ISO 811
ISO 811 is the international standard test for hydrostatic head, the measure of how much water pressure a fabric resists before water penetrates.
It specifies the rate at which water pressure increases and the water temperature, which is why results are only comparable between fabrics tested to the same standard.
What the test does
A fabric specimen is clamped and water pressure is applied to one face at a controlled rate, typically 60 cmH2O per minute, until water appears on the other side. The pressure at that point is recorded as a water column height in millimetres.
Why the rate matters
A faster ramp gives the fabric less time to fail and therefore produces a higher number. AATCC 127, the common American equivalent, uses different conditions, so an ISO 811 figure and an AATCC 127 figure from the same fabric are not interchangeable. A number quoted without its standard is not comparable to anything.
Censored results
Tests are often run only to a threshold. If all specimens survive it, the report reads "exceeds" that figure rather than giving a value. That is a floor, not a measurement, and it should not be ranked against another brand's precisely measured number. VEAR's own result is of this kind, as set out on our lab tests page.
Related terms
- Hydrostatic head: the property this standard measures
- Waterproof and breathability ratings explained: why cross-brand comparison fails
- Waterproof vs water resistant: what the number has to be paired with
- Glossary: all terms