a substitute for the gasket? Has anyone tried making air-tight seals with play dough? Is it suitable for high-vacuum (outgassing, porosity)? Are there any similar alternatives?

I will try placing the bell on the baseplate and surrounding the interface between the two with ample play-dough.

Cheers!

Please follow the forum rules for posting. At a minimum you need to use your real name.

Regarding play doh, mashed potatoes would be cheaper. There are lots of other household semi-solids you could try. What about flex seal? If it works you could make a commercial.

In all seriousness, I am myself willing to try unconventional alternatives to expensive vacuum equipment. If your research says that play doh will work then I suggest you try it and report back.

If your bell jar was indeed made for vacuum and you have a flat base plate then some silicon grease should work. If not then there are other problems.

Thanks, I changed my username.
I will get back if I get it working.

I tested it today, but could only reah up to 0.2 atm.

In all fairness, I was not starting with a functioning system to begin with. So, I may have other problems (like the pipe fittings) that are contributing. But, I heard hissing noises coming from the doh interface which leads me to believe that it does not work properly. If anyone else tried it, please let me know.

For reference, I am using a glass chamber similar to:
https://www.amazon.com/Whole-Housewares ... 510&sr=8-2
As pneumatic feedthrough, I am using something like this:
https://www.amazon.com/Pieces-Brass-Ref ... 71&sr=8-15

I would not use that as a vacuum bell jar. It is not designed for vacuum and failure could be catastrophic and dangerous.

Following on to Jim's comment...That is not a bell jar!! That is a floral or "my Pretty" display for the mantle. It is made of the wrong type of glass. It is far too thin. Beams from a plasma will see its untempered nature collapse, (Implode), throwing shards of glass everywhere. It is dangerous on so many levels.

Scientific bell jars are of Kimax glass at least 1/4" thick and properly tempered. They are tempered to relieve all internal stresses. They will have a 1/2 thick, flat, ground glass base for proper gasketing. They are very expensive when purchased new. Even they are not recommended for plasma work without a proper implosion shield.

Nuff said.

Richard Hull

Apiezon "Q" is pretty much like playdough and specifically designed for vacuum sealing. I tried using it to seal a bell jar to a metal plate.
It didn't work any better that a home made rubber gasket and was far less consistent.
Unfortunately this bell jar is an odd size (10 3/8 inches) and I have not found a commercial rubber "L" gasket that will fit.

But seriously, there are recipes for making play dough. The primary ingredient is flour. What are the odds it will seal?

One thing I have learned in the vacuum world. You have to be very meticulous. Murphy is lurking around every corner.

chuck

I don't think anyone here has ever even considered using play dough for a vacuum seal.

It is made with water.

Water in a vacuum chamber is the enemy. Bake outs are used to get rid of it (and other things) in very high vacuum systems.

If you already have real vacuum grease, why are you wasting your time making playdough?

Joe.

Without a proper bell jar containing a large, broad flat base, a proper mating gasket that is very lightly greased with a suitable vacuum grease, a deep vacuum is just not possible.

This assumes that you wish to run numerous vacuum tests where removing and modifying the internals of the system are part and parcel to the entire plan. Typically, this is the very reason for using a bell jar in the first place, be it a small glass bell jar or an industrial, one meter diameter stainless steel bell jar.

Casual work with a bell jar of questionable type, (mason jar, thin floral or display container, etc.), will yield casual, questionable results.

Richard Hull

update:

I gave up on using play dough, just could not reach low enough pressures after 2 days of testing. Thanks for the heads up on the vacuum-specific product!
I ended up grinding the bell jar with 80 grit sand-paper to create a somewhat flat lip and placed it on an Aluminum plate. Unfortunately, the plate was not very planar since it got slightly deformed during shipping and the seal had to be complemented with ample vacuum grease.
I tested the system (with ample safety measures in place) and achieved low pressures (0.00 atm on my crappy manometer), but not low enough to avoid having a plasma arc rather than a star. Maybe it is the fault of the bell jar, maybe of the high voltage feedthrough which is just a high voltage cable press-fitted through a hole in the Al plate, surrounded by a JB-weld seal (by the way, has anyone ever tried this type of feedthrough?). I agree with Richard, this solution is way too unreliable, and debugging the seals is just a nightmare.

How the hell does a bit of 0.5 inch aluminium get bent in the post

unless of course you have skimped and used something much thinner---read not suitable

you need to work out your active vacuum area in square inches and then the plastic modulus of your plate material

aluminium is not very strong / tough. Steel or stainless would have been much better even hard brass plate is superior

also 0.5 inch thick in aluminium is about a minimum for threaded seals from 1/8" up. You might get away with 3/8 plate stainless for the same use

Hello and please read up on vacuum systems and best practices; there are sources here in the FAQ's if you search carefully and there are books available on the subject. An excellent reference are old issues of Scientific American magazine that are available on line or at a library (old issues from the 1950's to 70's.) See their amateur scientist sections.

That said a wine bottle can make a great small but very serviceable "glass bell jar" like chamber. Simply cut the bottom off using an inexpensive scratching kit (these are sold on amazon and are bottle cutting systems with instructions on how to do it.) Wine bottles are thick and good quality glass and the upper end accepts a stopper that an electrode can be inserted thru (i.e. a piece of stripped copper wire.)

