I have a LARGE glass cylinder that I am considering using as a vacuum chamber,
mainly for low pressure gas+electricity experiments.
i.d. 160mm (6.3"), wall 5.33mm (0.2"), height 1207mm (47.5")
no visible cracks or chips, some mild abrasion scratches.
Online calculators suggest +4 barg (50 psig) as a maximum working pressure,
so -1 barg should be no problem......
how dangerous is this idea ?Heavy Walter - 26-5-2017 at 06:10
Hi Sulaiman
Always evacuating a glass container has a risk.
Mainly if you don't know its previous history (bumps, chemicals contained, etc).
Is it borosilicate glass? Would be nice to look at it with polarizers in order to detect stresses.
I would suggest a protective cover as a plastic or metal mesh in order to prevent flying glass in case of implosion.
Do you need to attach valves or electrodes? If doing so with glassblowing techniques you will need to release stresses in an oven.
Send pictures!
Good luck,phlogiston - 26-5-2017 at 07:22
Positive and negative pressure put different stresses on the container, and the compression and tensile strengths of glass are differen. You cannot
directly translate the pressure specification for overpressure into a spec for the negative pressure differential.
Fortunately, the compressive strength of glass is higher than its tensile strength, so it works in your favour in this case.
and +1 on the shielding. Having once seen the shrapnel from a piece of glassware that imploded, I'd not feel at ease around that tube while it is
under vacuum.Melgar - 26-5-2017 at 08:13
Fortunately, the compressive strength of glass is higher than its tensile strength, so it works in your favour in this case.
Eh, glass doesn't work like steel and aluminum. If the cylinder had been used, and there were significantly more scratches on the inside then
outside, (and the scratches were mostly horizontal) the reverse would be true to a small degree. If you take a small section of the glass on the side
of the cylinder, imagine it with the pressure on both the inside and the outside. The main difference is that the side with the lower pressure is in
tension, and the side with the higher pressure is in compression. Now, like you said, you'd get a small benefit if the lower pressure was on the
inside, because the glass would be in compression in the cross section perpendicular to its axis. However, with glass, the main problem is crack
propagation, so existing scratches on the glass would cancel out any benefits from compressive or tensile loading.
Sorry to be pedantic, but I wrote it already so I'm gonna leave it. OP: just make sure you check really good for scratches. Also if you've ever
torched it to burn off residue that you couldn't get off before, don't use it then either, because it'll have lots of residual stresses in it.Dr.Bob - 26-5-2017 at 10:46
I would just keep it behind a plastic shield and it should be OK. The ends are weak points, as would be the middle. That is a large vacuum
chamber, but likely fine. Praxichys - 26-5-2017 at 11:21
A healthy coating of clear plasti-dip couldn't hurt either.
I bought it on a whim, used via eBay, no history.
I suspect soda-lime glass but it was listed as 'scientific glass'
I have a 1m x 1m toughened glass screen that I can use as a safety screen,
the glass is just a tube, I will fabricate the ends, where fluid & electricity will feed through.
