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.
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.
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
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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
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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