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Author Topic: What's the best way to construct a good vacuum chamber?  (Read 5304 times)

Offline PmbPhy

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I'm thinking about constructing a vacuum chamber for scientific demonstrations which I'm going to use during tutoring. I also want to use it to show what happens when the US flag is unfurled in a vacuum like the astronauts did on the moon. I'd like some input into constructing one. Anybody have any idea about the best equipment to pull a hard vacuum like the one on the moon? What materials should I use to create the chamber? I'm thinking of using thick glass and an iron frame sealed with some sort of rubber gasket. I want it to be able to be disassembled for easy movement.

One of the experiments done on the moon was when one of the astronauts dropped a feather and a hammer and they hit the ground at the same time. They've sort of recreated something like this at the Boston Museum of Science. They have a tall glass cylinder in which all of the air has been taken out. They drop a feather and another object (e.g. a coin or a pool ball) and they fall at the same rate. A shorter version was done at MIT. See:
http://video.mit.edu/watch/feather-and-coin-in-a-vacuum-6407/

Thoughts?


 

Offline chiralSPO

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Re: What's the best way to construct a good vacuum chamber?
« Reply #1 on: 02/07/2015 22:44:51 »
I think thick metal walls and thick glass/plastic window could work. The hard part about constructing such a box would be making sure there are no leaks (this would probably require some welding...)

The harder part would be achieving the hard vacuum in the first place. We use a rotary pump ("roughing pump") to get down to pressures of about 10 mTorr (105 bar), which is perfectly sufficient for anything I need to do. One of my collaborators uses a rotary pump to get to this pressure, then opens a valve to an oil diffusion pump (https://en.wikipedia.org/wiki/Diffusion_pump) to get from there to 1010 bar, though supposedly it is possible to get down as low as 1012 bar... The pressure on the moon is about 1015 bar, which would be VERY hard to get in a home-made, or even laboratory-made, box.

You can get a used rotary pump on ebay for several hundred bucks, (http://www.ebay.com/itm/like/331583214895?lpid=82&chn=ps), up to a few thousand bucks (http://www.ebay.com/itm/Edwards-E2M30-Mechanical-Pump/121460692379?_trksid=p2141725.c100338.m3726&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20141212152715%26meid%3D8658c3021aac4dacb6f7173748b512f7%26pid%3D100338%26rk%3D9%26rkt%3D30%26sd%3D331583214895) and honestly, I would be skeptical of anything cheaper than $1200.

Diffusion pumps are a little cheaper (no moving parts), and you can get those on ebay for a few to several hundred dollars (just search for "diffusion pump"). The only thing is, you need to have a rotary pump and a diff. pump--if you try running the diffusion pump without first getting below 103 (ideally 105) bar, it will polymerize the oil inside and you'll need to get a new one, or spend hours scraping tar out of the pump.

If this sounds like an undertaking (and investment) you are willing to consider, go for it (though I also have to warn, catastrophic failure of a high vacuum system could be quite dangerous!)

Good luck  :)
 

Offline alancalverd

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Re: What's the best way to construct a good vacuum chamber?
« Reply #2 on: 02/07/2015 23:56:55 »
You don't need a particularly hard vacuum to demonstrate the "feather and hammer" experiment. The water jet pump you may recall from school chemistry labs will reduce the air density to less than 0.1 atmosphere so you will certainly see a table tennis ball fall faster as the pressure drops whereas a ball bearing will take pretty much the same time at any pressure. A decent rotary pump will take you below 0.01 atmosphere - lunar conditions if not deep space.

If you aren't concerned to get to millitorr, the best material for demonstration purposes would be Perspex (Lucite) which is available in tube form up to several inches in diameter and several feet long, with wall thickness of 0.3 inches or more. Any workshop should be able to cut, drill and turn it. I would make flat circular plugs for the ends, sealed with O rings - it's then easy to attach your pumps and any other apparatus  such as a trigger to release your test objects. 
 

