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The Artful Bodger
12-31-2013, 10:05 PM
I appears I have been landed with doing some sort of presentation to school children on aviation and meteorological matters.

One of the topics is a demonstration using a vacuum chamber to simulate atmospheric changes up to about 12,000', which I believe is about half sea level pressure.

So, and I am sure there are people here with this knowledge, will a typical glass preserving jar handle a half bar of depression? Will this weaken the jar and cause it to fail after a number of uses?

Thanks

Jpfalt
12-31-2013, 10:28 PM
Using a preserving jar as a vacuum chamber is not a good idea. You need to look for a Bell jar built for the job. The Bell jar is heavy glass formed into a smooth dome shape. Any geometry discontinuity in a preserving jar will be a stress concentrator that can result in an implosion that will end up throwing glass fragments all over.

Don Young
12-31-2013, 11:01 PM
It is common for preserving jars to be sealed with the contents at boiling temperature. When cooled they are under vacuum but I don't know what the pressure is.

Jpfalt
12-31-2013, 11:57 PM
The pressure in a canning jar should be the vapor pressure of water at room temperature. I checked a table and saw that at room temp, about 75 degrees, the vapor pressure of water is 20 mmHg which is about 1/2 psi. That means the canning jar is at about a 14 psi vacuum.

Looks like it might work. However, I would get a clear polycarbonate tube or shiel to put around it just in case the jar cuts loose. I would still get a Bell jar built for it.

dp
01-01-2014, 12:45 AM
I have a refrigeration vacuum pump and used a domed cake dish lid to do a similar demonstration. At nearly 1/4" thick it was more than up to the pressure. It was nearly identical to this: http://www.aliexpress.com/item/Cover-cake-pastry-glass-utensils-cake-pan-with-lid-glass-fruit-plate-glass-cake-pan-extra/991803968.html and I bought it from a Good Will store (a charitable store that resells gifted goods from citizen donors).

jhe.1973
01-01-2014, 12:49 AM
I'd only like to add to look over every bit of the surface of whatever glass you use to be sure there are no nicks or scratches.

Stress concentrations at vacuums or pressure can be dangerous!

macona
01-01-2014, 12:54 AM
Get a real bell jar. This is not something to mess around with. If a canning jar with food implodes nothing happens. An empty jar is a whole different story.

boslab
01-01-2014, 01:42 AM
Youve seen the 45 gallon drum implosion experiment i suppose, heavy glass vessels including tv tubes going bang send an awful lot of shrapnel around the room, even vacuum domes aka bell jars do go bang too!, we destroyed many in the lab, as posted it only needs a scratch, we ended up getting clear heat shrink tube to wrap them just in case, it worked, big polcarbonate box surruonding vacuum experiments where there may be a risk of implosion is a legal requirement in schools over here, along with the usual risk assesments, just control the risk with guarding, id be tempted to vacuum a jar down as far as possible to see how it behaves myself, depending on the vucuum pump you have, an Edwards was used in the lab which could go down fairly low.
Mark

macona
01-01-2014, 02:03 AM
As a professor at a local college told me, 'splosions are bad. It does not matter if they are im or ex, they are bad. He went on to tell me he has seen glass from a bell jar imploding go through a car door.

I have a full steel shatter guard around my bell jar:

http://farm8.staticflickr.com/7172/6821472445_d1727308f4_b.jpg (http://www.flickr.com/photos/67292116@N00/6821472445/)
Vacuum System (http://www.flickr.com/photos/67292116@N00/6821472445/) by macona (http://www.flickr.com/people/67292116@N00/), on Flickr

KMoffett
01-01-2014, 12:07 PM
You might consider an acrylic or polycarbonate tube capped with similar sheets . I constructed a 2meter long, 6" diameter, 1/4" wall acrylic vacuum chamber to demonstrate the fall of a super ball vs a feather in air and in a vacuum. We routinely pump it down to -27mmHg.

Ken

Edwin Dirnbeck
01-01-2014, 12:51 PM
You might consider an acrylic or polycarbonate tube capped with similar sheets . I constructed a 2meter long, 6" diameter, 1/4" wall acrylic vacuum chamber to demonstrate the fall of a super ball vs a feather in air and in a vacuum. We routinely pump it down to -27mmHg.

Ken

This sounds like a good idea to show my grandkids. Does the feather get slowed down by any kind of static electric charge that might be in an accrilic tube?Thanks,Edwin

Fasttrack
01-01-2014, 02:45 PM
What are you using to pump down the chamber? How big does it need to be? What will you be looking at in the chamber?

I suggest building your own vacuum chamber and using poly-carbonate or acrylic end plates for viewing. You will only be pumping down to about 350 torr, so you won't need to worry too much about the materials you use. You could make the chamber from ordinary steel. Just polish up the inside so there is no scale or other nasty on it and then clean it well with acetone followed by denatured alcohol before you pump it down for the first time. Check out the specs for ASA flanges. A simple o-ring flange is all that is needed to seal the windows on either end. Make sure your windows are thick enough to withstand the pressure differential, though.

The Artful Bodger
01-01-2014, 05:49 PM
I want to demonstrate lapse rate (temperature drop with increasing altitude and decreasing pressure). There will be school kids watching and an adult or two. Hopefully I will also be able to demonstrate the appearance of visible water vapour (i.e. 'clouds') and compare the altitude shown on an altimeter to the estimated height of clouds in the area. If all goes well we might even be able to demonstrate the principles behind the fohn winds we get in this area. A poly carbonate box seems a good idea! thanks.

darryl
01-01-2014, 06:26 PM
I agree with all the safety issues, and would also suggest a polycarbonate shield box to house the apparatus. Beyond that though, I think most any glass jar would be able to sustain a full vacuum. But you might want to use a more appropriate thing so you can see the cloud formation, etc without optical distortions. I would tend to think that a length of polycarbonate tube would be the better thing to use for a chamber. Even at 8 inches in diameter, it is easy to make end caps capable of handling the pressure. Those don't have to be see-through, and one would become the base of it anyway. A piece of 1/2 thick aluminum plate will be more than strong enough to resist caving in as the top cover.

