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Peter Clinch · 23-01-2013, 08:26 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:

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

Why? What's the physics behind that?

Bad phrasing on my part. What I'm getting at is that in order to /be/
staurated you need over a 4m water column providing pressure, and that
isn't going to happen to anyone using it in a normal application. In
other words, the hydrostatic head will be maintained precisely because
the pore structure cannot get saturated until you exceed it.
The pore structure won't be saturated however much the face is.

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.

It's surface tension, but you can't break the surface tension until you
exceed the hydrostatic head, which you won't do because it's too big.
So it won't leak until you've got a lot more pressure forcing the water
through than will happen in practical application.

Not really. Holding up a few metres of water on one side is NOT
saturation, and I am not disputing their claims. But once it gets
saturated, then there is liquid water both sides, and the surface
tension effect gives way to percolation.

We are in furious agreement. But /how/ it's going to get saturated in
normal use, given the amount of pressure you need to force water in to
the pore structure is what "oh it will leak, albeit slowly" doesn't address

I have measured this effect for several fabrics, including Goretex,
and it occurs for that as much as anything else. However, I never
managed to get more than a certain amount of dampness through the
Goretex, so all I could be sure of was that the percolation rate
was non-zero (but negligible). What I can't be sure is how much
would get through with the pumping caused by footwear and clothing
movement.

Most examples of "leaking" are condensation on the other side, which is
remarakably difficult to get rid of.

If it's easy to measure percolation through goretex then it would fail
the hydrostatic head tests that Gore use as the basis of their guarantee.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net http://www.dundee.ac.uk/~pjclinch/