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Water Vapor Permeability

Water Vapor Permeability

Diffusive transfer of water vapor through porous materials is often described by a modified form of Fick’s law:

Equation 1
w''v = -µ · dp/dx

Where:

w''v = mass of vapor diffusing through unit area per unit time, gr/h·ft2
dp/dx = vapor pressure gradient, in. Hg/in.
µ = vapor permeability, gr·in/h·ft2·in. Hg

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In engineering practice, permeance may be used instead of permeability. Permeance is simply permeability divided by the material thickness in the direction of vapor flow; thus, permeability is a material property, whereas permeance depends on thickness.

Permeability is measured with wet-cup, dry-cup, or modified cup tests. Specific test methods for measuring water vapor permeability are given in ASTM Standard E96.

For many engineering materials, vapor permeability is a strong function of mean relative humidity. Wet and dry cups cannot adequately characterize this dependence on relative humidity. Instead, a modified cup method can be used, in which pure water or desiccant in a cup is replaced with a saturated salt solution (Burch et al. 1992; McLean et al. 1990). A second saturated salt solution is used to condition the environment outside the cup. Relative humidities on both sides of the sample material can be varied from 0 to 100%. Several cups with a range of mean relative humidities are used to map out the dependence of vapor permeability on relative humidity.

In measuring materials of high permeability, the finite rate of vapor diffusion through air in the cup may become a factor. Air-film resistance could then be a significant fraction of the sample’s resistance to vapor flow. Accurate measurement of high-permeability materials may require an accounting of diffusive rates across all air gaps (Fanney et al. 1991).

Related:

References:

• ASTM Standard E96
• Fanney et al. 1991
• Burch et al. 1992
• McLean et al. 1990