Imagine a vertical tube with a fabric sample clamped at the base so that no water can escape – now add water to the tube. The height of the column of water in millimeters at the point where the water forces its way through the fabric/coating is called the hydrostatic head.
There is no internationally accepted standard for how high this should be for a ‘waterproof’ fabric.
In real-world terms, a dry bag intended for use inside a backpack will only require a modest hydrostatic head; a dry bag intended for paddling will need a much higher hydrostatic head.
Ultra-Sil tests at over 2000mm (more waterproof than some tent floors), which is perfectly adequate for waterproof storage inside a pack, but not sufficient for use in paddle situations. A dry sack intended for paddling will need a hydrostatic head of at least 10,000mm.
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