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Phil Cook · 23-01-2013, 08:37 AM posted to uk.rec.walking,uk.rec.gardening,uk.d-i-y

On 22/01/2013 21:12, wrote:
In article ,
Peter Clinch wrote:

Physics is not mocked. What I said is correct. You may well
believe in Maxwell's demon, but there's no such animal.

Once Goretex gets saturated, the surface tension no longer blocks
water from passing through, and it is no longer waterproof.

Gore have a ridiculous guarantee, "guaranteed to keep you dry". A
classic case of the large print giveth and the small print taketh away,
but in this case the small print points out the foundation to the claim
is a hydrostatic head well in excess of that considered to be
"waterproof" (MOD say 800mm, outdoor industry reckon 1m).

And that is hydrostatic head, which will be maintained whether or not it
is "saturated".

Why? What's the physics behind that? Its claims are nonsense (a
water molecule is NOT much larger than a steam molecule), and my
understanding is that the hydrostatic head is due to hydrophic
material.

Individual water molecules are the same as water vapour ones but they
are bonded together by relatively weak intermolecular bonds. The bonds
between the H and O atoms are not symmetric and so the H and O of
different molecules are attracted to each other. You can't get liquid
water molecules through the breathable membrane without putting energy
into it to separate the intermolecular bonds thus creating water vapour.

In real life you will not get water vapour to flow from a cool
environment outside your jacket to a warm one inside it.

The coating on the outer face of breathable fabrics is there to
attempt to prevent the face wetting out and creating a barrier to
water vapour molecules.

The reasons breathable fabrics fail to keep you dry are that they
either leak (at openings or places where the membrane has failed) or
that they are incapable of transmitting the amount of water vapour you
are sweating into them to the outside.
--
Phil Cook