Fluid permeance Permeance is closely related to permeability, but it refers to the extent of penetration of a specific object with given thickness by a liquid or a gas. It is the degee to which a materal or membrane transmits another substance. Units of permeance are volumetric output per unit membrane area per unit trans-membrane pressure. Permeance is also referred to as pressure-normalized flux. Fluid Mechanics Wikipedia ODM2 Units Type Controlled Vocabulary ODM2 Working Group A vocabulary for describing the type of the Unit or the more general quantity that the Unit represents.