On Thu, Nov 03, 2022 at 09:38:20AM -0500, David Wright wrote: > On Wed 02 Nov 2022 at 06:44:22 (+0100), tomas@tuxteam.de wrote: [...] > > 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). I understood that. My point was that a helium filled HD most probably hasn't that pressure, so the leak rate is bound to be lower. Besides, if it's just He one can't make tight containers for, it'd be that leaking out, but air staying out, so... > Yes, I've read anecdotes of walls collapsing from He's diffusive > escape, but IIRC they've involved semi-permeable materials. ...this is what ultimately would happen. The HD would fail earlier, but then from heads flying too low, for lack of pressure. > One symptom of an decreasing vacuum would probably be overheating, > but by the time that happens, the heads might have crashed anyway. Yep. It took me a while to interpret that "decreasing vacuum" in the right direction :) BTW, the Wikipedia says the driving factors for helium are less turbulence and lower friction; of course that is boud to correlate with lower flight height. [...] > We baked our mass spectrometers in removeable oven enclosures. [...] Nice stories :) I did theoretical physics, but of course, our faculty was chock full of crazy and interesting folks doing stuff like you described above. Searching for leaks in the apparatus consisted of... pumping it full of helium and sniffing for it with a sensor :-) > 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...." I didn't know, but I had the hunch that hydrogen is even nastier than helium to contain. Hydrogen readily makes hydrides with metals, i.e. whithin the metal matrix it's a proton in a sea of electros. An atom doesn't get much smaller as that. Helium, OTOH, holds pretty tight onto the two electrons it has (snobby noble gases, they are). Cheers -- t
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