Hydrostatic Head: Measuring Water Resistance Beyond Spray
Hydrostatic head quantifies how much water pressure a fabric resists, separating genuinely waterproof products from merely repellent ones.
This guide explains the underlying principle, the numbers that matter and the checks that keep results consistent.
Test principle
Water pressure is increased under a fabric sample until water penetrates, and the pressure at penetration is recorded.
It also reduces the risk of a mid-season change, which is where both cost and calendar damage usually originate.
Rating interpretation
Repellent fabrics sit low, while waterproof laminates reach several thousand millimetres of water column.
Document what happens when the result falls outside the range, including sorting, reworking and claim handling.
- Request test reports on production fabric, not on development samples
- Verify the sampling plan and the number of specimens
- Store reports with roll or lot identification for traceability
Seam and edge effects
Results on seams, zips and bonded joins differ from the flat fabric, so test the finished construction.
Include the parameter in the pre-production meeting so the whole team understands what will be checked.
Changes after use
Flexing, abrasion and repeated washing reduce performance, so retest after simulated use.
Compare results before and after the intended care regime, since performance often changes after the first wash.
- Agree tolerances in writing before the first sample is made
- Define the measurement condition and the direction measured
- Review the limit each season against real performance data
Standard methods
ISO 811 and AATCC 127 are the common methods, and the pressure rise rate should always be reported.
Agree how changes will be priced before the order starts, which avoids difficult conversations mid-production.
Summary
The technical content is straightforward. The discipline lies in writing it down, testing it on production fabric and repeating the result on the next order.
