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Re: Ovens or no ovens
Morgan wrote:
Jeff, I guess that my main area of concern is that since you are pumping with
an open stopcock and I'm guessing that you're down to a vacuum where you're
below laminar flow and into random molecule flow; since you're only heating
a small area of glass, do you think that this newly liberated stuff that
comes out of the hot glass is going to go out through the tubulation and not
just have these molecules go reattach themselves to another cooler part of
the tube?
Do you understand what I'm trying to ask?
Morgan
Yes, I do, but this has proven to be a very insignificant problem. When I
bombard, I often go back over areas that were previously heated, and when I
do I can see how much new gas is outgassed on my vacuum gauge in response to
the heating. When I initially induction heat the electrodes, or heat the
glass, the vacuum gauge quickly flies off scale. After a little while, the
vacuum slowly drops in general, but goes up and down in response to moving to
different parts of the glass with the torch. The response is very fast.
Remember, I do not use a Pirani gauge at moderate vacuum. I use a ion gauge
at high vacuum, using a diffusion pump with a base pressure of about 1
millimicrons (10E-6 torr). This is a very sensitive way to detect outgassing.
When the outgassing is complete, I do not get any additional releases of gas
as viewed by my vacuum gauge.
Compare this with the electric discharge method. The outgas products are
trapped inside the tube along with the discharge mixture until the valve is
re-opened. I have observed electrodes burn in outgassed oxygen, if the
released gas pressure was too high. The same thing can happen there in a much
greater proportion -- molecules re-settling back on to the walls of the
vessel, or the electrodes for that matter in the form of an oxide coating.
This is certainly not as good as my method.
Ovens do have the best advantage of the 3 methods in the respect that you
claim, no doubt about it. But the margin of superiority is very small, unless
you are doing crackle tubes. And the quality of the flame/RFI bombarded tubes
is already so high, it's hard to imagine a requirement for a higher standard.
They both for the most part produce the same results. But the inflexibility
and. slowness of ovens, both in heat-up and cool down, as well as setup, is
their biggest disadvantage as I see it.
Hope that explains the point.
Jeff Golin