Comment by ThePhysicist
7 years ago
Helium is very good at getting through the tiniest cracks due to its small atomic diameter. One uses it to find leaks in devices with ultra-high vacuum seals (e.g. in low-temperature physics) by attaching a pump with a simple mass spectrometer sensitive to Helium to the device under test and then using a small helium gun to test different parts of the device from the outside. When spraying it at the leak some of it is pushed inside by the pressure difference and can be detected by the pump spectrometer.
Hydrogen would be even better of course but it is rather dangerous when mixed with air.
Hydrogen would not be better. A helium atom is considerably smaller than a hydrogen molecule. Even if you could use atomic hydrogen, the helium atom's radius is significantly smaller due to the higher central charge that is only partially screened by the "other" electron.
Hydrogen binds with other hydrogen atoms to create H2 which is the only form of elemental hydrogen you’ll see outside of a lab helium is unique in that its singular atom is its own elemental molecule.
It's not unique in that. There are other single-atom molecules.
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Did it appear at any point that your parent doesn't know that with the really knowledgeable response he wrote? Why make a comment that doesn't add value?
Hydrogen would be better. Size isn't the most important factor. Gasses with low molecular mass have higher particle velocities. The velocity affects mixing, passage through holes (including filters), and the speed of sound. Hydrogen is about 2 AMU, 1 for each atom, while helium is about 4 AMU.
You can find the formulas in a typical college chemistry textbook.
If "hydrogen would be better", I guarantee companies that make highly-sensitive cryostats (like companies that manufacture MRIs) would do it.
They don't - even knowing He is more expensive that H, because helium is better at leak testing than hydrogen is
It gets worse than that when your cryo equipment has a “cold” helium leak. Ie, something leaking on the dilution fridge only at helium temperatures (4.2K) and lower, but no trace of a leak at room temp.
Repeated bouts of guessing where the leak might be and trying to plug it (usually by welding or re-machining some part), cooling down, checking if it worked, warming up, ripping some more of your hair out, repeat.
Yeah I had that as well, in that case it was best to replace all the Indium seals right away as like you said it takes 24 hours to do a full cooldown/warmup cycle anyway, so spending a day to make new seals is often faster. That's a part of low-temperature physics I definitely don't miss!
I worked on a system where I needed to reach and maintain pressures of 10^-3 tort, and that was hard enough. I can’t imagine having to deal with helium sensitivity.
10^-3 tort is very low. That will cause lawyers to explode.
Just having fun with your typo...
It's why companies that do stuff with ultra-low-temperature pressure vessels typically dramatically overengineer the devices.
Electronic parts also get helium exposure on the launch pad in a rocket. I know for example there can be helium sensitivity issues on gyros for CubeSats.
Wouldn’t hydrogen only work if it was atomic hydrogen? I thought molecular hydrogen gas was larger than helium.
Hydrogen is soluble in most metals. It will just diffuse right into it, like air flowing through a foam filter.
Added bonus: if the metals in question form chemical bonds with hydrogen, (titanium in particular) it will happily do so, even if it's deep inside the metal. The metal will turn very brittle very quickly if it's exposed to molecular hydrogen.
https://en.wikipedia.org/wiki/Hydrogen_damage
Even a single hydrogen atom is bigger than a single Helium atom. The charge of the helium nucleus is higher and thus "pulls" the two electrons further inward than Hydrogen's single proton pulls its single electron.
Hydrogen exists as H2 in its elemental form you’ll you’ll need to ionize it to separate it and keep it in an ionized or other way isolated form to prevent it from binding to other hydrogen atoms.
If you are talking about ionized hydrogen, you are no longer talking about a gas. If it’s ionized, it is a plasma.
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As a bonus hydrogen is higly explosive, helium is not.
Hydrogen is no more explosive than other flammable gasses when mixed with ambient air. To get the explosive effect, you need to mix it with oxygen in the right proportion (2:1, molecule-wise) and not have any filler gas such as nitrogen.
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