It was my understanding that rubber was a very good material to used to seal things to make them airtight and thats why I used it. Apparently it is not, so I will not use it. I also ask what is wrong with a wood surface? The base of the chamber will be metal but I am talking about having the whole setup on wood, how would that effect anything? I am not intent on using a wood and rubber base plate I was just asking whether I should keep it as I assumed that the jar would be more airtight if the rim of it was resting on rubber rather than wood or metal.
Secondly, I do want advice on my real problems, if i didn't then I would not be addressing the current problems that have been sent to me by people on this forum. The pump is one of the problems, that is why I am asking about it. If i did not ask the questions I asked in the previous post then it could lead to my fusor not working, I just want help picking my pump. I am being very thorough this time as I had asked on another forum weather the current bad pump im am using is ok for fusor operation and they said it would be fine, it isn't. Also please respect that I am new to working with pressure and pumps so I am likely to get things wrong.
Thanks for the help!
My fusor is only arcing and not creating the purple glow
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Chris Bradley
- Posts: 2930
- Joined: Fri May 02, 2008 7:05 am
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Re: My fusor is only arcing and not creating the purple glow
OK, so the main thing I was trying to encourage you to consider is that wood is no good as a material for a vacuum envelope. Effectively, you are using that stiff hard rubber as your vacuum envelope. Neither wood nor rubber are suitable materials for that purpose.
Consider, as the chamber pressure drops that hard rubber sheet will have to be taking the atmospheric load, because the wood is not fit to be a vacuum envelope so will bleed air in behind the rubber. This makes the rubber distort, and any hard effort you may or may not have made a good seal with will then be instantly undone because everything will flex and change shape.
Rubber is fit for making seals from, not for making vacuum chamber envelopes from. Also, you need to pick the right sort of rubber, as they come in all shapes and sizes, or to be specific in various material types, surface finishes and hardnesses. Yours is a very hard rubber and not the sort you'd choose for seals.
Also, not that this yet concerns you, but you can't really let polymerics get exposed to a plasma because the free ions will start reacting with the rubber and will outgas reaction products spectacularly, as well as change the physical properties of the rubber. The total surface area of rubber to the chamber volume should be kept to a minimum, and it is also worth while 'shadowing' any rubber with earthed metalwork where possible which reduces exposure to free ions/electrons. If you have an experimental setup where you can assure that the rubber is protected - for example you might create some metal box within the belljar that contains the plasma - then there are ways to accomplish that sort of thing. However, this is adding complexity when the most appropriate remedy might simply be to use a metal base plate.
Consider, as the chamber pressure drops that hard rubber sheet will have to be taking the atmospheric load, because the wood is not fit to be a vacuum envelope so will bleed air in behind the rubber. This makes the rubber distort, and any hard effort you may or may not have made a good seal with will then be instantly undone because everything will flex and change shape.
Rubber is fit for making seals from, not for making vacuum chamber envelopes from. Also, you need to pick the right sort of rubber, as they come in all shapes and sizes, or to be specific in various material types, surface finishes and hardnesses. Yours is a very hard rubber and not the sort you'd choose for seals.
Also, not that this yet concerns you, but you can't really let polymerics get exposed to a plasma because the free ions will start reacting with the rubber and will outgas reaction products spectacularly, as well as change the physical properties of the rubber. The total surface area of rubber to the chamber volume should be kept to a minimum, and it is also worth while 'shadowing' any rubber with earthed metalwork where possible which reduces exposure to free ions/electrons. If you have an experimental setup where you can assure that the rubber is protected - for example you might create some metal box within the belljar that contains the plasma - then there are ways to accomplish that sort of thing. However, this is adding complexity when the most appropriate remedy might simply be to use a metal base plate.
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Dan Tibbets
- Posts: 578
- Joined: Thu Apr 17, 2008 1:29 am
- Real name:
Re: My fusor is only arcing and not creating the purple glow
Your pump may be fine. At this stage don't worry about a better pump. The sound in the video is consistent with a large leak as mentioned. As you start pumping, the pump should sound like this, but the sound should quickly change to the mentioned "lub-Lub" sound. That it doesn't is a strong indicator of a major leak. You should have a valve between the pump and chamber. Close it, turn off the pump, then listen for hissing and try to localize. The gasket looks like it may be a open cell foam. It will never make a good seal if this is indeed the case. Even a closed cell or solid gasket may not seal well if the connecting edges are too broad. The atmospheric pressure pushing on the outside pushes the pieces together, but the resultant pressure needs to be concentrated to a small area. That is why narrow knife edge dimensions are used for the best seals.Remove the gasket and try some ointment (grease and rubber seals do not work well together in my limited experience). Petroleum jelly or hydrocortisone ointment (not cream) worked for me when I was playing around. Do not use a scented ointment like Vicks Vapor Rub.This ointment/ grease may make an adequate seal if your jar and base plate are smooth enough and nor warped relative to each other. The grease can seal small leaks, but not larger leaks. This is not a good sealing material but it may far exceed the dismal seal you presently have. A true vacuum grease works better but is also very much more expensive.
Also, keep in mind that you could have a leak anywhere anything penetrates the chamber. With the power off and disconnected (for safety) if you can hear a hiss, run your finger over the seal areas of the base, feed throughs, etc. If you hear a change in the hiss you have possibly located a localized major leak.
The arc is consistent with a mild vacuum. The pressure may be ~ 10,000 to 50,000 Microns. For a good glow discharge you will need to get below several thousand Microns, hopefully into the hundreds of Microns, and even lower for bugle, e- beam and star modes.
A gross test of your jar and base plate. Without a gasket, try to rock the bell jar on the base plate. If you can obtain a noticeable rocking motion, the matting surfaces are not flat and level enough. You might never get a good seal in this case. The solution is to machine the surfaces, possibly a major undertaking. An alternate flatter base plate may serve provided the glass base itself is not too uneven.
Dan Tibbets
Also, keep in mind that you could have a leak anywhere anything penetrates the chamber. With the power off and disconnected (for safety) if you can hear a hiss, run your finger over the seal areas of the base, feed throughs, etc. If you hear a change in the hiss you have possibly located a localized major leak.
The arc is consistent with a mild vacuum. The pressure may be ~ 10,000 to 50,000 Microns. For a good glow discharge you will need to get below several thousand Microns, hopefully into the hundreds of Microns, and even lower for bugle, e- beam and star modes.
A gross test of your jar and base plate. Without a gasket, try to rock the bell jar on the base plate. If you can obtain a noticeable rocking motion, the matting surfaces are not flat and level enough. You might never get a good seal in this case. The solution is to machine the surfaces, possibly a major undertaking. An alternate flatter base plate may serve provided the glass base itself is not too uneven.
Dan Tibbets