Listen to the advice, Chris, before you pump it down.
Either that, or 'stand well back'.
I'm very interested in how this 'pans out'.
A 'multi-purpose' chamber with multiple HV HF feedthroughs.
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Chris Bradley
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Ash Small wrote:
> Listen to the advice, Chris, before you pump it down.
> Either that, or 'stand well back'.
What do you want me to do, then?
This isn't a regular cylinder, it's a cooling-tower shaped thing and is clearly designed to mitigate a 'flat-collapse'. It is so thick that were it to deflect at all then I think its stiffness would, indeed, cause a fracture and a bit of bracing would be far *less* stiff and have no effect. Were it a commercial glass then Doug has put in place the right mitigations. I think this would be useless for this bit of glass. I'm not saying I know that I'm right, but it seems a moot point. If it goes, it goes.
if you handled this thing, you'd see that the idea of just standing back is funny - this is a half-inch thick glass and is about *3 times* as thick as a UHV bell-jar I have - if this were to actually shatter, the pieces would be enormous and distance alone wouldn't protect you! Solid steel plate is the only practical protection, and this is done. I am guessing that this sat in an industrial process line as a visual check and had to be able to take sudden pulses of high rates of change of pressure.
I was contemplating trying out a polycarbonate cylinder and reinforcing it internally. But it seems a redundant thing to attempt given this bit of kit. Whether it has some inherent weaknesses due to its prior life or not, we'll find out. It's moot - all I can do is pump it down behind plate steel and we'll see what happens. I'm far more concerned about the viability of the feedthroughs!
The one thing I might do is not bother to take up some of the compressive load with the external struts. As Dave says, the actual contact pressure would help to stabilise its location. I'll also forego a smear of vacuum oil I was going to apply and just leave the silicone's frictional forces to do their bit. If it leaks, I can throw some Apiezon 'mud' at it.
> Listen to the advice, Chris, before you pump it down.
> Either that, or 'stand well back'.
What do you want me to do, then?
This isn't a regular cylinder, it's a cooling-tower shaped thing and is clearly designed to mitigate a 'flat-collapse'. It is so thick that were it to deflect at all then I think its stiffness would, indeed, cause a fracture and a bit of bracing would be far *less* stiff and have no effect. Were it a commercial glass then Doug has put in place the right mitigations. I think this would be useless for this bit of glass. I'm not saying I know that I'm right, but it seems a moot point. If it goes, it goes.
if you handled this thing, you'd see that the idea of just standing back is funny - this is a half-inch thick glass and is about *3 times* as thick as a UHV bell-jar I have - if this were to actually shatter, the pieces would be enormous and distance alone wouldn't protect you! Solid steel plate is the only practical protection, and this is done. I am guessing that this sat in an industrial process line as a visual check and had to be able to take sudden pulses of high rates of change of pressure.
I was contemplating trying out a polycarbonate cylinder and reinforcing it internally. But it seems a redundant thing to attempt given this bit of kit. Whether it has some inherent weaknesses due to its prior life or not, we'll find out. It's moot - all I can do is pump it down behind plate steel and we'll see what happens. I'm far more concerned about the viability of the feedthroughs!
The one thing I might do is not bother to take up some of the compressive load with the external struts. As Dave says, the actual contact pressure would help to stabilise its location. I'll also forego a smear of vacuum oil I was going to apply and just leave the silicone's frictional forces to do their bit. If it leaks, I can throw some Apiezon 'mud' at it.
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Doug Coulter
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Chris,
Knowing it's that thick and now what it's made of, you're probably fine unless you get a stray beam of something hitting it (easier than you'd think). You still need a lip to keep the o-rings from sucking in though. I was using 3 liter Gallo bottles, not known for thickness, uniformity and so forth, and hey, got to drink the contents first. I didn't break enough of them to need to drink too awful much, they actually work well in most cases.
You're going to find I think you didn't make the end-plates thick enough, flat plates don't do as well with that stress level. My books say, make it thicker by about 2x if you can stand a fair amount of flex, more if not. But they probably won't break, just flex and leak if I'm right.
