Theory and Method
Water movement through a geotextile follows Darcy's law, which relates flow velocity to hydraulic gradient through a coefficient describing the material. Flow velocity is discharge divided by the clamped test area, and the ø50 mm bore gives a defined area of roughly 19.6 cm² through which every measured litre must pass. Hydraulic gradient is the head differential divided by the fabric thickness. Rearranging gives the permeability coefficient directly from measured volume, elapsed time, test area, head, and thickness.
Thickness is where cross-plane testing of fabrics gets awkward, and it explains why permittivity exists as a separate parameter. A nonwoven geotextile compresses under load, so its thickness is not a single number but a value tied to whatever pressure it sits under. Permittivity sidesteps the problem by expressing flow per unit area per unit head, which drops thickness out of the calculation entirely. The same test run on this apparatus yields both quantities, and laboratories report whichever their specification calls for.
Two conditions have to be controlled for either number to mean anything. Water viscosity falls as temperature rises, so a sample measured on a warm afternoon passes more water than the identical fabric measured on a cold morning, which is why the thermometer feeds a correction back to a reference temperature. Dissolved air is the second issue: as water warms inside the apparatus, air comes out of solution and lodges in the pore structure, progressively choking flow and drifting the reading downward through the test. Deaired water prevents that, and a constant head keeps the driving pressure steady so the discharge rate reaches a stable value before the operator starts timing.
