Comment by comboy
7 years ago
Anybody can provide a reason why SiT1532 wouldn't be hermetically sealed? They are talking about high accuracy, intuitively it would seem that if helium affects it this badly, then it would be susceptible to some atmospheric pressure changes (unless the seal worked against air but not helium).
They do have a strong hermetical seal on them, but the older ones used by Apple are still susceptible to “small gas molecules”.
They must have some seal in place and fortunately our air is Nitrogen, O2, and a little bit of Argon. These can’t get through the seal! Helium is only found in minuscule amounts naturally and apparently this concentration is so low it doesn’t matter.
Helium is special because is it has the smallest kinetic diameter of all the noble gases. You can compare its diameter to other gases here: https://en.wikipedia.org/wiki/Kinetic_diameter#List_of_diame...
Helium is frequently used in leak tests since it has a way of working through even the tiniest of leaks. Elemental Hydrogen is of course smaller, but I believe you only find Hydrogen as H2 which is a larger molecule than Helium.
According to that table, Helium is 260, and water is 265, and there are a few more under 300.
Does that mean that water (in some form) also acts like helium and would be able to get through that seal?
I think the answer is that water has polarity! Though we’re definitely hitting my chemistry knowledge limit...
While water’s overall charge is neutral, its charge isn’t distributed symmetrically ans tbis makes it a rather polar molecule. This means part of molecule have different charges which leads to attraction to other water molecules thanks to van der waals forces. Basically water naturally clumps up and won’t squeeze through tiny holes even if it’s kinetic diameter would technically allow it. (Though I believe very high pressure can force it through)
Water’s polarity is what causes surface tension and droplets! For comparison, gasoline is relatively nonpolar, though not as nonpolar as He. If you’ve ever spilled some you can see it spread out immediately and avoid the droplet formation and cohesion you’d see with water. Similarly, water tight containers might not be gasoline tight. (but don’t test this out...)
To correct my comment from earlier: Helium is special because of its small kinetic diameter AND because it’s naturally found in a monoatomic and nonpolar form, as are all noble gases. This lack of polarity leads to minimal intermolecular attraction that would cause clumping and limit flow.
(I found a reference that helium can be measured flowing through a 1.0nm hole, albeit at a very low rate. I wonder how close to the 0.260nm kinetic diameter a hole can get for He to still flow)
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Sealing against Helium is vastly more difficult than sealing against air, as air is made out of molecules of rather large atoms, while Helium gas consists of single small atoms. That is why divers watches often have a "helium escape valve". The watches are sealed against high pressures of water or air. But when worn in pressure chambers where divers live on a mix of high pressure helium and oxygen, the helium gets into the watch. When depressurizing at the end of the pressure chamber time, the helium pressure could blow the watch crystal from the watch if not released by the escape valve. For a similar reason pilot watches have special fastening of the watch crystal, so that underpressure doesn't lift it off.
Those helium escape valves are just for marketing overpriced toys to pretentious wannabes. Most working saturation divers haven't used fragile, expensive mechanical wristwatches for decades now.
The seal works against air but not hydrogen or helium because the molecules are so small they'll diffuse through pretty much anything.
It's really hard to contain helium. It'll leak through pretty much any solid material.
is mylar special? a good mylar balloon can float for weeks
It's not special, just less porous compared to latex. Mylar balloons still lose gas after time.
It's the aluminium on the mylar.
>(unless the seal worked against air but not helium).
That is exactly the case. From the article, helium (and to a lesser degree, hydrogen) have very very small molecules and are able to slip past very small imperfections in hermetic seals.