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Permeation of water vapor through seals: vacuum vs dry gas

  1. Mar 3, 2016 #1
    From reading about the permeation of gasses through polymer seals, I am led to believe that given two identical sealed chambers, one containing a vacuum and the other pressurized perfectly dry nitrogen, water vapor would permeate through the seals and into the chambers at the same rate. This would occur because permeation rates depend only on the partial pressure of the gas in question.

    Is my understanding of the phenomena correct? Intuitively, I would have thought that the positive pressure of the second chamber would lead to a slower ingress of water vapor than the negative pressure of a vacuum.
  2. jcsd
  3. Mar 8, 2016 #2


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    I am not read up on the subject but your intuition presupposes that the water molecules colliding with the gas molecules is a major factor. Within the seals the water molecules are principally scattering through the polymer matrix. The gas vs vacuum question only comes into play at the inner surface of the seal and there I imagine the water is very much less likely to be scattered back into the polymer rather than out into the chamber. Indeed water vapor inside will not affect this either except that equilibrium will arise where as much diffuses out as in.

    Disclaimer: I am guessing here based on my understanding of the processes and may be totally wrong.
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