Offline PmbPhy

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Re: What's the best way to construct a good vacuum chamber?
« Reply #3 on: 03/07/2015 01:28:49 »
Quote from: alancalverd
You don't need a particularly hard vacuum to demonstrate the "feather and hammer" experiment.
I'm merely trying to reproduce the environment in the which they happened on the moon.
 

Offline chiralSPO

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Re: What's the best way to construct a good vacuum chamber?
« Reply #4 on: 03/07/2015 02:16:27 »
Quote from: alancalverd
You don't need a particularly hard vacuum to demonstrate the "feather and hammer" experiment.
I'm merely trying to reproduce the environment in the which they happened on the moon.

Two issues:

(1) you can't "merely" reproduce the environment on the moon here on Earth--it takes effort!

(2) you don't necessarily need such high vacuum to reproduce the phenomenon you're after.
 

Offline alancalverd

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Re: What's the best way to construct a good vacuum chamber?
« Reply #5 on: 03/07/2015 13:36:17 »
You "merely" need about 0.1g vertical acceleration. The atmospheric pressure is much less of a problem. 
 

Offline PmbPhy

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Re: What's the best way to construct a good vacuum chamber?
« Reply #6 on: 03/07/2015 19:13:39 »
I'd like to make something clear. I have no interest on whether you think that the experiments will work or whether there will be problems or not or whatever. I'm far from being ignorant so I'm fully aware of all the problems that will occur between what I will do and what I'm comparing it to. I've already made the decision to go ahead with it. All I'm asking is about construction methods. So please let's stick with the subject at hand and not take this thread off subject by your concerns about your comparing the real experiment and the real life events that I'm comparing them to. Okay?  Thanks.

Quote from: chiralSPO
Two issues:

(1) you can't "merely" reproduce the environment on the moon here on Earth--it takes effort!
Nobody suggested otherwise. The term merely as I used it here is defined as being nothing more than which means that I'm doing nothing more than creating a vacuum sufficient to reproduce certain events that occurred on the moon like the waving of the American flag as a result of vibration rather than "wind" that the nutcases claimed was the case.

Quote from: chiralSPO
(2) you don't necessarily need such high vacuum to reproduce the phenomenon you're after.
I know that, of course. But when one takes into consideration that the nutjobs who claimed we never went to the moon will use any excuse to "diss" an experiment. So I'm doing what I can to get as close as I can to that vacuum. If it's too problematic then I won't make it that hard of a vacuum. It's best to get as closer to the environment as reasonable when trying to reproduce such effects. And I know it doesn't have to be exact or even close. But that's my choice of a starting place.

You "merely" need about 0.1g vertical acceleration. The atmospheric pressure is much less of a problem.
The acceleration is not relevant to what I'm doing. I'm reproducing what happened to the flag as it was unfolded. Since I can't control the acceleration I won't attempt to try so I'll see what happens with an acceleration of 1g. If I get the flag to "wave" then I'll have demonstrated the effect. Otherwise it was for nothing. The other is to show that a hammer and a feather fall at the same rate. And one last experiment, which will be very difficult, will demonstrate that a rocket engine can work in a vacuum.
« Last Edit: 03/07/2015 19:21:44 by PmbPhy »
 

Offline PmbPhy

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Re: What's the best way to construct a good vacuum chamber?
« Reply #7 on: 03/07/2015 19:17:05 »
You "merely" need about 0.1g vertical acceleration. The atmospheric pressure is much less of a problem.
The acceleration is not relevant to what I'm doing. I'm reproducing what happened to the flag as it was unfolded. Since I can't control the acceleration I won't attempt to try so I'll see what happens with an acceleration of 1g. If I get the flag to "wave" then I'll have demonstrated the effect. Otherwise it was for nothing. The other is to show that a hammer and a feather fall at the same rate. And one last experiment, which will be very difficult, will demonstrate that a rocket engine can work in a vacuum.
 

Offline alancalverd

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Re: What's the best way to construct a good vacuum chamber?
« Reply #8 on: 04/07/2015 06:58:31 »
The first task, flagwaving, can best be demonstrated in a bell jar, i.e. a chamber with a near-unity aspect ratio, and again Perspex (also marketed as Lucite in the US) is adequate and easy to machine, and you don't need a particularly hard vacuum.

