to ascertain the garment’s ability to keep out water.
This is measured in PSI (pounds per square inch) or
Hydrostatic Head. Hydrostatic Head is measured in mm
and pressure is measured in pounds per square inch.
The Hydrostatic Head is determined by a test
whereby a fabric is held taut underneath a sealed
tube of water 1 inch in diameter. Over 24 hours it is
observed to see how many millimetres of water the
fabric can withstand before it leaks through. When the
fabric begins to seep water, the “height” in millimetres
is noted. This is known as the fabric’s ‘hydrostatic
head’ which literally means: Hydro - Water, Static - Not
moving, Head - Height.
The PSI relates to the amount of water pressure
that a garment can withstand both from the pressure
within the garment (the wearer) and the external
conditions (the weather).
To be 100% waterproof, the British Standards of
3 PSI has to be met, which means that a fabric can
withhold 1500mm of water pressure. Pressure is not
just as simple as the pressure of a rain storm, simply
moving a garment when it is worn will exert pressure
on the fabric.
Let’s put this in perspective though; if you were
to sit on a wet, flat surface the mean pressure you
generate is around 10,000mm! If you were kneeling in
a firing position then the point of the knee generates
15-17,000mm of pressure (smaller surface area,
greater pressure) so let’s face it, a minimum legal
rating of 1500mm just does not do the job. It does
begin to explain though how a garment can be legally
classed as “waterproof”, but in reality it really isn’t.
Back to GORE-TEX; what’s the hydrostatic head I
hear you ask? Simple one this, as all GORE-TEX products
have a MINIMUM testing level of 28,000mm… that’s
right, 28 metres! Put another way I’ve seen top end
GORE-TEX fabrics tested at over 70 PSI and not leak
until they quite literally explode! Belt and braces time
here folks.
balancing act. Behind it all is a law of nature, the Law
of Partial Pressures. This law states that moisture as a
vapour will naturally move from a warm, high pressure
environment to a cooler low pressure one. If we
are keeping a well balanced temperature inside our
clothing system then on a foul weather day moisture
will move away from our skin through the pores in
our clothing and escape to the outside. Sometimes we
call this the “Temperature Gradient” as it sounds like
moisture vapour is moving “down” from hot to cold.
So does a breathable fabric technology work in a rain
forest? Yes is the answer, but due to the temperature
inside the garment and outside the garment being
more in equilibrium the transfer rate will obviously
slow down. We all know we can work with Ma Nature,
but never beat her!
This of course is also subject to testing; many
methods can be used here to establish a Moisture
Vapour Transmission Rate (MVTR) or a Resistance to
Evaporative Transmission (RET) figure. Of the two I
prefer an RET figure as it just plain makes more sense
to me.
Skin is 100% breathable and has an RET of Zero. As
soon as you cover it though you build up a resistance
that slows down the rate at which moisture can be
transmitted away; the thicker the barrier the more
resistance and the slower the transmission. In essence
this means that as you cover the skin with layers your
system becomes less breathable so we will always be
looking for technical fabrics with as low an RET figure
as possible.
In some cases the clever folks at Gore have got the
RET figure well down into single figures, especially on
their bespoke technologies and the GORE-TEX 3 Layer
Pro garments. They’ve developed a completely new
and innovative lining fabric here called a Micro Grid
Backer which is thinner, but more durable, than the
industry standard linings available elsewhere. Thinner,
less resistance, right?
Getting the Balance Right!
Keeping you Comfortable
Comfort is a relative thing, but if you’re too hot, that
can lead to heat exhaustion; too cold, hypothermia.
Controlling the micro-climate inside your clothing
system helps to prevent either. The body regulates
itself effectively; as activity levels increase you sweat
and the evaporation of this moisture helps to cool the
skin. If moisture is not allowed to move away you will
become “uncomfortable” inside your clothing system.
We’ll get properly into the layering system in a future
article.
Letting moisture escape from the system is a tricky
The third and final part of the precarious balancing
act is putting this membrane technology together
in a garment with a lining fabric and an outer face
fabric that will ensure long term durability whilst not
degrading performance. These fabrics are chosen with
extreme care and tested for all kinds of heat, cold, flex
and abrasion. The tests are carried out both by Gore
themselves and also independently.
So, with our GORE-TEX fabrics in place we have
exceptionally durable garments which have at least a
28+ metre hydrostatic head and which can sit right
at the very top of the of the “Extremely Breathable”
category. They are put together under minute scrutiny
by Gores many Brand partners from the very latest
and finest face and lining fabrics.
The fact of the matter is that many people find
GORE-TEX garments expensive but at the risk of being
blunt they must have no real conception of what is
going on behind the scenes! When you’re paying for a
GORE-TEX garment you’re getting “Formula 1” levels of
performance and that doesn’t come cheap. Of course,
you can always buy a cheap “hatchback” but if your
life may end up depending on the performance of
your clothing system we have to ask the question
“how much is too expensive”?
pmcimagazine.com