How is waterproofness measured?

A hydrostatic head test determines the waterproofness of a fabric coating. To be classified as waterproof a material must have a hydrostatic head rating of at least 1,500mm.In a hydrostatic head test a section of the fabric is pulled tight and sealed in a chamber. Water is then added (in mm) and the fabric is observed over a 24 hour period to see how much it withstand before droplets seep through. A jacket with a hydrostatic head rating of 2,000mm, for example, can withstand 2,000mm (32 feet) of rainfall in a single day before water will penetrate through.

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