Of course no matter what, the amount of energy you effectively store in a container at vacuum is purely related to volume and about 14.7 psi max, and frankly, I've broken some glass stuff about that size and been kind of disappointed that I didn't get a better show out of the large brand new 25" picture tube I dropped and which came out of my paycheck when that mattered a lot more. Glad I wasn't in the path of the main blast though, those were some sharp bullets -- lousy aerodynamics in flight, so the "Stand well back" isn't a stupid idea. Glass bits stuck into the high ceiling.
Looks like your shield will handle that easy anyway, as you say. It's only so many foot pounds.
Chemical stuff can either add or subtract stress from glass, you can't assume it's the better for it.
This is covered extensively in some references I have. Depends on which side the chemical changed the expansion coefficient on for example and compared to which side the tension is on.
One very good ref is Walter H. Kohl's "Handbook of materials and techniques for vacuum devices", and another is the older RCA tube designer's handbooks --these are not how to design *with* tubes, but how to design the tubes, used to be internal documents to bring new guys up to speed. The former is frankly better and has more of that "guild secret" stuff in it, but is an expensive AIP publication, copyright current, while the other ones can be found free on the web.
You will have the odd beam leak if you have a way to use the mark one eyeball to see whats going on inside your stuff. Do use a sacrificial transparent piece of stuff (glass, quartz, mica) in there as you can shatter pyrex quite easily with a sudden hit of a decent beam at a point. It's good, but won't stand anything. I've easily shattered completely unstressed pyrex by accident. It was enough mess without it having been the containment vessel itself.
The polarizer suggestion is good, and fun besides, even with some fairly cheap film types readily available surplus. A real eye opener for this beginning glass blower-- who then got an annealing oven rigged up fast and a lot of problems went away.
All this may not strictly apply to Chris here -- I suspect other people lurk and need to know this stuff, who won't have Chris' precise setup, which however presents a good example of this class of thing for discussion.
And good to see you getting this thing built at last -- hat's off.
Knowing it's that thick and now what it's made of, you're probably fine unless you get a stray beam of something hitting it (easier than you'd think). You still need a lip to keep the o-rings from sucking in though. I was using 3 liter Gallo bottles, not known for thickness, uniformity and so forth, and hey, got to drink the contents first. I didn't break enough of them to need to drink too awful much, they actually work well in most cases.
You're going to find I think you didn't make the end-plates thick enough, flat plates don't do as well with that stress level. My books say, make it thicker by about 2x if you can stand a fair amount of flex, more if not. But they probably won't break, just flex and leak if I'm right.
Of course no matter what, the amount of energy you effectively store in a container at vacuum is purely related to volume and about 14.7 psi max, and frankly, I've broken some glass stuff about that size and been kind of disappointed that I didn't get a better show out of the large brand new 25" picture tube I dropped and which came out of my paycheck when that mattered a lot more. Glad I wasn't in the path of the main blast though, those were some sharp bullets -- lousy aerodynamics in flight, so the "Stand well back" isn't a stupid idea. Glass bits stuck into the high ceiling.
Looks like your shield will handle that easy anyway, as you say. It's only so many foot pounds.
Chemical stuff can either add or subtract stress from glass, you can't assume it's the better for it.
This is covered extensively in some references I have. Depends on which side the chemical changed the expansion coefficient on for example and compared to which side the tension is on.
One very good ref is Walter H. Kohl's "Handbook of materials and techniques for vacuum devices", and another is the older RCA tube designer's handbooks --these are not how to design *with* tubes, but how to design the tubes, used to be internal documents to bring new guys up to speed. The former is frankly better and has more of that "guild secret" stuff in it, but is an expensive AIP publication, copyright current, while the other ones can be found free on the web.
You will have the odd beam leak if you have a way to use the mark one eyeball to see whats going on inside your stuff. Do use a sacrificial transparent piece of stuff (glass, quartz, mica) in there as you can shatter pyrex quite easily with a sudden hit of a decent beam at a point. It's good, but won't stand anything. I've easily shattered completely unstressed pyrex by accident. It was enough mess without it having been the containment vessel itself.
The polarizer suggestion is good, and fun besides, even with some fairly cheap film types readily available surplus. A real eye opener for this beginning glass blower-- who then got an annealing oven rigged up fast and a lot of problems went away.
