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Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure the "accessible void", the total amount of void space accessible from the surface (cf. closed-cell foam).

There are many ways to test porosity in a substance or part, such as industrial CT scanning.

The term porosity is used in multiple fields including pharmaceutics, ceramics, metallurgy, materials, manufacturing, hydrology, earth sciences, soil mechanics and engineering.

Optical method of measuring porosity: thin section under gypsum plate shows porosity as purple color, contrasted with carbonate grains of other colors. Pleistocene eolianite from San Salvador Island, Bahamas. Scale bar 500 μm.

Several methods can be employed to measure porosity:

• Direct methods (determining the bulk volume of the porous sample, and then determining the volume of the skeletal material with no pores (pore volume = total volume − material volume).
• Optical methods (e.g., determining the area of the material versus the area of the pores visible under the microscope). The "areal" and "volumetric" porosities are equal for porous media with random structure.[5]
• Computed tomography method (using industrial CT scanning to create a 3D rendering of external and internal geometry, including voids. Then implementing a defect analysis utilizing computer software)
• Imbibition methods,[5] i.e., immersion of the porous sample, under vacuum, in a fluid that preferentially wets the pores.
• Water saturation method (pore volume = total volume of water − volume of water left after soaking).
• Water evaporation method (pore volume = (weight of saturated sample − weight of dried sample)/density of water)
• Mercury intrusion porosimetry (several non-mercury intrusion techniques have been developed due to toxicological concerns, and the fact that mercury tends to form amalgams with several metals and alloys).
• Gas expansion method.[4]

Several methods can be employed to measure porosity:

• Direct methods (determining the bulk volume of the porous sample, and then determining the volume of the skeletal material with no pores (pore volume = total volume − material volume).
• Optical methods (e.g., determining the area of the material versus the area of the pores visible under the microscope). The "areal" and "volumetric" porosities are equal for porous media with random structure.[5]
• Computed tomography method (using industrial CT scanning to create a 3D rendering of external and internal geometry, including voids. Then implementing a defect analysis utilizing computer software)
• Imbibition methods,[5] i.e., immersion of the porous sample, under vacuum, in a fluid that preferentially wets the pores.
• Water saturation method (pore volume = total volume of water − volume of water left after soaking).
• Water evaporation method (pore volume = (weight of saturated sample − weight of dried sample)/density of water)
• Mercury intrusion porosimetry (several non-mercury intrusion techniques have been developed due to toxicological concerns, and the fact that mercury tends to form amalgams with several metals and alloys).
• Gas exp

where

VV is the effective volume of the pores,
VT is the bulk volume of the sample,
Va is the volume of the container containing the sample,
Vb is the volume of the evacuated container,
P1 is the initial pressure in the initial pressure in volume Va and VV, and
P2 is final pressure present in the entire system.
The porosity follows straightforwardly by its proper definition
${\displaystyle \phi ={\frac {V_{V}}{V_{T}}}}$.
Note that this method assumes that gas communicates between the pores and the surrounding volume. In practice, this means that the pores must not be closed cavities.
• Thermoporosimetry and cryoporometry. A small crystal of a liquid melts at a lower temperature than the bulk liquid, as given by the Gibbs-Thomson equation. Thus if a liquid is imbibed into a porous material, and frozen, the melting temperature will provide information on the pore-size distribution. The detection of the melting can be done by sensing the transient heat flows during phase-changes using differential scanning calorimetry – (DSC thermoporometry),[6] measuring the quantity of mobile liquid using nuclear magnetic resonance – (NMR cryoporometry)[7] or measuring the amplitude of neutron scattering from the imbibed crystalline or liquid phases – (ND cryoporometry).[8]