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Mainifold and "flushing"
>> Tom - elaborate on your "manifold side" stopcock. <<
The idea is that you keep a clean side and a dirty side to the pumping system.
Basically, that means that the part of the manifold that has the pump/stopcock
and the outside air stopcock along with the tube to be processed are kept away
from the clean side ( which contains the filling apparatus and gauges ) by
means of a stopcock. This means that when I finish filling a tube with rare
gas I shut this stopcock to prevent that part of the system from coming in
contact with outside air again. It is always in either a vacuum or whatever
pressure of rare gas the last fill was for the tube last pumped. I originally
did this so I didn't have to outgas this part of the system during warm up.
But then came the realization that there was this extra bit of left over
filling gas at around 12mm pressure that would normally be wasted. So I
included it in the pumping procedure as a flush cycle. Right after the tube is
done bombing and is coming down from the heat I open this stopcock up and
"rinse" the tube. The theory is that the pumps have a difficult time pulling
the residual molecules of impurities out when there are so few of them. By
introducing more molecules (this time inert "clean" molecules) to dilute the
remaining ones into the mix you can then pump them out with the rare gas
introduced. I find that it does help with difficult repairs like ones with a
lot of salt in them. You probably don't have that problem where you are but
it makes a good test bed. The hydrogen would also be more effective than
simple inert gas flush because it would also have the added benefit of being
highly reactive and acting as a "getting" compound does as a pump. I doubt
that it is that necessary unless you have a considerable amount of impurities
in which case you better get new pumps.
Hope I explained the process and my theory behind it. If not, ask more
questions.
E-mail from: Tom Biebel, 03-Oct-1995
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