Comment by Triesault
7 years ago
Hermetically sealed by definition is airtight, i.e. "excludes the passage of air, oxygen, or other gases". [1] With that said, the diffusion rate depends on both the properties of both the seal and the gas.
7 years ago
Hermetically sealed by definition is airtight, i.e. "excludes the passage of air, oxygen, or other gases". [1] With that said, the diffusion rate depends on both the properties of both the seal and the gas.
Sometimes, definitions aren't the most mechanically sound constructs available.
Very few air seals protect against helium, the smallest gas. Most materials, even metals, are like sponges to helium. It'll seep through slowly.
I was about to correct you and say that hydrogen is the smallest, but apparently, hydrogen's atomic radius is actually a bit bigger. Helium's radius is smaller due to the larger charge of the nucleus making for a tighter electron cloud.
I too had the same thought, and then thought maybe hydrogen molecule is being considered. Read up to see that hydrogen atom has bigger radius. TIL.
Hydrogen creates molecules H2.
Yup. Larger atomic radius due to less nuclear charge, and it's only stable in molecular form - H2.
Helium can cause problems with the metals containing fusion reactors - He bubbles can get inside the metal structure and weaken it considerably:
"The sun makes energy by fusing hydrogen atoms, each with one proton, into helium atoms, which contain two protons. Helium is the byproduct of this reaction. Although it does not threaten the environment, it wreaks havoc upon the materials needed to make a fusion reactor.
"Helium is an element that we don't usually think of as being harmful," said Dr. Michael Demkowicz, associate professor in the Department of Materials Science and Engineering. "It is not toxic and not a greenhouse gas, which is one reason why fusion power is so attractive."
However, if you force helium inside of a solid material, it bubbles out, much like carbon dioxide bubbles in carbonated water.
"Literally, you get these helium bubbles inside of the metal that stay there forever because the metal is solid," Demkowicz said. "As you accumulate more and more helium, the bubbles start to link up and destroy the entire material.""
https://phys.org/news/2017-11-fusion-energy.html
Case in point, helium filled balloons that deflate over a couple days.
That’s just because rubber balloons are extremely porous, especially when stretched. They don’t hold other gases well either.
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Air filled balloons also deflate over a couple days.
Compare with aluminium coated foil. They don't deflate that fast.
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I thought hydrogen was the smallest gas, not helium.
Hydrogen normally comes as a molecule (H2), not an atom.
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Depends on your definition of smaller, but apparently it's not clear-cut: https://www.quora.com/Which-element-is-bigger-an-atom-of-hyd...
Well, air is also compound of rare gases of which Helium is one.