Water transmission rates, definition

Two different definitions of water vapor permeability are used in the BS and ASTM standards, and this can cause confusion. BS 3177 and BS 2782 define the water vapor transmission rate as the mass transfer rate of water vapor per unit area (g m 24h) (see Section 1). but they call it "permeability," When the material is homogeneous and the transfer rate is inversely proportional to thickness. BS 3177 defines an "equivalent permeability" as the "permeability" multiplied by the thickness of the test piece in thousandths of an inch (g mil m 24h). 

The ASTM standards adopt definitions that are consistent w ith the equivalent definitions for gas transmission. Water vapor transmission rate is the mass transfer rate of water vapor per unit area (g nr 24h). Permeance is the ratio of the water vapor transmission rate to the difference in vapor pressure between the surfaces of the test piece measured in mm of mercury this unit is known as the metric perm (g nr 24h mmHg). This is equivalent to the gas transmission rate. Permeability is the product of the permeance and the thickness of the test piece, assuming that the permeance is inversely proportional to thickness for homogeneous materials this unit is known as the perm-centimetre (g cm nr 24h mmHg). Since the adoption of SI units, the water vapor permeability may also be expressed in the units of microgram meter per newton hour (pgm N h or pgm m Pa h ). 

The dynamical proposal can be expressed in terms of several alternative definitions. Yet, in order to exhibit dynamical contribution to catalysis, any definition must find a different magnitude of dynamical contributions to the rate constant in the enzyme and in water. All the reasonable definitions result with very small dynamical contributions.40 Rigorously speaking, it is agreed in the chemical physics community (see references in Ref. 4) that all the dynamical effects are contained in the transmission factor, that corrects the absolute rate theory for re-crossing of the reactive trajectories (see Ref. 40 for a clear definition). All reported simulation studies (e.g., Refs. 4,103) found that the transmission factors are similar in enzymes and in solution, and do not differ much from unity in the enzyme (e.g., Refs. 4,125). 

Permeability is defined as the product of the diffusion constant and the solubility coefficient. Water vapor permeability (WVP] is defined as the time rate of water vapor transmission through unit area of flat material with a definite unit thickness induced by unit vapor pressure difference between two specific surfaces, and under indicated temperature and humidity conditions. (ASTM E 96, Standard Test Method for Water Vapor Transmission of Materials]. 

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