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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
by ThePhysicist 8y 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.
- klodolph 8y agoWouldn’t hydrogen only work if it was atomic hydrogen? I thought molecular hydrogen gas was larger than helium.
- mamon 8y agoAs a bonus hydrogen is higly explosive, helium is not.
- athenot 8y agoHydrogen 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.
- kens 8y agoOne of the dangerous things about hydrogen is its wide flammability range: from 4% to 75% concentration of hydrogen in air. Hydrogen will detonate in concentrations between 18% and 59%. In comparison, gasoline has a surprisingly small flammability range of 1.4% to 7.6%.
- monochromatic 8y agoWhat drives this wide range of flammable concentrations? Is it just an issue of low activation energy?
- namibj 8y agoAlso the upper end is impacted by the low number of oxygen atoms each hydrogen molecule can react with, in this case it's just one, compared to gasoline with say 8 carbon and 18 hydrogen atoms which bounds to 25 oxygen atoms. Mind you, the size of a gas molecule doesn't affect how many you get per volume at a certain pressure and temperature. So 50% gasoline vapors and 50% hydrogen are both molecule counts, with the former requiring 25 times as much oxygen for full combustion, compared to the latter.
- deleted 8y ago[deleted]
- dogma1138 8y agoHydrogen 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.
- nwallin 8y agoHydrogen 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 https://en.wikipedia.org/wiki/Hydrogen_damage
- Nimelrian 8y agoEven 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.
- wnoise 8y agoHydrogen 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.
- deleted 8y ago[deleted]
- dogma1138 8y agoHydrogen 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.
- jazoom 8y agoIt's not unique in that. There are other single-atom molecules.
- monochromatic 8y agoIt's not unique. It's just the smallest.
- jazoom 8y agoThat point has already been well-established in all the other comments.
- rocqua 8y agoIm guessing it's the only one that's gaseous at room temperature and pressure? Edit: Never mind, all the nobel gasses.
- dogma1138 8y agoIt’s not all the Nobel gases, elemental Xenon is Xe2, so are the rest I think they are also not that Nobel anymore everyone of them besides helium was found to form compounds. Edit: apparently even helium can form compounds it’s just freaking hard to force it to play nice with others - https://en.m.wikipedia.org/wiki/Noble_gas_compound https://en.m.wikipedia.org/wiki/Noble_gas_compound
- planteen 8y agoElectronic 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.
- jeffwass 8y agoIt 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.
- ThePhysicist 8y agoYeah 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!
- justwalt 8y agoI 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.
- ConceptJunkie 8y ago10^-3 tort is very low. That will cause lawyers to explode. Just having fun with your typo...
- warrenm 8y agoIt's why companies that do stuff with ultra-low-temperature pressure vessels typically dramatically overengineer the devices.