Darcy (Unit)

The darcy (D) is a non‑SI unit of permeability describing how easily a fluid flows through a porous medium; 1 D ≈ 9.869×10⁻¹³ m². In practice, permeability is often given in millidarcies (mD) or in SI units (m²). Permeability is a property of the medium alone and, via Darcy’s law, relates volumetric flux to pressure gradient and fluid viscosity under laminar flow. For woven meshes and cakes, reported permeabilities depend on porosity and tortuosity and can change with compression and fouling, so test conditions (fluid, temperature) must be stated. When converting between D and m², apply the constant precisely; mixing up porosity‑dependent ‘hydraulic conductivity’ (which folds in fluid properties and is common in hydrogeology) with permeability causes specification errors. In procurement and QA, quote permeability together with air/water permeability tests and mean‑flow pore size to provide a consistent hydraulic picture.