It is used in the measurement of gas permeance. Materials have a low permeance/porosity. The inverse of reluctance is called "permeance". Another commonly expressed unit is Gas Permeance Unit (GPU). In electromagnetism, permeance is the inverse of reluctance. Permeance can be expressed as the ratio of the permeability with the thickness of membrane. In materials science, permeance is the degree to which a material transmits another substance.