A gasket can be made from a rubber sheet (again, amazon and cheap.) Since you can grind the bottle surface (the cut section) so it is smooth, the gasket will seal w/o any vacuum grease. A cheap steel plate like a saw blade can be used as a base plate - the large hole can be used as a vacuum port via using epoxy to seal an appropriate sized copper tube (a good vacuum port.) Then with a proper vacuum gauge (that reads torr, not atm!) you can get a feel for real vacuum systems and an idea on what your pump can really do. Hooking up a neon sign transformer to create a plasma is then simple.

I used such a design in high school to make my first successful sputtering system.

I frankly worry about threads like this. I admire the overall desire to try for plasma, but there seems to be a lack of knowledge prerequisite to assembly of the project and adding energy in the forms of vacuum and high voltage.
Jamming a conductor through a hole in the baseplate and surrounding it with jb weld will not work. Using a bell jar desugned to display a flower arrangement will not only not work, but it might get you or someone else hurt. I don't mean to talk down at all because my own knowledge was once far below what was needed.
Use play dough, sure...of course it wasn't going to work, but it it doesn't hurt to learn a few things the hard way. My biggest problem is the continued string of bad decisions. I personally refuse to render more advice until you spend more time with the FAQ's and commit to working with safe equipment. I don't want you to be hurt and I don't want any hint of liability. I wish you the best.

Guglielmo, I have to agree with Jim’s response to the highest degree. If for nothing else, your safety hinges upon reading the FAQ’s many times over until you get a firm grasp on what’s required.

Also, post an intro about yourself in the “Please Introduce Yourself” forum as required by the site rules (it’s supposed to be your first post before anything else). If we know more about your experience, member responses can be tailor fit to your background and experience level.

Mark Rowley

Thanks all for the comments and the advice!

Just to be clear, I am trying to see how cheaply I can build a system that shows clearly a plasma ball (so the wine bottle or a jar would not work because of all the visual distortions). So, the hacky approach is entirely by design (It would be all too easy to buy chamber and feed-throughs from Kurt J Lesker...). I do have some experience working with high voltage systems, and I am fully aware of the risks that come when combining vacuum and high voltage and how to mitigate them.

Thanks again!

I have relocated this post the appropriate forum. To be honest, I wonder why the post even exists and almost dumped the entire thread but I shall let it stand for instructional purposes.

Sanding a fire polished surface to flatten it puts many fine notches into the glass. Theses are all weak points for the system to fail under vacuum. You say you are experience but I am not seeing it in your posts.

If plasma is what you want, then why vacuum at all? These plasma globes are ubiquitous for under $20 USD. They do not use high vacuums but rather gas mixtures that give plasma at much higher pressures.

I agree! leave this up as an example of how not to do vacuum work within a demo fusor. It points out over a long and tedious group of postings that using all the wrong materials and little understanding about how even a well sealed insulated plastic lead wire and even the best vacuum rated epoxies and greases under ion and electron bombardment in vacuo will outgas and kill the vacuum. Never let any plastics, epoxies or greases exist in even tiny amounts within a vacuum where bombardment is possible.

Richard Hull

If Guillermo's aluminum plate had been properly smooth and flat, and 3/8 or 1/2 inch thick, I bet it would work fine.

I once had marginal success with a 8" bell jar and baseplate of 3/8" thick HDPE plastic, which deflected about 1/2 mm from the suction (IIRC).
The Young's modulus (stiffness) of that material is about 1 GPa, compared to about 70 GPa for all aluminum alloys and about 200 GPa for all steels.

That HDPE accepted tapered 1/8"-NPT threads for a vacuum gauge port and a low voltage electrical feedthrough.
In fact 3/8" is approximately the normal length that 1/8" NPT pipe nipples thread into fittings.

Rich,

Did you see the glass "bell jar" that he was using? It was thin and untempered. ANY deflection from a warped base plate of such a fragile piece of glass from bending would be catastrophic.

A heavy wall tempered glass proper jar is at least double the thickness of these display glass canopies. Stiffness goes up with the cube of thickness so double the thickness is 8 times the rigidity and strength in this case.

Couple that with the misconception that a poor vacuum does not exert as much stress on the bell jar as a high vacuum and there is a problem. High vacuums of 1 x 10^8 torr are essentially the same forces on a container as a 10 torr vacuum! Nine orders of magnitude of vacuum and essentially the same forces on the enclosing container!

He is focused on sealing rather than safety. In an earlier post in this thread I noted the dangers in using the wrong type of bell jar. 10 microns of air pressure in a demo fusor will not only give you a beautiful round ball of plasma in a symmetrical grid, but also star mode rays. His bell jar is dangerous and his sealing attempts abortive.

Proper vacuum rated bell jars and their current prices can be found at

https://www.thomassci.com/Supplies/Bell ... ass%20Bell

For bargain basement prices go to Duniway at

https://www.duniway.com/part/10x12
Duniway' viton L gasket for the jar is a steal at $50.00

I actually own this 10" Duniway bell jar that I bought in 1999 with L gasket. I will be happy to let it go for a mere $500

Richard Hull

Made a "proto-putty" type silicone seal for my bell jar - lots of info on it on the innerweb.
Just used soapy water & GE silicone 1 - not the starch method.
It's gooo-ey, gooey, then sets within 30 seconds
The "seal" is still out-gassing acetic acid ..weeks later.
Hmm..