Vacuum chamber and the characteristics of glass?

[The Artful Bodger]

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]

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]

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]

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]

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...991803968.html and I bought it from a Good Will store (a charitable store that resells gifted goods from citizen donors).

[jhe.1973]

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]

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]

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]

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:


Vacuum System by macona, on Flickr

[KMoffett]

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]

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]

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]

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]

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]

Or just buy this: http://www.ebay.com/itm/Bell-in-vacu...item3cd01cc5cb