Theory and Method
Hydrostatic resistance testing applies water pressure to one face of a clamped specimen while the opposite face rests on a perforated support plate that drains to a collection vessel. The pump raises pressure in steps and holds each step for a set interval. An intact barrier passes water only through slow molecular diffusion, so almost nothing accumulates downstream. Once the applied head exceeds what the material can withstand, water forces its way through pores, pinholes, or weak seams, and the collected volume climbs sharply. That pressure step becomes the reported hydrostatic resistance.
Permeability testing works below the breakthrough point and measures steady flow rather than failure. Darcy's law connects that flow to the permeability coefficient through k = QT / (AtΔH), where Q is the permeated volume, T the specimen thickness, A the exposed area, t the elapsed time, and ΔH the head difference across the specimen. Holding pressure and temperature constant over a long interval lets the flow rate settle, which is what makes the calculated coefficient repeatable between operators and laboratories.
The mechanical design follows from both mechanisms. Perforated plate holes of ø3 mm on 6 mm centers drain permeated water while supporting the specimen against 2.5 MPa, so the membrane bulges into small openings instead of failing across an unsupported span. The ø200 mm clamping diameter exposes more than 200 cm² of material, a large enough area for flaw density in the sample to represent the product rather than one favorable patch. Water temperature changes viscosity and therefore flow rate, so the instrument tracks it throughout the run.