Polycarbonate tubing isn't cheap, but the protective box would just use flat panels. You could use a larger tube as the protective cover for the actual 'vacuum' tube, but you'd probably have to take out a second mortgage on your house to be able to afford it :)

macona
01-01-2014, 08:10 PM
Or just buy this: http://www.ebay.com/itm/Bell-in-vacuum-jar-sound-physics-demonstration-demo-water-boil-air-pressure-New-/261189584331?pt=LH_DefaultDomain_0&hash=item3cd01cc5cb

boslab
01-01-2014, 08:17 PM
Stick a smoke detector in there and you can watch particle trails too!
Am241 worked for me!
Mark

KMoffett
01-01-2014, 08:24 PM
This sounds like a good idea to show my grandkids. Does the feather get slowed down by any kind of static electric charge that might be in an accrilic tube?Thanks,Edwin
Never had a static problem with the demo. It never ceases to amaze me to watch the ball drop and the feather float down in ambient, and both " drop like rocks" in the vacuum.

Ken

The Artful Bodger
01-01-2014, 08:55 PM
Stick a smoke detector in there and you can watch particle trails too!
Am241 worked for me!
Mark

I need to know more please!

The Artful Bodger
01-01-2014, 08:58 PM
The ball and feather demonstration might be appropriate for us too as we are trying to get young children (12-14 year olds) interested in things related to atmosphere, weather and of course aeroplanes! But I guess the ball and feather demo would require a much harder vacuum than my half atmosphere?

How thick would the wall have to be and an 8" polycarbonate tube to be safe at all practically achievable vacuum levels?

Don Young
01-01-2014, 09:34 PM
Isn't the force on a vacuum chamber essentially 14.7 PSI at absolute vacuum and pretty close to that at most 'normal' vacuums? There is not much difference between 28" HG and 30" Hg. I never thought that force was a real concern in high vacuum work.

darryl
01-01-2014, 09:38 PM
For an 8 inch poly tube, I think 3/8 wall thickness would be lots- a 1/4 inch wall would probably never show the strain either. One of the pieces in my vacuum system uses an 8 inch diameter plumbing fitting- I don't think that gets any thicker than about 1/4 inch. End caps are 3/8 thick polycarbonate, but I've also blown them into a dome shape. They take both the highest vacuum I can make, and up to 150 psi pressure. I can pump down to about 1 psi, which is about 95% of a full vacuum. In terms of the ability of something to withstand a vacuum, there's no difference between that and a high vacuum.

It's equivalent to anything sealed and placed within another chamber which is pressurized to 15 psi. That might be a good test, actually, since it's way easier to achieve some relatively low positive pressure than a full vacuum. Of course the chamber you use would have to be rated to take the pressure.

You could use water of course- tap water will be at least 30 psi, often up to 50 or so. If you really want to test a canning jar or similar for it's ability to withstand it, just rig up something you can seal and pressurize from your tap.

macona
01-01-2014, 10:35 PM
Isn't the force on a vacuum chamber essentially 14.7 PSI at absolute vacuum and pretty close to that at most 'normal' vacuums? There is not much difference between 28" HG and 30" Hg. I never thought that force was a real concern in high vacuum work.

Atmospheric pressure may not seem like a lot but that is still a lot of energy.

Don Young
01-02-2014, 10:41 PM
I was just commenting that the force is almost the same at moderate vacuums as it is at 'deep' vacuums. I agree that the force can be quite high but I don't think that you can break a canning jar even with a 'perfect' vacuum.

Atmospheric pressure may not seem like a lot but that is still a lot of energy.

darryl
01-03-2014, 12:48 AM
The canning process will get you pretty close to a vacuum, as Jpfalt suggests. Getting rid of the final 10% of air will not get you any closer to a catastrophe. If you were considering a canning jar in the first place, that suggests that a diameter of 4 inches would be enough for the project- 8 inch tube would just up the cost for you. Maybe compromise at 6 inch polycarb tubing at 1/4 inch wall. The base and the top can be grooved for the ends of the tubing, and that will prevent it from losing its round shape and possibly collapsing. For more safety you could also machine a round hole in a piece of flat sheet and place that over the tube at about the center point. Make that from the same stuff you use for the outer enclosure.

Personally I'd have no problem capping a 6 inch diameter tube of polycarbonate and pumping it down. I'd be standing right beside it.

As far as the feather and ball, I see the need to get below 1/2 atmosphere, but 1psi remaining air isn't going to screw up the demonstration.

macona
01-03-2014, 02:48 AM
I was just commenting that the force is almost the same at moderate vacuums as it is at 'deep' vacuums. I agree that the force can be quite high but I don't think that you can break a canning jar even with a 'perfect' vacuum.

I wouldn't be so sure, I have seen canning jars shatter on cool down. Not every one is going to be perfect. Why mess with on when you can get a real jar and base plate so cheap. Not worth the potential injury, especially around kids.

The Artful Bodger
01-03-2014, 03:55 AM
I would be very dubious of a canning jar 'shattering' on cool down however I have seen jars crack on cooling, just a crack but no violent implosion which of course would be impossible when the jar is almost filled with liquid.

EVguru
01-03-2014, 05:10 AM
Charity shop presssure cooker with acrylic lid; job done.

macona
01-03-2014, 01:55 PM
You need about 3/4 acrylic to withstand that pressure. That's more than the bell jar and plate I mentioned.