Theory and Method
Water moving through a geotextile perpendicular to its plane follows Darcy's law while flow stays laminar, so discharge rises in proportion to the head across the fabric. The QualiGTX™ GPT D clamps the specimen in a Ø50 mm aperture, holds a measured head difference, and records the volume passing through against elapsed time. Reading that discharge at a known head is the whole measurement. Everything else in the instrument keeps the head, the temperature, and the water condition from drifting while it happens.
Permittivity is what the method reports. Dividing the flow rate by the specimen area and again by the hydraulic head yields a value in reciprocal seconds, which describes the fabric independently of its thickness. That independence is the point: nominal thickness varies with packaging, shipping, and handling, so a permeability coefficient tied to thickness compares two fabrics poorly. Multiplying permittivity by nominal thickness recovers a permeability coefficient when a design calculation calls for one.
Water viscosity falls as temperature rises, so an uncorrected result reflects bath temperature as much as the fabric. The method corrects to a 20 °C reference, and that correction needs a temperature reading the operator can trust. The QualiGTX™ GPT E heats or cools between 5 °C and 60 °C and resolves temperature to 0.1 °C. A dissolved oxygen device provides the second check, since gas coming out of solution forms bubbles that block pores and depress the measured flow.
