Note Water Vapor Transmission

Note that vapor resistance represents the resistance of a material to water vapor transmission. 

Figure 4. Water Permeability for Polypropylene and Polycarbonate. Polypropylene measurements were made on 9.5 mil extruded sheet polycarbonate measurements were made on 10 and 20 mil extruded sheet. Note that permeability has water pressure in the denominator which is distinctly different from a WVTR (water vapor transmission rate).

An interesting note in the development of silicone sealants is that their high rate of water vapor transmission has been exploited to make a vapor permeable (breathable) coating for roofs and other surfaces. This sealant is applied in the form of a solvent dispersion to attain the necessary consistency for easy application. 

Moreover, transport phenomena for the sweat case are much more complicated than the water vapor case because wetting of the surface by liquid water precedes water wetting of the surface by liquid water precedes water vapor transmission. Note that there is an important difference in water absorbing eharacterizing of wool and cotton, although both fibers have relatively high water vapor absorption rates. Because of the hydrophobic surface of wool fibers, a liquid droplet in contact with a wool fabric does not spread out laterally within a fabric layer. The water vapor transport rate, in the sweat ease, ean be indicated by the size of liquid water spread out on the surfaee or within a fabric matrix. 

NOTE Opacity readings (covering power or reduction in light transmission) may also be high where water vapor in the stack gases condenses in cold weather. 

Of alcohols, methanol has been the most popular inhibitor, due to its cost and its effectiveness. Katz et al. (1959, p. 218) indicated that the inhibition ability of alcohols increases with volatility, that is, methanol > ethanol > isopropanol. Typically methanol is vaporized into the gas stream of a transmission line, then dissolves in any free water accumulation(s) where hydrate formation is prevented. Makogon (1981, p. 133) noted that in 1972 the Soviet gas industry used 0.3 kg of methanol for every 1000 m3 of gas extracted. Stange et al. (1989) indicated that North Sea methanol usage may surpass the ratio given by Makogon by an order... 

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