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HYDROSTATIC HEAD
a measurement of waterproofing
For any fabric to be considered fully
waterproof it must be able to withstand the
pressure of a column of water 1000mm high
without leaking. This is classed as a
hydrostatic head rating of 1000.
To measure the hydrostatic head of a fabric
a column of water is pressed against it. The
height of the column is increased until the
water penetrates the fabric.
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The term hydrostatic head is
used to denote the amount of
pressure of water that is
required in order to penetrate a
given fabric. |
Some products, such as tents and jackets are
by their very nature intended to protect the
user from the wind, rain, snow, sun and so
on. Different conditions would require
different levels of protection. A mild
climate with a few light showers would
require the hydrostatic head to be less than
if the climate was rougher with lashing
rain.
The British Ministry of Defence
definition for a waterproof fabric is that
it must resist a column of water at least
eight hundred millimetres high.
Pressure to the surface will increase the
need for a higher hydrostatic head. Examples
are a groundsheet in a tent is going to
expect people to walk on it Likewise, a
rucksack will increase pressure to the
surface of a jacket at that area.
Of course, this is compensated for in the
level of hydrostatic head. As mentioned
above, the MOD consider 800mm as waterproof.
Most tents start around 1500 which is
substantially higher.
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