THE KNOWLEDGE

The main measure of a waterproof is pressure rating called hydrostatic head.

A piece of the jacket material is clamped across the bottom of a vertical tube, which is then slowly filled with water. Observers then monitor how high the column of water gets before the material lets drips through. A hydrostatic head of 3000mm means the column was 3 metres tall before any water came through. Interestingly, Gore-Tex don’t use the hydrostatic head test as they say it isn’t very realistic: instead they replicate a full range of outdoor conditions in their lab and unleash them on a completed jacket.