Creating a contained vacuum. Ideas?

[Demonis]

First off, I'm new here. Greetings! I should start off by saying I'm not a genius, as I'm sure some people here may be. I don't have a degree in any of the fields on this website, but all interest me. I like to think I have a general understanding of how some things work, but I know I always need to learn more. Which of course, is why I am here.

I've been working on an experiment for around two years now. Designed it during spare time in college and just haven't really dedicated time or money to it until now. I'm at a point where I need to make a contained vacuum, preferably the most efficient vacuum possible for the best price. Unfortunately I do not know where to begin, I know there are ways to force extraction of air from a container by a pump, and ways to do it chemically. I really need as little air resistance as possible within my container.

Another setback is the container (at least the original design) will be made from a transparent material, likely Lexan.

The container will not likely be much bigger than a 5 gallon bucket, and will have many things inside of it that will take up space. Metal, ceramics, ect ect. (That may be important for chemical vacuum ideas?)

I just need a direction to start some research in on costs, I've drawn a blank!

Thanks to anyone who volunteers their precious time to help me!

 

[berkeman]

First off, I'm new here. Greetings! I should start off by saying I'm not a genius, as I'm sure some people here may be. I don't have a degree in any of the fields on this website, but all interest me. I like to think I have a general understanding of how some things work, but I know I always need to learn more. Which of course, is why I am here.

I've been working on an experiment for around two years now. Designed it during spare time in college and just haven't really dedicated time or money to it until now. I'm at a point where I need to make a contained vacuum, preferably the most efficient vacuum possible for the best price. Unfortunately I do not know where to begin, I know there are ways to force extraction of air from a container by a pump, and ways to do it chemically. I really need as little air resistance as possible within my container.

Another setback is the container (at least the original design) will be made from a transparent material, likely Lexan.

The container will not likely be much bigger than a 5 gallon bucket, and will have many things inside of it that will take up space. Metal, ceramics, ect ect. (That may be important for chemical vacuum ideas?)

I just need a direction to start some research in on costs, I've drawn a blank!

Thanks to anyone who volunteers their precious time to help me!

Welcome to the PF. For your initial experiments, you can probably just use a Bell Jar like this:

http://scientificsonline.com/product.asp_Q_pn_E_3071338

And a suitable vacuum pump. You can get basic vacuum pumps for $100-$200 or so.

http://en.wikipedia.org/wiki/Vacuum_pump

http://www.usvacuumpumps.com/welcome/

You can use a google search to find other places to buy online or local to you.

 

[Demonis]

Sadly the jars won't work, it's going to have to be a custom fabricated container... which should be fun to make.

Thanks for the link to the pumps though. I have not come across that page yet.

 

[Mapes]

Can you tell us why existing bell jars won't work? (E.g., a need for feedthroughs, etc.) This will help people give relevant advice.

I tried to assemble a 1' x 1' x 1' Lexan container several years ago to perform solder experiments in a reducing atmosphere. I can't remember whether the wall thickness was 1" or 1.5", but it was definitely inadequate at withstanding a vacuum (15 psi). It never failed, but the walls deflected noticeably and the edge seals were poor. In the end, it was necessary to fabricate a welded stainless steel container with ceramic feedthroughs (>$10K) to perform the experiments repeatably.

 

[Demonis]

The build of the container will not be a length of cylindrical container all with the same diameter. It's going to contain multiple 'staged' sizes. That is discouraging hearing about the lexan. Though my lexan design is only going to be a prototype tester model. If it worked, then presumably, future models would be cast from aluminum.

I'm fortunate enough to have a machinist friend who can fabricate much of what I need to do these things, however he does not have much knowledge of vacuums either.

 

[Demonis]

Can you tell us why existing bell jars won't work? (E.g., a need for feedthroughs, etc.) This will help people give relevant advice.

I tried to assemble a 1' x 1' x 1' Lexan container several years ago to perform solder experiments in a reducing atmosphere. I can't remember whether the wall thickness was 1" or 1.5", but it was definitely inadequate at withstanding a vacuum (15 psi). It never failed, but the walls deflected noticeably and the edge seals were poor. In the end, it was necessary to fabricate a welded stainless steel container with ceramic feedthroughs (>$10K) to perform the experiments repeatably.

I would like to know how you chose to assemble the lexan, if you don't mind. So that I do not make the same mistakes. The only port that will be accessable to the container will be that of the pump.

 

[DaveC426913]

It seems to me, you are concentrating on the wrong part. You are letting the machinations dictate the shape of the chamber and then trying to get the vacuum chamber to work reliably.

It seems to me, the functionality of your vacuum chamber will be a bottleneck in the design. I think you need to give it priority, or you will spend the lion's share of your time fixing leakage and bracing problems. I suggest you make an optimal vacuum container first, and then figure out how to make your machinations operate in that space.

