permeance

P is the permeance of the space occupied by the flux.

In Materials science, permeance is the degree to which a material transmits another substance.

The inverse of reluctance is called permeance.

Perm (unit), a unit of permeance (gas or vapor transmission) in materials and membranes.

Properties of Magnetic Materials (units of magnetic permeance)

The base normal SI unit for permeance is the kilogram per second per square meter per pascal.

Magnetic permeance is defined as the reciprocal of magnetic reluctance (in analogy with the reciprocity between electric conductance and resistance):

A perm is a unit of permeance or "water vapor transmission" given a certain differential in partial pressures on either side of a material or membrane.

The SI unit of magnetic permeance is "webers per ampere-turn", that is Wb A.

A magnetic circuit almost acts as though the flux is 'conducted', therefore permeance is larger for large cross sections of a material and smaller for longer lengths.

American building codes have classified vapor retarders as having a water vapor permeance of 1 perm or less when tested in accordance with the ASTM E96 desiccant, or dry cup method.

Air barrier material - Building material that is designed and constructed to provide the principal plane of airtightness through the environmental separator and which has an air permeance rate no greater than 0.02 L/(s-m2) at a pressure difference of 75 Pa when tested in accordance with ASTM E 2178.