All this may not strictly apply to Chris here -- I suspect other people lurk and need to know this stuff, who won't have Chris' precise setup, which however presents a good example of this class of thing for discussion.
And good to see you getting this thing built at last -- hat's off.
Why guess when you can know? Measure!
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Chris Bradley
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Doug, Thanks for the replies and the things to double-check;
Doug Coulter wrote:
> You still need a lip to keep the o-rings from sucking in though.
The lip is a half-inch so call it 15si for the seal area, assuming ~10" diameter. The total vertical force due to the vacuum will be some 1100lbs, so there will be 75psi on the seal. If the coefficient of friction of the seal is >0.2 then I reckon it'll stay there, and I think it is?
> You're going to find I think you didn't make the end-plates thick enough, flat plates don't do as well with that stress level. My books say, make it thicker by about 2x if you can stand a fair amount of flex, more if not. But they probably won't break, just flex and leak if I'm right.
I did look at this, but worth a check. Bear in mind this is Kite [electrical] tufnol and is the stiffest with 6.9GPa. Compare, for example, aluminium with 69GPa so this is equivalent to a 15mm aluminium plate (as flexure is proportional to 1/t^4). But if there are any issues - including sealing of the feedthroughs - there is a 'plan b': I will add a second layer of tufnol with a sandwich of gasket and the feedthroughs off-set and connected by foil within that sandwich. I could only get Kite to 25mm, but did debate the merits of the mechanical grades that I could get to 50mm. I made a decision figuring it was likely to be close to satisfactory... we'll see!!... In any case, a pretty simple remedy. Now that I know 'what it looks like' and how to make it, and that the standoffs are already made, it will only be a couple of hours to remake them from new tufnol blocks. No risk...all useful findings!
> Chemical stuff can either add or subtract stress from glass, you can't assume it's the better for it.
I was thinking temp rather than chemical duress. You're right. I'll contemplate that when picking the glass shards out of the internal teflon parts! The thing was very clean when I got it and *presumed* chemicals just for safety as various nasties can go through these. It got a fine abrasive clean under a small selection of solvents.
> Do use a sacrificial transparent piece of stuff (glass, quartz, mica) in there as you can shatter pyrex quite easily with a sudden hit of a decent beam at a point.
I'll definitely be doing this where beams are a risk. I would plan to use polycarbonate internally where I am not expecting, but there is a risk of, beams or 10mm glass where I half-expect it.
As you say, it is a different setup to usual so hopefully there will be learning points anyway. It's a materially cheap set-up and, I would suggest, if it all works out, might therefore have useful outcomes for the amateur.
Doug Coulter wrote:
> You still need a lip to keep the o-rings from sucking in though.
The lip is a half-inch so call it 15si for the seal area, assuming ~10" diameter. The total vertical force due to the vacuum will be some 1100lbs, so there will be 75psi on the seal. If the coefficient of friction of the seal is >0.2 then I reckon it'll stay there, and I think it is?
> You're going to find I think you didn't make the end-plates thick enough, flat plates don't do as well with that stress level. My books say, make it thicker by about 2x if you can stand a fair amount of flex, more if not. But they probably won't break, just flex and leak if I'm right.
I did look at this, but worth a check. Bear in mind this is Kite [electrical] tufnol and is the stiffest with 6.9GPa. Compare, for example, aluminium with 69GPa so this is equivalent to a 15mm aluminium plate (as flexure is proportional to 1/t^4). But if there are any issues - including sealing of the feedthroughs - there is a 'plan b': I will add a second layer of tufnol with a sandwich of gasket and the feedthroughs off-set and connected by foil within that sandwich. I could only get Kite to 25mm, but did debate the merits of the mechanical grades that I could get to 50mm. I made a decision figuring it was likely to be close to satisfactory... we'll see!!... In any case, a pretty simple remedy. Now that I know 'what it looks like' and how to make it, and that the standoffs are already made, it will only be a couple of hours to remake them from new tufnol blocks. No risk...all useful findings!
> Chemical stuff can either add or subtract stress from glass, you can't assume it's the better for it.
