Melting Alumina tube

I want to be able to seal the ends of several alumina tubes, for
thermocouples, maybe 6mm OD x 4mm ID, by melting the end into a blob,
using oxy/acetylene.
Did some trials, but my shade 5 goggles wouldn't do - blindingly bright.
Ordered some shade 8, but - is this a practical idea?
Anybody done similar?
Thanks,
-- Peter F
Reply to
Peter Fairbrother
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Turns out, yes, you can. :(
Molten alumina is a bit weird, I'm not sure whether it's thick and sticky or thin and sticky. I guessed the first, the latter seemed more in tune with what happened - but I haven't worked it out. Getting the tube to blob and seal over was unexpectedly hard, or rather it didn't go as expected - but by no means impossible.
To stop the tube cracking, get an insulating firebrick, grade 26 or better (grade 23 won't do), drill a deep hole a couple mm larger than the alumina tube, and insert the tube so only a cm or two are showing. Surround the tube end at a distance of an inch or so with some extra bits of firebrick to form a hearth. It helps to be able to turn/rotate the tube/hearth.
Oxy/MAAP (real MAPP, not propylene), 1/4 by 1/8 Omegatite 450 99%+ impervious alumina thermocouple protection tube, j26 firebrick.
For reference, if anyone is interested.
Now, how do you weld miniscule 0.005" 0.0125mm tungsten/rhenium thermocouple wires together?
Anyone know of a source of tungsten tube, about 3mm OD, in 6" lengths? They make 1" lengths for fishermen...
-- Peter
Reply to
Peter Fairbrother
The traditional method is to twist the ends together and use capacitor discharge between both cores commoned together and a piece of platinum, tungsten, graphite, whatever is appropriate.
Voltage and capacitor size are subject to experimentation.
regards Mark Rand
Reply to
Mark Rand
I've done it successfully in the past using an oxy-acetylene flame, but with fine wires you do have to be careful not to melt the lot !
Andrew
Reply to
Andrew Mawson
When I've looked into it what I found indicated a capacitor discharge between the thermocouple wires and typically a graphite plate shielded in a tube with an argon atmosphere.
Reply to
David Billington

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