You will never convince an idiot of the second task. Deep space, they will remind you, is full of cosmic dust and planets for the rocket to push against. Or each exhaust molecule is pushing against the previous one. It is a well known fact that the moon landings were faked, "sat nav" is just an accelerometer moving over a map, satellite TV is the Devil's method of spreading pornography and misinformation, and Newtonian physics is bunk because Aristotle had a proper beard, like God. Those Who Know The Revealed Truth have no interest in physics, and by the time they get elected to the Presidency and have their finger on the button, it's too late to do anythng about it. 

We pass this way but once, Pete. Don't waste it.
 
 

Offline PmbPhy

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Re: What's the best way to construct a good vacuum chamber?
« Reply #9 on: 04/07/2015 08:50:02 »
It just occurred to me that if I had one of those rooms in an old building which has concrete walls, ceiling and floor then I could make the room into a vacuum chamber. For the fun of it I could invite a bunch of these crackpots who claim that a rocket engine won't work in a vacuum to come and watch the demonstration. I'm looking into whether an Estes rocket engine will work on a vacuum. I think it does. The nice thing about having the chamber the size of a room is that the pressure in the room won't change by a noticeable amount so nobody could claim that when the rocket started it provided the gas on which to push off of. Then again I don't see why they don't use that to explain how a rocket can get into orbit. After all a crackpot is a crackpot is a crackpot, right? Lol!

I think a lot of useful experiments might be able to be done by creating such a chamber. Who knows? I have a friend who's one of their top physicists at MIT. He might help me get a room to convert. The physic faculty might enjoy having it too.

Any thoughts on this? E.g. what experiments using a vacuum chamber that large might a physics student might find very educational?
« Last Edit: 08/07/2015 03:49:59 by PmbPhy »
 

Offline alancalverd

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Re: What's the best way to construct a good vacuum chamber?
« Reply #10 on: 08/07/2015 15:14:24 »
An interesting question! Most teaching experiments are either reasonably unaffected by a bit of air (though anticonvection baffles are often useful e.g. in determining G) or are so sensitive to the odd molecule that we have to resort to small glass chambers (cathode ray tubes etc) or continuously-pumped UHV systems. By the time you get to room-size experiments you are talking about serious facilities like JET and CERN, which take months to pump out and the presence of an Estes rocket would cost thousands of hours and zillions of dollars to clean up.

You could put your rocket motor on a force balance and run it at sea level and 10,000 ft altitude (it's an excuse to visit the Grand Canyon) and demonstrate that the force is independent of ambient air pressure. Then the question of "in a perfect vacuum" doesn't arise and your critics will have to explain why your horizontal straight line of reaction force vs altitude doesn't curve.
 

Offline chiralSPO

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Re: What's the best way to construct a good vacuum chamber?
« Reply #11 on: 08/07/2015 15:31:40 »
Any thoughts on this? E.g. what experiments using a vacuum chamber that large might a physics student might find very educational?

I can think of one, but you'll also need to track down a spherical cow...  ;)

It just occurred to me that if I had one of those rooms in an old building which has concrete walls, ceiling and floor then I could make the room into a vacuum chamber. For the fun of it I could invite a bunch of these crackpots who claim that a rocket engine won't work in a vacuum to come and watch the demonstration. I'm looking into whether an Estes rocket engine will work on a vacuum. I think it does. The nice thing about having the chamber the size of a room is that the pressure in the room won't change by a noticeable amount so nobody could claim that when the rocket started it provided the gas on which to push off of. Then again I don't see why they don't use that to explain how a rocket can get into orbit. After all a crackpot is a crackpot is a crackpot, right? Lol!

I think a lot of useful experiments might be able to be done by creating such a chamber. Who knows? I have a friend who's one of their top physicists at MIT. He might help me get a room to convert. The physic faculty might enjoy having it too.