I was thinking temp rather than chemical duress. You're right. I'll contemplate that when picking the glass shards out of the internal teflon parts! The thing was very clean when I got it and *presumed* chemicals just for safety as various nasties can go through these. It got a fine abrasive clean under a small selection of solvents.
> Do use a sacrificial transparent piece of stuff (glass, quartz, mica) in there as you can shatter pyrex quite easily with a sudden hit of a decent beam at a point.
I'll definitely be doing this where beams are a risk. I would plan to use polycarbonate internally where I am not expecting, but there is a risk of, beams or 10mm glass where I half-expect it.
As you say, it is a different setup to usual so hopefully there will be learning points anyway. It's a materially cheap set-up and, I would suggest, if it all works out, might therefore have useful outcomes for the amateur.
Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Chris, Doug is right. You don't know what might happen. You don't know the history of the chamber, stray beams could cause localised heating, etc.
I don't know if lining it with a metal gauze would stop stray beams, I don't even know what beams you will be using.
As I said before.....Stand well back.....
Maybe an external polycarbonate or similar shield would be worth considering.
It would make a very nice plasma light if you keep the voltage below xray levels.
I don't know if lining it with a metal gauze would stop stray beams, I don't even know what beams you will be using.
As I said before.....Stand well back.....
Maybe an external polycarbonate or similar shield would be worth considering.
It would make a very nice plasma light if you keep the voltage below xray levels.
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Chris Bradley
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Ash Small wrote:
> You don't know the history of the chamber, stray beams could cause localised heating, etc.
> It would make a very nice plasma light if you keep the voltage below xray levels.
Difficult to know how to respond to your posts, Ash. The support is nice but the insipid commentary is distracting. ....maybe you could spend time to go figure out why there won't be stray beams and high voltage in the first experiments I'm planning.
All that I will be doing I have previously posted about over the last 18 months, I've simply not yet put it all in one place to spoon feed you...and if you can't [be bothered to] figure it out or don't think there's enough information for you, then chill out and just wait for some actual results. I'm sure everyone else has already figured I'll go over that stuff again and in one place when there are actual results to discuss along with it, else I'll just be doing yet more armchair blah-de-blah. I was hoping this thread was just about the actual chamber construction. I'd suggest it'd be better for you to put discussions and speculations of the actual experiments in the threads where I've discussed those potential experiments.
Carl Willis wrote, on 2009-11-05 18:32;
> Do you think we're in nursery school, Ash? I understand this was offered in service of some kind of rhetorical point, but the sheer insipidity makes me wonder if you think your audience here walks around in diapers!
> You don't know the history of the chamber, stray beams could cause localised heating, etc.
> It would make a very nice plasma light if you keep the voltage below xray levels.
Difficult to know how to respond to your posts, Ash. The support is nice but the insipid commentary is distracting. ....maybe you could spend time to go figure out why there won't be stray beams and high voltage in the first experiments I'm planning.
All that I will be doing I have previously posted about over the last 18 months, I've simply not yet put it all in one place to spoon feed you...and if you can't [be bothered to] figure it out or don't think there's enough information for you, then chill out and just wait for some actual results. I'm sure everyone else has already figured I'll go over that stuff again and in one place when there are actual results to discuss along with it, else I'll just be doing yet more armchair blah-de-blah. I was hoping this thread was just about the actual chamber construction. I'd suggest it'd be better for you to put discussions and speculations of the actual experiments in the threads where I've discussed those potential experiments.
Carl Willis wrote, on 2009-11-05 18:32;
> Do you think we're in nursery school, Ash? I understand this was offered in service of some kind of rhetorical point, but the sheer insipidity makes me wonder if you think your audience here walks around in diapers!
Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
What's wrong with the suggestion about metal gauze, Chris?
Plans change and details change, Chris. I doubt you''l do 'everything' you've previously discussed.
Personally, you've got me thinking about glass chambers for some plasma lighting. If I assemble them inside a vacuum chamber, I won't have to pump them down.
We all know the level of mentality of some contributors to this site, Chris, but I'm trying my best not to lower myself to it.
When do you hope to achieve 'first light'?.....I'm looking forward to the pictures.
Plans change and details change, Chris. I doubt you''l do 'everything' you've previously discussed.
Personally, you've got me thinking about glass chambers for some plasma lighting. If I assemble them inside a vacuum chamber, I won't have to pump them down.
