Helium and Permeability

Name: Jure
Status: student
Grade: 9-12
Location: N/A

Question: I have searched your archives for a similar question, but
I did not find it. I have a question concerning helium gas. As it
goes helium permeates through certain solids due to its size. So a
balloon will slowly deflate, lose all of its helium content. So one
way of trying to stop the leakage of helium is using low helium
permeability materials as Mylar and composites. If hypothetically we
have a balloon (a membrane that is very good at preventing air from
leaking in or out) that is filled with helium at a slightly lower
pressure (contrary to balloons normally) than the surrounding air,
does that stop helium from leaking through the membrane as it would
have to actively move from a place of lower pressure to the outside
with higher pressure? Is there something I am missing or is this
true?
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Jure,

You are correct in stating that the diffusion of helium is a function of its 
size and the permeability of the membrane. However, effusion (as opposed to 
diffusion) is not a function of the overall external pressure but rather the 
partial pressure of the particular gas in the outside.

Think of this hypothetical experiment: have two compartments separated by a 
removable wall. On one side (A) we have helium at a very low pressure (say 0.1atm) 
and on the other side (B), we have air (a mixture of all sorts of gases, but with 
very little helium in it) that is at a higher pressure (say 1atm) but since 
constitutes less than 0.01% of air, then the pressure due to helium must be less 
than 0.01atm. So now, we remove the partitioning wall. Although there will be a 
net transfer of gases from B to A, the net effect for the helium alone would be 
that the helium will move from A to B even though B is at an overall higher 
pressure.

So in your balloon hypothetical, there will still be a net transfer of helium from 
the balloon to the outside - because the partial pressure of helium in the 
atmosphere is very low.

Greg (Roberto Gregorius)
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Jure,
 
You are basically correct.  Some helium will work its way out of the balloon 
through the Mylar sheet, a very small number to be sure.  But over time some 
helium will escape.  Probably the largest leaks will occur at the knot that 
closes the balloon.  

Warren Young
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