I don't know how well concrete can hold a vacuum... I would worry about outgassing and leaking (concrete is porous), and I know some materials may be very strong in some ways, but not in others, so I would check with some engineers to see how concrete might hold up against such challenges.

How are you planning on evacuating such a large room?
 

Offline PmbPhy

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Re: What's the best way to construct a good vacuum chamber?
« Reply #12 on: 09/07/2015 02:19:55 »
Quote from: chiralSPO
I don't know how well concrete can hold a vacuum...
It doesn't have to hold it that long. But if there's a problem there might be a way to seal the walls so that we can maintain a good vacuum. I do need to check on whether the walls can tolerate such strong forces. I forgot how my force would be on the wall after integrating over the entire surface and ceiling.

If I have a real desire to do this in real life then I'll find someone who knows how to determine what walls will tolerate such forces. Civil engineers are good at that. After all a wall 10 ft by 10 ft is subject to over 57,000 lbs of force. See what happens when you don't put pen to paper first. No wonder people don't do it like this. Lol!

Back to the drawing board.

Quote from: chiralSPO
How are you planning on evacuating such a large room?
Any vacuum pump will do. The pump can't tell how large a room is. The only thing different will be how long the pump needs to operate.
 

Offline chiralSPO

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Re: What's the best way to construct a good vacuum chamber?
« Reply #13 on: 09/07/2015 03:51:01 »

Quote from: chiralSPO
How are you planning on evacuating such a large room?
Any vacuum pump will do. The pump can't tell how large a room is. The only thing different will be how long the pump needs to operate.

You must be more patient than I am! The pumps that I use take about 5 minutes to get a 500 mL volume from atmospheric pressure to 0.5 Torr, and about 10 more minutes to get to 0.05 Torr (if it's sealed very well). If we used the same pump on your 10x10x10 foot room (2.8316 x 107 mL), it would take almost 200 days to get to 0.5 Torr.

And if the sides of the room have any amount of leakage (or if there is any leakage anywhere), then the strength of the vacuum will be limited by the size of the pump. Given the large surface area (a 10x10x10 foot room will have 600 square feet of potential leaks), you will need to be extra careful about the material used. You may be able to seal the surface of the concrete with a coating of polyurethane or sodium silicate (water glass).

I don't mean to be downer here. I think it's a cool idea, I'm just trying to identify potential problems at the outset. Definitely consult a civil engineer with any questions about concrete or other building materials!

I know your dream is possible, and has been realized before:
http://facilities.grc.nasa.gov/spf/
http://www.nasa.gov/multimedia/imagegallery/image_feature_1281.html
http://facilities.grc.nasa.gov/documents/TOPS/TopEPL.pdf

 It might be worth checking out how they did this. At first glance, it looks like they made the interior out of aluminum (to ensure the seal), and have a warehouse full of pumps!
 

Offline PmbPhy

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Re: What's the best way to construct a good vacuum chamber?
« Reply #14 on: 09/07/2015 04:23:21 »
I don't understand. What pumps do you use? Anyway, as I said, what I think up in my mind doesn't translate easily into reality. Thus the contemplation process. And that's what this thread was for. I've decided I can't afford to do this on my own. I've decided to ask MIT if I can use theirs. I'm sure they don't wait 200 days and they get down to 10-7 torr    See: http://web.mit.edu/aeroastro/labs/spl/facilities.html

re - I don't mean to be downer here.

No worries. If you know me by now then you know I can take it. After all I'm a physicist and I know how these things go. Sometimes things are hard to hear. I just totally underestimated the force from the pressure (never calculated it until today - should have know better anyway) and the cost of doing this (e.g. cost of a good pump, materials and labor).

You simply cannot be a physicist and not accept the kind of information you gave me with thanks. So a very warm thanks to you. :)   I realized it was beyond my means earlier today.
 