We all know the level of mentality of some contributors to this site, Chris, but I'm trying my best not to lower myself to it.
When do you hope to achieve 'first light'?.....I'm looking forward to the pictures.
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Chris Bradley
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Ash Small wrote:
> I'm looking forward to the pictures.
I'm rather hoping that you won't have much to see. As I have said before in my posts (have you looked yet), if you can see unit-eV recombination going on then I don't see how it will be efficient enough to be a net fusion device.
> I'm looking forward to the pictures.
I'm rather hoping that you won't have much to see. As I have said before in my posts (have you looked yet), if you can see unit-eV recombination going on then I don't see how it will be efficient enough to be a net fusion device.
Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
There are some of your posts that I've been planning to read 'in depth' at a later date.
I noticed you don't seem to have any gas feedthroughs, Chris.
I noticed you don't seem to have any gas feedthroughs, Chris.
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Chris Bradley
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Ash Small wrote:
> I noticed you don't seem to have any gas feedthroughs, Chris.
It's the bolt on top with the mudguard washer and the complaint washer underneath. Just 'plugged' for now - atm gas [and whatever else outgasses!] will be the first test gas.
> I noticed you don't seem to have any gas feedthroughs, Chris.
It's the bolt on top with the mudguard washer and the complaint washer underneath. Just 'plugged' for now - atm gas [and whatever else outgasses!] will be the first test gas.
Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
OK.....Just looks like another HV feedthrough, unless you know what you are looking for.
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Chris Bradley
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
The chamber sealed, but initially leaked like a sieve. Spraying a bit of IPA around, I could actually see the solvent being sucked up into the homemade silicone seals, even though I had clamped the top and bottom down quite firmly. So I pulled those out and tested the flatness with vac grease. I guess one advantage of glass enclosures is you can actually see where the surfaces are or are not in close contact, if there is a fluid between them.
With a bead of Apiezon Q around both ends to minimise doubt from leaks there, I was still getting no better than a miserable 1 torr. The culprit looks to have been the ASA flange on the top bulhead, which also had one of my 1mm thick silicone gaskets under it. Once under vacuum I could further clamp it down, presumably meaning the bulkheads had flexed compared with unloaded, and confirms Doug's thoughts, though measuring flatness by eye and a straight edge, there didn't appear to be any flex at all.
On clamping the flange down more firmly, I was in the 100s micron range, with continuous pumping. My conclusion is, essentially, that the silicone wasn't thick enough to accommodate the irreguarities of the tufnol along with the effects of the vacuum load as well. I now intend to obtain some viton sheeting with enough thickness to do the job better. Not sure what thickness is optimum, perhaps I should take regular viton seals as the standard and go for 4 or 5 mm thick. Any opinions?
With a bead of Apiezon Q around both ends to minimise doubt from leaks there, I was still getting no better than a miserable 1 torr. The culprit looks to have been the ASA flange on the top bulhead, which also had one of my 1mm thick silicone gaskets under it. Once under vacuum I could further clamp it down, presumably meaning the bulkheads had flexed compared with unloaded, and confirms Doug's thoughts, though measuring flatness by eye and a straight edge, there didn't appear to be any flex at all.
On clamping the flange down more firmly, I was in the 100s micron range, with continuous pumping. My conclusion is, essentially, that the silicone wasn't thick enough to accommodate the irreguarities of the tufnol along with the effects of the vacuum load as well. I now intend to obtain some viton sheeting with enough thickness to do the job better. Not sure what thickness is optimum, perhaps I should take regular viton seals as the standard and go for 4 or 5 mm thick. Any opinions?
Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Why viton sheet instead of seals?
A cylindrical seal will seal better than a flat one.
A cylindrical seal will seal better than a flat one.
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Chris Bradley
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Whilst getting an order together for more suitable seal materials, I found some 'bits of old' rubber sheet in the garage that I cut to shape. No idea what the actual material is, but it is very evident that the lack of sealing was, indeed, from the excessively thin silicone bits I had previously made.
