> Would 1 Torr outside pressure on a say, 3-500Z which I presume, has a very
"hard"
> vacuum,
> dramatically slow the ingress of air into the tube over a long period of
time?
>
> If so, then someone could help frugal hams by designing a similar, but re
usable, home-
> evacuatable can that
> could vacuum pack 3-500Zs and the like, extending their life during/after
long times of
> storage.
>
> best, 73, de Pat AA6EG
>
************************
Hi Pat,
Theoretically, if you reduced the outside pressure on a glass (or metal)
vacuum tube to, say, 1/10 atmospheric pressure, that would reduce the
diffusion of air molecules into the interior of the tube to 1/10 the rate.
However, I doubt that would extend the life of the vacuum tube much. My
guess (and it really is only a guess) is that vacuum degrades inside a tube
mostly because of outgassing of absorbed molecules on the surface of the
interior parts of the tube, and not diffusion through the tube walls.
When I was a grad student, we used to do experiments inside
ultra-high-vacuum vessels. A single thumbprint inadvertently left on an
interior supporting structure would ruin the vacuum, requiring a thorough
(and costly) tear-down of the equipment.
That said, gases do diffuse into any vacuum-containing enclosure. Another
example from my grad student days was doing experiments at very low
temperatures, using liquid helium in glass dewars, which were basically
thermos bottles with a vacuum jacket between the inner and outer glass
walls. Helium atoms are small enough that they diffuse through the glass
walls and quickly contaminate the vacuum. For that reason, the vacuum jacket
had to be continuously pumped during an experiment.
73,
Jim Garland W8ZR
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