Fabric Air Permeability Testing
topic
Air permeability testing (ISO 9237, ASTM D737, TEXTEST FX3300, SDL Atlas M021S) measures the volumetric flow rate of air passing perpendicularly through a known fabric area at a defined pressure differential — characterising the porosity and flow resistance of fabric structures for filtration, breathability, parachute, and sail applications. ISO 9237 procedure: circular test head 20 cm² area, fabric specimen clamped over test aperture, differential pressure maintained at 100 Pa (ISO 9237) or other specified pressure using variable flow valve, air flow measured by rotameter (analogue) or mass flow meter (digital, ±1% accuracy, range 0.1–200 L/m²/s). Result reported as L/m²/s at 100 Pa pressure differential — multiply by 3.6 for m³/m²/h, divide by 3.28 for cfm/ft². Multi-pressure testing (20, 50, 100, 200 Pa) provides permeability-pressure relationship for non-Darcy flow regime analysis in filter fabrics. Air permeability variation along roll: coefficient of variation CV% < 8% for uniform fabric; CV% > 15% indicates finishing tension variation (stentering width variation causes porosity changes). Construction effects on air permeability: plain weave 40 × 38 per cm cotton Ne 40: 150 L/m²/s; same fabric calendered (2 passes, 80°C, 150 kN/m): 60 L/m²/s (60% reduction from fibre compaction); coated with 30 g/m² PU: <1 L/m²/s (>99% reduction). Parachute fabric porosity (ASTM D737 standard: 1/2 inch H₂O = 125 Pa): F-111 low-porosity canopy specification 3 cfm/ft² maximum = 15 L/m²/s at 125 Pa — exceeding this specification causes excessive air bleed through canopy reducing drag coefficient and increasing descent rate above safe limits for landing impact.
Role
Air permeability testing is the functional performance test for all porous textile applications including filtration, breathable clothing, parachute canopies, and geotextile separation — with permeability specification defining whether a parachute slows descent to safe landing speed, whether a geotextile drains at the design hydraulic gradient, and whether a breathable garment transmits perspiration vapour at the rate required for thermal comfort during exercise.