I can't vouch for the calibration accuracy of my PKR251, but it seems to pump down within minutes to around the 80 micron range (indicated) and it stays there whilst pumping away (E2M2) and very slowly (hours) pulls maybe a little more, progressively. On isolating the valve it appears to recover within 10's of minutes to around the units of mbar range where it stays, steady, for days.
I am presuming that what I'm looking at is a 'severe' case of water outgassing from the tufnol (bakelite), as was expected. I'll not be trying too hard to get the vacuum lower than this until I have good seals (of at least *known* rubber!) and the first experimental bits are installed. Perhaps I might get rid of most of this with a long pump.
I'll say that the teflon stand-offs appear to have visibly lost their surface 'gloss'. The teflon rod I used appears at least a little bit shiny in strong light, but now they appear quite matt in the chamber. Together with the tufnol surfaces, I guess these all add up to some very significant sources of outgassing humidity. I might consider some alternatives to the teflon standoffs, if these are likely ot contribute significantly. Any suggestions welcome - e.g. I am debating if melting vac wax all over the tufnol surfaces might help out and I'd be heating the surface up as well in that process, which can only help.
The graph is pressure (Pa) wrt time (s) after isloation.
I can't vouch for the calibration accuracy of my PKR251, but it seems to pump down within minutes to around the 80 micron range (indicated) and it stays there whilst pumping away (E2M2) and very slowly (hours) pulls maybe a little more, progressively. On isolating the valve it appears to recover within 10's of minutes to around the units of mbar range where it stays, steady, for days.
I am presuming that what I'm looking at is a 'severe' case of water outgassing from the tufnol (bakelite), as was expected. I'll not be trying too hard to get the vacuum lower than this until I have good seals (of at least *known* rubber!) and the first experimental bits are installed. Perhaps I might get rid of most of this with a long pump.
I'll say that the teflon stand-offs appear to have visibly lost their surface 'gloss'. The teflon rod I used appears at least a little bit shiny in strong light, but now they appear quite matt in the chamber. Together with the tufnol surfaces, I guess these all add up to some very significant sources of outgassing humidity. I might consider some alternatives to the teflon standoffs, if these are likely ot contribute significantly. Any suggestions welcome - e.g. I am debating if melting vac wax all over the tufnol surfaces might help out and I'd be heating the surface up as well in that process, which can only help.
The graph is pressure (Pa) wrt time (s) after isloation.
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Frank Sanns
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
I don't think a sheet of rubber will be any workse than your Bakelite. Melting wax over the Bakelite will not gain you anything. Wax is not a whole lot better than kerosene for vapor pressure/outgassing with linear low density polyethylene being better and HDPE and UHMWPE being the best of the linear alkanes.
Also, you do not really need to bolt the top and bottom halves together as the vacuum will supply far and enough force to keep them in place. Aluminum plate would have worked with feedthroughs especially if the bottom were insulated from the table with the Bakelite.
Frank Sanns
Also, you do not really need to bolt the top and bottom halves together as the vacuum will supply far and enough force to keep them in place. Aluminum plate would have worked with feedthroughs especially if the bottom were insulated from the table with the Bakelite.
Frank Sanns
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Chris Bradley
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Frank S. wrote:
> Melting wax over the Bakelite will not gain you anything. Wax is not a whole lot better than kerosene for vapor pressure/outgassing
I was meaning a proper vacuum wax. I'm thinking of Apiezon W, with a 20C vapour pressure in the e-9 torr range and is sold for use as a permanent vacuum sealing. It also has getting properties and is advertised as offering a "total seal" against water vapour and atmospheric humidity. I will probably be able to forego a separate seal as well, if I can cover the area uniformly and seal directly against it.
I might well move to metallic bulkheads, but initially I want to get to understand what happens when there is no conductive surface in the chamber.
> Melting wax over the Bakelite will not gain you anything. Wax is not a whole lot better than kerosene for vapor pressure/outgassing
I was meaning a proper vacuum wax. I'm thinking of Apiezon W, with a 20C vapour pressure in the e-9 torr range and is sold for use as a permanent vacuum sealing. It also has getting properties and is advertised as offering a "total seal" against water vapour and atmospheric humidity. I will probably be able to forego a separate seal as well, if I can cover the area uniformly and seal directly against it.
I might well move to metallic bulkheads, but initially I want to get to understand what happens when there is no conductive surface in the chamber.
