Your explanation about helium defusing because it is inert is correct, and I
should have mentioned that. However, the diffusion is also because the atoms
are so small, which is why neon, argon, and krypton, also inert, don't
diffuse through glsss.
Interesing story about the lamps.
Jim
> -----Original Message-----
> From: Fuqua, Bill L [mailto:wlfuqu00@uky.edu]
> Sent: Thursday, May 12, 2011 11:51 PM
> To: 4CX250B
> Subject: RE: [Amps] Vacuum Packaging, storage for Glass tubes
>
> It is interesting that the explaination of helium atoms diffusing thru
materials due to their
> size is still around.
> Hydrogen does not diffuse as quickly as Helium. The real reason is that
the helium is not
> reactive with the walls.
> It is an inert gas. Most other light gases will bind to the wall material
before it can diffuse
> thru it.
> By the way, most chemical and physics professors will say the same
regarding the size
> until they stop to
> think about it. Just like the vacuum infilated balloon. It is something we
have heard over
> and over when we
> were young.
>
> I once got caught in a situation that really upset the apple cart in
our Physics Department
> at the Univerity of KY.
> A student asked be to explain a demonstration that was done the previous
day.
> The demo consisted of two coils and a long rod made of steel welding
rods taped
> together.
> The first coil had a lamp in series with it and was connected to a
120VAC source. The
> lamp lit up
> not real bright but maybe about half the intensity. When the rods were
removed from the
> coil it got
> very bright. That was OK.
> The second coil was to be used as a secondary winding and had a much
lower wattage
> lamp connected to it.
> With the rods in place in the primary and the primary coil's lamp glowing
about half
> brightness the secondary coil
> is added. It's lamp got very bright and the primary lamp became dimmer.
The professor as
> homework wanted the
> students to explain what they saw. He had his own somwhat convoluted
explaination of
> what had happened.
> I told the student that it was impossible that the primary lamp got
dimmer, it should have
> gotten brighter.
> The girl did her homework assignment and got a ZERO.
> I went to the guy who set up the demos and had him show it to me. And
indeed it appeared
> that the primary
> lamp got dimmer. Then I asked the guy to put a card board box over the
secondary's lamp.
> And we did it again.
> Guess what, the primary lamp got brighter. The students' eyes read
adjusted to the very
> bright secondary lamp
> when it came on and it made the primary's lamp appear to get dimmer.
> The older professor refused to change the girls grade. She went on to
med school and is
> now a doctor and A's in all
> her classes except one, Physics.
> 73
> Bill wa4lav
>
> ________________________________________
> From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf
Of Jim
> Garland [4cx250b@muohio.edu]
> Sent: Thursday, May 12, 2011 3:20 PM
> To: amps@contesting.com
> Subject: Re: [Amps] Vacuum Packaging, storage for Glass tubes
>
> > 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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