permeability

/perr'mee euh bil"i tee/, n.

1. the property or state of being permeable.

2. Also called magnetic permeability. Elect. a measure of the change in magnetic induction produced when a magnetic material replaces air, expressed as a coefficient or a set of coefficients that multiply the components of magnetic intensity to give the components of magnetic induction.

3. Geol. the capability of a porous rock or sediment to permit the flow of fluids through its pore spaces.

4. Aeron. the rate at which gas is lost through the envelope of an aerostat, usually expressed as the number of liters thus diffused in one day through a square meter.

5. Naut. the capacity of a space in a vessel to absorb water, measured with reference to its temporary or permanent contents and expressed as a percentage of the total volume of the space.

[1750-60; PERMEA(BLE) + -BILITY]

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▪ physics

      capacity of a porous material for transmitting a fluid; it is expressed as the velocity with which a fluid of specified viscosity, under the influence of a given pressure, passes through a sample having a certain cross section and thickness. Permeability is largely dependent on the size and shape of the pores in the substance and, in granular materials such as sedimentary rocks, by the size, shape, and packing arrangement of the grains.

      The standard unit of permeability is the darcy, equivalent to the passage of one cubic centimetre of fluid (having a viscosity of one centipoise) per second through a sample one square centimetre in cross-sectional area under a pressure of one atmosphere per centimetre of thickness.

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