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Doug Coulter
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Chris,
I have and have used some apeizon wax W, and it's indeed pretty good at room temperature.
It seems to be highly refined coal tar of some sort.
It is also brittle at that temperature, but makes gas quick when warmed up some, you can smell it when you're heating it in a spoon for application. I'd not use it for large area exposure to vacuum, especially on something bound to get warm. Very good for things like sealing tiny leaks or sticking two pieces of glass together if something else handles the mechanical part of that job. If used in a situation where there's a big thermal expansion mismatch, it won't save you, it will crack and leak.
I don't know about household paraffin wax, but beeswax as sold industrially is pretty good, and doesn't get brittle. I also made my own "vacuum grease" which I use to pack valves and treat o rings from this and some low vapor pressure forepump oil, works great. Still kind of wax like at room temp, I didn't use much oil in it, just enough so you can dig some out with a screwdriver easily.
I think your bakelite might be fine, if it isn't oiled-up much, anyway (degrease with a solvent, like acetone). May take some time to start getting down to good vacuum, but you'll probably get there.
No matter what, everything you put into a tank (or make one of) is somewhat conductive -- it's just a matter of degree.
A lot of times, the really good insulators (like quartz) this makes big problems with surface charge pickup from stray particles, which then affects motions of other ones due to the unintended field. Pyrex is a couple orders magnitude more conductive than quartz and has less, but not zero, troubles with this. Most other glasses fall in between. I have had real troubles with another good insulator, mica, in the tank. If it's not thick enough to withstand the max volts it could ever see, it picks up enough charge on one side to arc through, which makes quite a mess, and since it's high D, it stores enough energy to blow holes in itself. Teflon also does "funny things" as does PEEK.
This is one that isn't obvious, seems rational to want a perfect insulator, but actually with some experience it's just what you DO NOT WANT in almost every case. I know, you'll try it anyway, and should, and I await the tales of unexpected things happening that will result from that. For nearly everything, it's better to have a good conductor and control the fields, not just let them develop any old way.
I have and have used some apeizon wax W, and it's indeed pretty good at room temperature.
It seems to be highly refined coal tar of some sort.
It is also brittle at that temperature, but makes gas quick when warmed up some, you can smell it when you're heating it in a spoon for application. I'd not use it for large area exposure to vacuum, especially on something bound to get warm. Very good for things like sealing tiny leaks or sticking two pieces of glass together if something else handles the mechanical part of that job. If used in a situation where there's a big thermal expansion mismatch, it won't save you, it will crack and leak.
I don't know about household paraffin wax, but beeswax as sold industrially is pretty good, and doesn't get brittle. I also made my own "vacuum grease" which I use to pack valves and treat o rings from this and some low vapor pressure forepump oil, works great. Still kind of wax like at room temp, I didn't use much oil in it, just enough so you can dig some out with a screwdriver easily.
I think your bakelite might be fine, if it isn't oiled-up much, anyway (degrease with a solvent, like acetone). May take some time to start getting down to good vacuum, but you'll probably get there.
No matter what, everything you put into a tank (or make one of) is somewhat conductive -- it's just a matter of degree.
A lot of times, the really good insulators (like quartz) this makes big problems with surface charge pickup from stray particles, which then affects motions of other ones due to the unintended field. Pyrex is a couple orders magnitude more conductive than quartz and has less, but not zero, troubles with this. Most other glasses fall in between. I have had real troubles with another good insulator, mica, in the tank. If it's not thick enough to withstand the max volts it could ever see, it picks up enough charge on one side to arc through, which makes quite a mess, and since it's high D, it stores enough energy to blow holes in itself. Teflon also does "funny things" as does PEEK.
This is one that isn't obvious, seems rational to want a perfect insulator, but actually with some experience it's just what you DO NOT WANT in almost every case. I know, you'll try it anyway, and should, and I await the tales of unexpected things happening that will result from that. For nearly everything, it's better to have a good conductor and control the fields, not just let them develop any old way.
Why guess when you can know? Measure!
