What does hydrostatic head mean?

The colder and wetter seasons are here again. Wind, rain, cold, snow … some people would call it “bad” weather But if you, like us, love the outdoors, you won’t easily be kept indoors, right?. And there’s really no need; how “bad” you experience will be depends in large part on how good your clothes are. Or to quote the original insight by Alfred Wainright’s. “There is no such thing as bad weather, only unsuitable clothing.”

The weather type that will most spoil your pleasure, if you are not suited up well, is rain. Wet equals cold equals misery. So suitable clothing must, first and foremost, be watertight.

When you do go looking for new winter sports kit, or a good raincoat, you will soon come across the term “hydrostatic head”. But what does that mean?

Water repellent vs waterproof

First of all, there is a difference between waterproof and water repellent. Water repellency can handle a few drops of water or even a mild rainfall. But with a longer time spent in the rain, or heavier rain or snow, you will quickly get wet

Waterproofness is the extent to which a product (for example a jacket) really keeps the water away from your body, or conversely the extent to which the material let’s through water.

To express how waterproof a fabric is an often used standard is “hydrostatic head”. This measurement unit is displayed on many (technical) jackets and pants, with indications of 5,000 to 30,000.

What does “hydrostatic head” mean?

Measuring water tightness with alternative test set-up.

The indication of water column is derived from a test setup where a tube of 2.5 cm diameter is placed on a piece of test material. Water is poured into the tube and measured to what extent this water can rise in the tube before the pressure on the material becomes so large that it leaks through the material. 20,000 mm, for example, means that this water column should be 20m high for the pressure to be too big and for the water to leak through the fabric. Because 28-meter tubes are not always handy, there are variants in this test setup where pumps increase water pressure on dust to the equivalent of the pressure that comes from the water column.

Because 28-meter tubes are not always handy, there are variants in this test setup where for example, pumps increase the water pressure on the test fabric to the equivalent of the pressure in a water column set-up.[/vc_column_text]

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Watercolumn value’s

So what doe the various value’s actually stand for.

  • 0 – 1.000 mm: The substance is water repellent, not yet waterproof.
  • 1.000 – 5.000 mm: The fabric is waterproof but it can handle little or no pressure. If you sit down and have a seat, lean up to a something, you will get wet.
  • 5.000 – 15.000 mm: The fabric is waterproof and stays that way for the most forms of pressure and during regular intensity weather.
  • 15.000 – 30.000 mm: The fabric is completely waterproof. The fabric can handle heavy rain over a prolonged period of time, wet snow is no problem, even if you’re going to go underwater, water would not go straight through immediately.
  • 30.000 mm and higher: In effect, we are talking about completely solid materials that will never allow water to pass.

More about waterproofness and hard shells in GearGuide Hardshells.

Editors Squad
Editors Squad
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