Re: Helium [was: t-bird screwing up]
On Wed 02 Nov 2022 at 06:44:22 (+0100), tomas@tuxteam.de wrote:
> On Wed, Nov 02, 2022 at 12:06:16AM -0500, David Wright wrote:
> > On Tue 01 Nov 2022 at 06:49:09 (+0100), tomas@tuxteam.de wrote:
>
> [...]
>
> > > 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.
>
> But not a harddisk. My point was somewhat tongue-in-cheek: even assuming
> He diffuses out of the harddisk, we've got very good at keeping air out
> (cf. vacuum tubes), so you'll be left with... vacuum. Heads would fly
> lower, not higher. Actually heads would roll on the tarmac :-)
>
> Of course, if you've got 200 or 500 bar, something might leak.
Yes, that referred only to the gas cylinders (snipped from the above).
> > And vacuum tubes do contain a getter to deal with outgassing, which
> > will help mitigate slight leaks.
>
> Oxygen. Not nitrogen, AFAIK. Talk nerds getting off-topic :-)
Gettering nitrogen is tough.
On Wed 02 Nov 2022 at 07:51:35 (+0000), Tim Woodall wrote:
> 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.
Gene said Monel, and it's generally selected for properties like
remaining ductile at very low temperatures, strength, corrosion
resistance, low permeability etc.
> 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.
Yes, I've read anecdotes of walls collapsing from He's diffusive
escape, but IIRC they've involved semi-permeable materials.
One symptom of an decreasing vacuum would probably be overheating,
but by the time that happens, the heads might have crashed anyway.
> 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.
We baked our mass spectrometers in removeable oven enclosures.
Obviously we'd slide the magnet out of the way on compressed air,
as they weigh about a ton. After removing the ovens, the flight tube
(which would otherwise cool quickly) would be kept hot with one of
those tapes like people use to keep pipes in the roof from freezing
(but hotter).
During and after that, it's down to pumps of various sorts, oil- or
Hg- diffusion (yuk, never used those), Ti-sublimation, turbos, sputter
ion, plus a cold finger trap using liquid nitrogen. Whenever the
source was opened, an isolation valve maintained the vacuum in the
flight tube and detectors, while dry nitrogen was fed into the source
to prevent laboratory air from entering.
All that was just for ~10-7 mbar vacuum or so. (Any better was
pointless, as the sample itself, being incandescently evaporated,
would ruin it.) I'm sure the vacuums at CERN are far, far higher.
On Wed 02 Nov 2022 at 10:25:39 (-0000), Curt wrote:
> On 2022-11-02, David Wright <deblis@lionunicorn.co.uk> wrote:
> >
> > 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.
>
> I always thought vacuum tubes weren't filled with anything but a "high
> vacuum" and electrodes.
Yes, so the electrodes are surrounded by a vacuum; it's the vacuum
/tubes/ that are bathed in air. That's easier for the seals to cope
with than He would be.
> Gas-filled tubes, on the other hand, are filled with a gas.
How interesting :)
On Wed 02 Nov 2022 at 07:56:02 (-0400), gene heskett wrote:
> On 11/2/22 01:07, David Wright wrote:
> > On Tue 01 Nov 2022 at 06:49:09 (+0100), tomas@tuxteam.de wrote:
> > > On Mon, Oct 31, 2022 at 06:32:17PM -0400, gene heskett 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!
>
> We had such large bottles at Stellardyne Labs, in [San Diego] in the
> early 1960 time frame, used to test the
> ullage pressure regulators that kept the Atlas from collapsing under
> its own weight when it was fully[^H^H^H^H^H un-] fueled
> and ready to give John Glenn his first orbital ride. The procedure
> for the end of the night shift was to pump
> it all into this bank of bottles with a huge cardox 6 stage compressor
> in the back yard, putting them at around
> 7200 psi. All recorded in an electronic log on rolls of graphic
> printer paper.
>
> These bottles had monel walls about 2" thick, 10 feet tall, a dozen
> of them. Pipes to/from were about 3" in
> diameter, monel with a 1" bore.
>
> 8 hours later when the day shift clocked in, those bottles were down
> to 5800 lbs. That place used 95% of the
> US production of Helium at the time, with a new truckload at about
> $100k worth of helium nominally 2 x a
> week to keep it operational.
Do you think they cared? Forget John Glenn: his ride was just the
consolation prize. You have the US Government in a panic to produce
an ICBM worthy of the name, and they have a virtual monopoly on the
world production of He. Feel free to mark the waste down as another
of the project's failures if you like.
> > The operative word is seal, not the thickness of the monel walls.
> > Seal—and no cracks.
>
> You still don't get it, the helium molecule is so small it wiggles
> thru a steel walls huge molecules
> like they were a layer of felt. Monel alloy is denser but it still leaks.
I haven't done the experiments, but others, like this pair, have.
https://www.sciencedirect.com/science/article/abs/pii/0042207X61900185
I haven't paid for the full PDF, but a year earlier, the same pair
"investigated the permeation of helium and hydrogen through 0.25 mm
thick tubes at 1023 K. Hydrogen permeation was detected through Monel,
304 stainless steel, Kovar, Inconel, nickel, and 52 Alloy (Fe50 Ni50)
even at temperatures approaching room temperature. The same metals
were not permeated by helium at temperatures as high as 1123 K."
R. Collins and J. Turnbull. Degassing and permeation of gases
in tube materials. In D. Slater, editor, Advances in Electron
Tube Techniques, Proceedings of the Fifth National Conference,
pages 186–190. 1960.
(reported in a dissertation by Daniel Schultheiß, Augsburg, 2007)
Cheers,
David.
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