 

[Mapes]

I would like to know how you chose to assemble the lexan, if you don't mind. So that I do not make the same mistakes. The only port that will be accessable to the container will be that of the pump.

Sheets of Lexan, screwed together and sealed at the seams with a low-outgassing epoxy. It was very poor.

I agree (in hindsight) totally with DaveC426913 above: the glass or plastic bell jar is ubiquitous for a reason, because it works. More unusual vacuum chamber shapes are the result of many tens of thousands of dollars worth of design and welding (and I haven't mentioned yet that the welding needs to be specialized; look up "virtual leaks"). It may be easier to redesign your system than to troubleshoot a new vacuum container design. Sorry to be negative here.

 

[Demonis]

Hrmmm.. this could be a bigger problem than I'd hoped.

The idea of an ideal container would be something in the shape of this:

(Sorry, its a paint drawing since I'm at work. Dimensions are subject to change, but not greatly.)

http://img12.imageshack.us/img12/714/bbbbhjk.jpg

And yes, the top piece is necessary. I've been looking around for a bell jar of sorts that could match/replace that design, but no luck thus far.

 

[Hologram0110]

Hi,
I spent a fair amount of time trouble shooting vacuum equipment for various undergraduate labs in the past. What you need to decide is the quality of vacuum you want to obtain.

If you just need a vacuum with 10^-1 torr or do you need a system capable of 10^-6 torr or even lower? This will dictate what materials you can use because of out gassing, vapor pressure(temp). Low quality vacuums can be easily obtained with mechanical pumps, while higher quality vacuums will need vapor diffusion or turbo pumps which can cost significantly more. I suggest you determine how good is of a vacuum is good enough before you invest a lot of time and money into the system.

I've seen a vacuum system with composed of two sheets of Plexiglas and an Aluminum tube work very well (pressures around 10^-7 torr).

Other considerations are the volume, pump-rate, temperature requirements your system may be important quantities to consider. With out more information it would be hard to offer advice.

 

[Demonis]

Hi,
I spent a fair amount of time trouble shooting vacuum equipment for various undergraduate labs in the past. What you need to decide is the quality of vacuum you want to obtain.

If you just need a vacuum with 10^-1 torr or do you need a system capable of 10^-6 torr or even lower? This will dictate what materials you can use because of out gassing, vapor pressure(temp). Low quality vacuums can be easily obtained with mechanical pumps, while higher quality vacuums will need vapor diffusion or turbo pumps which can cost significantly more. I suggest you determine how good is of a vacuum is good enough before you invest a lot of time and money into the system.

I've seen a vacuum system with composed of two sheets of Plexiglas and an Aluminum tube work very well (pressures around 10^-7 torr).

Other considerations are the volume, pump-rate, temperature requirements your system may be important quantities to consider. With out more information it would be hard to offer advice.

I'm trying to achieve the best vacuum possible for as little money. I'm using my own pocket money to do this experiment, and need as little air resistance within the container as possible. The temerature range would be around room temperature, the volume should be around as much as a 5 gallon bucket would hold, give or take a few.

 

[Hologram0110]

Air resistance to mechanical motion? In that case you need to decide how much resistance is acceptable (based on speed, timing, measurement accuracy ect). That will govern what type of vacuum system you need. I would think since this is a personal project that a mechanical pump (good ones can go down to 10^-3 torr range) would be sufficent but that is just a guess.

You should ask someone with a fluids background what kind of pressure you should be shooting for before the gas-effects become insignificant (I unfortunately can't help you there). If you mange to get a target pressure then you can start planning accordingly.

 

[Demonis]

Air resistance to mechanical motion? In that case you need to decide how much resistance is acceptable (based on speed, timing, measurement accuracy ect). That will govern what type of vacuum system you need. I would think since this is a personal project that a mechanical pump (good ones can go down to 10^-3 torr range) would be sufficent but that is just a guess.

You should ask someone with a fluids background what kind of pressure you should be shooting for before the gas-effects become insignificant (I unfortunately can't help you there). If you mange to get a target pressure then you can start planning accordingly.

Well, I don't expect the prototype to be completely flawless. If it works properly, then I might get some outside financing to fund a bigger, more efficient version. I just need to minimize overall air resistance. I will see if I can find some information on what you said. I am sure I do not need anything close to a 'perfect vacuum' or anything too pricey to get a decent space going with some parts moving inside.

 

[Hologram0110]

Sorry I can't give you a definitive answer, but without knowing more about the experiment it is hard to know what kind of pressure is significant. Can you describe what you are trying to do with the device?