Offline Ethos_

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Re: What's the best way to construct a good vacuum chamber?
« Reply #15 on: 09/07/2015 04:53:51 »
How about a small sealed glass container, perhaps only one cubic foot in volume? Attach a string to the center of the top side of the container to which a small bottle rocket is allow to hang horizontally slightly off the bottom of the container. Supply wires running from outside the container attached to an electric igniter inside attached to the fuse of the bottle rocket. Evacuate the chamber with a vacuum pump and ignite the bottle rocket in the airless environment. Watch as the bottle rocket swings in an arc in the opposite direction of it's exhaust.

If that doesn't convince the naysayer's, then we must assume their intellect is deficient.

« Last Edit: 10/07/2015 00:49:39 by Ethos_ »
 

Offline PmbPhy

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Re: What's the best way to construct a good vacuum chamber?
« Reply #16 on: 09/07/2015 06:56:30 »
Quote from: Ethos_
How about a small sealed glass container, perhaps only one square foot in volume?
You'd be surprised how expensive they are. See: http://www.sanatron.com/products/COO-ACVSS-121212/12x12x12-inch-square-acrylic-vacuum-chamber-gas-spring-supported-lid.php?gclid=CjwKEAjwlPOsBRCWq5_e973PzTgSJACMiEp2TWncUbK_eA7IvcGB6G2rw75ox9SrlP75V2um23JIyBoCVTDw_wcB

I also didn't realize how expensive the pumps are: http://www.edwardsvacuum.com/promo/certified/us/index.html?gclid=CjwKEAjwlPOsBRCWq5_e973PzTgSJACMiEp21J1CEG-GHfXRUAXw75NOa-MpraGc0IXiSHrkbLV2fRoCno_w_wcB

That's why I gave this up and am seeking a large chamber that's already been built like the one at MIT.

Quote from: Ethos_
If that doesn't convince the naysayer's, then we must assume their intellect is deficient.
I'd wager that they'd claim that the gas from the rocket fills the chamber with gas so fast that it starts pushing on its own gas. That was always the purpose of having a large chamber.

It's a given that they naysayer's are intellectually deficient.
« Last Edit: 09/07/2015 07:05:15 by PmbPhy »
 

Offline PmbPhy

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Re: What's the best way to construct a good vacuum chamber?
« Reply #17 on: 09/07/2015 18:10:14 »
One of the things that most physicists aren't aware of is how, sometimes, when you look into something nutty you can find something interesting in the process. Now that doesn't mean that I should study all sorts of nonsense so that I'm destined to find something interesting. What it means is that you start to answer questions in more detail and rigor because they can't accept what most of us can by simply logic. In doing so you might be lead to interesting research that you normally wouldn't have otherwise. This has happened to me a few times and it happened again today. Had I not thought about rocketry more than the typical rocket equation then I never would have sought out a text on rocketry and learned that they've actually tested nuclear rocket engines. And they work quite differently than I'd ever have imagined.

For example; see: https://en.wikipedia.org/wiki/NERVA
 

Offline alancalverd

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Re: What's the best way to construct a good vacuum chamber?
« Reply #18 on: 10/07/2015 00:38:36 »
Now the nuclear rocket engine could be a good longterm investment. It seems to me that once in space, you could collect and use almost anything as a propellant - the analogy of throwing stones out of a boat can be realised. I've no doubt that the Society for the Protection of Asteroid Ecology and Native Culture would object, but vaporising dust and rocks would be one heck of a way to accelerate large human habitats over very long distances. 
 

Offline PmbPhy

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Re: What's the best way to construct a good vacuum chamber?
« Reply #19 on: 10/07/2015 21:59:31 »
Now the nuclear rocket engine could be a good longterm investment. It seems to me that once in space, you could collect and use almost anything as a propellant - the analogy of throwing stones out of a boat can be realised. I've no doubt that the Society for the Protection of Asteroid Ecology and Native Culture would object, but vaporising dust and rocks would be one heck of a way to accelerate large human habitats over very long distances.
I haven't put any numbers in since it's too complicated for me at this point. Do you have any idea if it could work by scooping up the hydrogen that is interstellar space?
 

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Re: What's the best way to construct a good vacuum chamber?
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