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Chris Bradley
- Posts: 2930
- Joined: Fri May 02, 2008 7:05 am
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Further to my recent good fortune in acquiring turpos pumps, I have now added the Varian V60 to my setup. The backing/roughing pump is stowed under the table surface and the pump sits very neatly, inverted, on top. Unfortunately there is no valve between chamber and turbo, but I suppose that aids flow rate, though reduces more flexible operation.
The actual time for roughing is quite a bit longer through the little KF16 port and the turbo, but I turned the turbo on around 1mbar and it spools up slowly and is still accelerating as the pressure drops through 0.1mbar. It sucked straight down to 2e-4 torr within a quarter hour.
Given my slack attention to cleanliness and the limited choice of available materials, I'm quite pleased with that and am sure it can be easily improved on. I'll give it a run for a few hours, but what I would like to do is give the insides a *proper* clean (I had been using terps and kitchen paper to clean off some apiezon q before assembling this!) and then a 'bakeout'. Clearly I can't do a bakeout as might be easy with a metal chamber (one of the limitations with this non-metallic setup, I guess) but was thinking that, perhaps, I could shine an IR light at it to heat up the internal surfaces; would that work? Any other suggestions?
The natural rubber seals are also an obvious source of high outgassing, so I'll now look into some of the links people have provided on suppliers.
I expect that e-4 torr range is fine for my experiments now (MFP 20-50cm), but this quest for the 'best vacuum' is a somewhat addictive hobby/activity in itself!!
The actual time for roughing is quite a bit longer through the little KF16 port and the turbo, but I turned the turbo on around 1mbar and it spools up slowly and is still accelerating as the pressure drops through 0.1mbar. It sucked straight down to 2e-4 torr within a quarter hour.
Given my slack attention to cleanliness and the limited choice of available materials, I'm quite pleased with that and am sure it can be easily improved on. I'll give it a run for a few hours, but what I would like to do is give the insides a *proper* clean (I had been using terps and kitchen paper to clean off some apiezon q before assembling this!) and then a 'bakeout'. Clearly I can't do a bakeout as might be easy with a metal chamber (one of the limitations with this non-metallic setup, I guess) but was thinking that, perhaps, I could shine an IR light at it to heat up the internal surfaces; would that work? Any other suggestions?
The natural rubber seals are also an obvious source of high outgassing, so I'll now look into some of the links people have provided on suppliers.
I expect that e-4 torr range is fine for my experiments now (MFP 20-50cm), but this quest for the 'best vacuum' is a somewhat addictive hobby/activity in itself!!
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Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Am I correct in assuming your vacuum guage is connected to a multimeter, Chris?
While the meters for guages generally have compensators for ambient temperature, it is a very simple solution, and probably accurate enough for now.
While the meters for guages generally have compensators for ambient temperature, it is a very simple solution, and probably accurate enough for now.
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Chris Bradley
- Posts: 2930
- Joined: Fri May 02, 2008 7:05 am
- Real name:
Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
In this pic is a PKR251 compact full range gauge. All the electronics is inside, it just puts a straight voltage out. Those who recognise it will figure out I've scrounged the gauge (on account of my bodged connector arrangement as I don't have the right connector and that it is properly out of my budget range). No controller came with it, and likely I won't ever get a controller given the huge costs of these things (for a glorified voltmeter!). The Pirani stage can be adjusted, but only really adjusts the >0.01atm, the adjustment doesn't make much difference below e-2. The cold cathode stage is factory preset and works, or it doesn't.
I paid 10 pounds for this - works fine! You could look the price up for new kit, but I'm quite sure it is all the wrong side of my monthly pocket money!
I paid 10 pounds for this - works fine! You could look the price up for new kit, but I'm quite sure it is all the wrong side of my monthly pocket money!
Re: A 'multi-purpose' chamber with multiple HV HF feedthroughs.
Hi-
nice setup!
Just one thing you should think off (even though your Glass is rock solid)-
if the Glass shatters (I also expect this can easily happen due to thermal stress from ebeam,...) you probably will have a nice new doorstop:
your turbo- pump....!
Greets
Roman
nice setup!
Just one thing you should think off (even though your Glass is rock solid)-
if the Glass shatters (I also expect this can easily happen due to thermal stress from ebeam,...) you probably will have a nice new doorstop:
your turbo- pump....!
Greets
Roman