Re: Helium [was: t-bird screwing up]
On Wed, 2 Nov 2022, David Wright wrote:
Whatever, even I with an 8th grade diploma, knows you cannot keep helium
anyplace for very long. Put it in a monel metal
bottle with walls an inch thick and its molecules's are so small that 10% of
it is gone in 6 or 7 hours.?
So the He cylinders that we used after a few months in storage
really contained nothing at all!
I assume this is down to the type of metal. Similar to the fact that you
cannot inflate tyres using CO2 other than as an emergency measure as CO2
escapes remarkably quickly. N2, a smaller molecule, has no such
problems.
And these
jerks thought they could seal it up in a drive housing 1/16" thick?
The operative word is seal, not the thickness of the monel walls.
Seal?and no cracks.
This is only a half-truth. You know what goes out faster than helium?
Vacuum. And there was a whole glorious epoch in electronics which did
rely on keeping vacuum "in". You should have some fond memories of
that.
To be fair, most vacuum tubes aren't bathed in helium, but air, and
then only at a one atmosphere differential pressure. A gas cylinder
might be as high as 500 atmospheres.
And vacuum tubes do contain a getter to deal with outgassing, which
will help mitigate slight leaks.
I don't recall ever talking about this in my student days and I cannot
begin to guess how to calculate it now, but I'd expect a porous to He
enclosure but otherwise sealed would only lose He to the point that
there was a partial vacuum. Beyond that point, any He that entered the
walls would return to the enclosure with very high probability.
I did do some vacuum stuff at college, although not about vacuum tubes
specifically, and I'd assume the getter is just there to catch the
outgassing you cannot bake out and even a small leak would quickly
render the tube useless.
I've wondered how CERN deals with outgassing - perhaps the magnets act
as a cold trap.
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