January 20, 2010

Terms of confusion, Take 2

by Bob Murphy

Chemical resistance is one the most important qualities in protective linings in relation to protecting the infrastructure. Permeability is often overlooked aas a criteria for chemical resistance of protective linings

“What’s the difference between transmission, permeance, and permeability?! I thought there were just ‘perms’.” Don’t worry – I’ll get to all of that later, but there’s some background work to be covered first.

Moisture vapor transmission is a measure of the passage of water vapor through a substance.

Permeability is a measure of the ability of a porous material to transmit fluids.

By the end of this series of posts you’ll have a better understanding of the test methods used, why and when they are used, the equations used to report water vapor transmission, permeance, and permeability and how they’re all different, possible sources of error and variability in the tests, and intentional or unintentional ways to alter the way in which the data is presented.

There are two primary ASTM test methods used to calculate the passage of water vapor through materials: ASTM D1653, Water Vapor Transmission of Organic Coating Films and ASTM E96, Water Vapor Transmission of Materials. Often times, you’ll see them referred as D1653-93 or E96-92. In ASTM convention, the number following the test method number is the revision year. Therefore D1653-93 was revised in 1993.

The difference between the two test methods lies in what materials are typically “covered” by each method. D1653 states that “these test methods cover the determination of the rate at which water vapor passes through films of paint, varnish, lacquer, and other organic coatings. The films may be free films or they may be applied to porous substrates”. E96 states “these test methods cover the determination of water vapor transmission (WVT) of materials through which the passage of water vapor may be of importance, such as paper, plastic films, other sheet materials, fiberboards, gypsum and plaster products, wood products, and plastics.

There are four things you should always ask yourself when evaluating product data or even comparing one set of data to another:

  1. What test method was used? ASTM D1653? E96? Some other method? Dry cup or wet cup?
  2. Are the terms the same? Am I comparing WVT to WVT or WVT to WVP?
  3. Are the units the same? Is it WVP in English units or metric units?
  4. What were the test conditions?
    • For WVT, the temperature, humidity and dry film thickness are needed to replicate the results
    • For WVP, the dry film thickness is needed

Also, a little knowledge of the calculations used to generate the data can be extremely useful.

Next: The calculations and their meaning

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