Safety check of an old glass bell jar before use

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Mark Aren
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Safety check of an old glass bell jar before use

Post by Mark Aren » Tue Jan 28, 2020 8:44 am

Hello all,

I have acquired a 300 (dia) x 400 (tall) thick walled glass bell jar and would like to run through some safety checks before I go much further. Unit came from an SEM, maybe 30+ years ago, and is complete with a (plastic ?) base seal.

The thought of such an item imploding terrifies me so I am planning a thorough optical inspection, with specific interest in cracks and abrasions. If you unit passes inspection then I plan to set it up an a 20mm ali base and use the roughing pump to pull it down as far as can easily be achieved. During this process the unit will be covered in a couple of sturdy blankets, and the pump left running for 30 mins. The bell jar will then be (gently) vented to atmosphere, and the process repeated.

Any comments appreciated.

Thanks, Mark

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Rich Feldman
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Re: Safety check of an old glass bell jar before use

Post by Rich Feldman » Tue Jan 28, 2020 5:13 pm

Sounds about like the one in my college lab for depositing films by evaporation.
Implosion risk similar to that of a large CRT television, eh?
I bet the peak stresses, in glass due to vacuum, are smaller in the bell jar because of near-ideal geometry.
The number of large bell jar years in service might be only 10^6 instead of 10^10.
bell2.png
bell2.png (50.2 KiB) Viewed 1212 times
Now the CRT is probably moved around less frequently, and protected on all sides but one, and might have a protective layer laminated onto the faceplate.
Time to shut up and let people who have worked with bell jars in this century speak up.
All models are wrong; some models are useful. -- George Box

John Futter
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Re: Safety check of an old glass bell jar before use

Post by John Futter » Tue Jan 28, 2020 6:04 pm

We use these daily at work in evaporation work.
We use a wire basket scatter shield when pumping down and when under vacuum.

The only time one has broken is during cleaning by some ham fisted idiot dropping it

your technique with the blankets is valid if you do not have a scatter shield --a wire type waste paper bucket would work well

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Rich Feldman
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Re: Safety check of an old glass bell jar before use

Post by Rich Feldman » Tue Jan 28, 2020 7:11 pm

John, is there also a policy that people working around the bell jar wear eye protection?
All models are wrong; some models are useful. -- George Box

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Re: Safety check of an old glass bell jar before use

Post by John Futter » Wed Jan 29, 2020 12:53 am

Rich
Interesting question
but no the SOP says you can only take the wire basket off when the chamber is up at atmosphere (I wrote the SOP)
The black thing on top of the chamber is the wire basket belljar underneath and aluminium foil in side the belljar to limit how much film gets on glass
DSC01703.JPG

Mark Aren
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Re: Safety check of an old glass bell jar before use

Post by Mark Aren » Wed Jan 29, 2020 8:54 am

Thank you again gentlemen.

Time to invest in an ali mesh safety guard...

Regards,

Mark

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