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NEON- Suck in's and such

Tom W. wrote:

 >> However, I'm not sure what I did to cause it in the first
 place. Can anyone out there explain this?  Recreate this? Come on, lets see
some hands. OK you there with the south cakilaky accent.

 Tom <<

 First of all, let me say there are far too many Tom's on this list. There goes
my minority status <g>.

 The question was on the suck-in's during bombarding on the inside of
double-backs. You also mentioned that it usually happened with smaller
diameter tubing which is quite often the case. This is caused by too high a
pressure in the glass when it is bombarding leading to the glass melting from
the heat of the resistance of the gas to the high current of bombarding.
Because the arc is taking the path of least resistance (in this case the short
cut on the short side of the turn on the double-back) it heats up the inside
surface the most. I think it also happens on double backs that aren't blown
out quite as much and the diameter is slightly smaller at that point. This is
why I mentioned in an earlier post that some benders over blow double backs
because of bombarding problems with the smaller diameter that results from an
under blown bend. It could also be that the arc is simply contacting the glass
more at this point than on a straight section.

 To avoid this, bomb at a slightly lower pressure. This is also common if you
bombard with the closed stopcock method and don't relieve the pressure
build-up as the gasses are evolved from the impurities outgassing and not
being removed by the pump. I usually don't have this problem as we are from
the open stopcock camp but sometimes the tubes will outgass impurities even
faster than the open pump stopcock can withdraw them and this can happen. On
tubes where we might suspect this will happen (like on 8mm w/tight bends) we
put a dab of temperature indicating paint on the double-backs. If that paint
melts then the glass will soon melt also and we stop bombing and let the pump
catch up for a bit. ...... Hope that makes sense.

 BTW, I used to live in Mobile, Alabama. Does that count for that accent you
mentioned? I know who you meant, though and he does have a nice one.

  E-mail from: Tom Biebel, 30-Aug-1996