Permeances

Permeances is a term used in several scientific fields to describe the ease with which something passes through a barrier or path, typically as a ratio of a flux to a driving force. In membrane science and chemical engineering, permeance measures how readily a gas or liquid passes through a barrier per unit driving pressure. It is defined as P = J/Δp, where J is the permeation rate per unit area and Δp is the pressure difference across the barrier. Common SI units are mol m−2 s−1 Pa−1 or m3(STP) m−2 s−1 Pa−1, and the gas permeation unit (GPU) is frequently used in industry. Permeance is inversely related to resistance to flow, and membranes with higher permeance permit faster transport for a given driving pressure.

In magnetism and electrical engineering, permeance is the reciprocal of reluctance and describes how readily a magnetic path conducts flux. It is defined by P = Φ/F, where Φ is magnetic flux and F is magnetomotive force (mmf). The unit is Weber per ampere-turn (Wb/At), and for simple geometries it approximates P ≈ μA/l, with μ the permeability, A the cross-sectional area, and l the path length. In circuit models, permeance is used to build analogies with electrical conductance, aiding the design of transformers and inductors.

The term emphasizes the relationship between a driving force and the resulting flow or flux, and its exact meaning depends on context. See also permeability and reluctance.