For example, Say you were trying to show that a feather and ball hit the ground at the same time when dropped in a vacuum. In this case how good of a vacuum you need would depend on what your time tolerances are. If your just eying it than pretty much any vacuum would probably do. However if your using some computer controlled timing device than you would likely be able to see the effects of aerodynamic drag at different pressures.

If indeed you settle on a mechanical pump it would greatly simplify the whole system. Since the vacuum is relatively poor anyways, you don't really need to worry about out-gassing for example (which becomes a problem at lower pressures).

 

[Demonis]

It's hard to explain, the experiment is almost like running an alternator inside of a vacuum, but there's a lot of modification to it. I'm trying to cut down on mechanical resistance (I have made many adjustments to minimize friction) as well as any air resistance.

 

[Mapes]

Ahh, this is a whole other story. If this is your only pressure constraint, you can definitely accommodate leaks; hell, if you reached 29 inches Hg, you'd have eliminated >95% of the air resistance. This is within reach of a cheap mechanical pump and a prototype enclosure. (Note that machinery can get very hot without the cooling due from air convection, though.)

There's a huge range of vacuum quality needed in practice, from torr to <0.001 microtorr. The more details you give, the less time it generally takes for the answers to converge to useful information.

 

[Hologram0110]

In that case you will want a simple mechanical pump. You could even use the Lexan (as long as it is sufficiently thick enough).

I do for see some potential stumbling blocks though that you may want to consider:

Heat dissipation: Have you considered where the heat that is generated will go? (friction, resistive heating ect) You may get significantly higher temperatures than you expected because you lack a thermal sink. (Until your sure about the thermal response you may want to run it only for short periods of time to prevent heat damage)

Vapor Pressure: The alternator will undoubtedly be lubricated to reduce friction and wear. You should consider what may happen to the lubricant in a vacuum. (Will it evaporate? if it does will it harm the pump? Will a loss of lubricant affect the alternator ect).

Good luck. I'm interested to find out if the mechanical drag is significant.

 

[Demonis]

Well, that's part of why I have made extreme efforts to reduce friction, to reduce heat and wear on the mechanical parts. So under a machine pump vacuum I should be able to eliminate most of the resistance you think? I would have thought 1.5" lexan would hold a moderate vacuum, that stuff is pretty stiff. Unfortunately, my original design may be a box instead of round, unless I can find an effective cylinder shell.

 

[Hologram0110]

For implosion-resistant-strength cylinder would definitely be stronger than a square. It would also be less likely to deform and crack at a seal.

During undergrad, some of my friends made a vacuum chamber for a cyclotron from a large aluminum pipe (radius ~15-20 inches) and two thick sheets of Plexiglas. On both ends of the pipe they machined a grove to fit it with an o-ring for a vacuum tight seal. When the vacuum pump was engaged it would self-compress the o-ring against the plexiglas. This allowed you to easily open the vacuum chamber between pump downs. You should be able to do something similar for your design. Don't forget to vacuum grease the o-rings.

Remember that not all heat generated in an alternator comes from mechanical friction. You will still have a lot of resistive heating. I would still suggest short duty cycles and some sort of temperature gauge till your sure you won't destroy the enamel on the wires.

 

[Demonis]

Short cycles are the initial plan, however, one of my modifications is moving the wires outside of the vacuum, the stator assembly (which I'm going to have to build) that I designed is going to be on the outside of the chamber. Which is why the chamber needs to be a non-metal. I don't want any magnetic interference.

 

[rplatter]

If your experiment does not require air on the exterior side of the container, you could place the entire experiment inside a larger vacuum container. It might be easier to find a large container that can fit the entire of your experiment.
I would also investigate the decompression chambers used for Dive accidents. They are fairly large and usually have view ports. I'm not sure the will support high exterior pressure, but I think they should, and they already have ports for evacuating gas. You could probably rent time on one or might even get time on one free. These things sit around waiting for an accident, so are frequently not in use at any particular time. You may need to risk the early termination of the experiment, but that is a detail you can work out with the owner.

 

[Demonis]

I don't think a chamber that large would benefit me much.

 

[Robert Hansel]

Hello, dear (excuse me!) vacuum heads,

Consider a long glass tube, like that of a mercury barometer.

1. Connect the vessel to undergo evacuation to the top of the barometer.
2. Fill the "barometer" and its connected vessel with mercury or some other dense liquid.
3. Now, wright the "barometer" and its connected vessel in such a way that the vacuum forms above the level of the mercury.
4. Your vacuum is complete.
5. Seal the neck of the vessel.
6. Concerned with mercury vapor? Place a getter in part of the evacuated space and heat the getter with transmitted RF.
7. If you choose to use water for your barometric vacuum generator, use a desicant-getter in a container that has a closure mechanism that seals by, for example, epoxy remote vibration

Robert Hansell, Pennsburg, Pennsylvania