March 10, 2010

Summing up the “perm” Series

by Bob Murphy

Final Odds ‘n’ Ends

There are four things you should always ask yourself when evaluating competitors’ 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. Another useful bit of information is the conversion factor from one term to another:

WVT:
Covert from Metric* to English units: Multiply by 1.434
Convert from English to Metric* units: Multiply by 0.697
*Assuming the metric units are per hour, not per day

WVP:
Covert from Metric* to English units: Multiply by 36.4
Convert from English to Metric* units: Multiply by 0.027
*Assuming the metric units are per hour, not per day

Permeability:
Covert from Metric* to English units: Multiply by 75.0
Convert from English to Metric* units: Multiply by 0.013
*Assuming the metric units are per hour, not per day

If you want to “convert” from WVP to WVT or Permeability to WVT, you’ll have to make some assumptions regarding the test conditions and/or dry film thickness or you’ll have to know these variables.

In order to convert from WVT to WVP or vice versa, you need to know the test temperature and relative humidity.

At 25ÂșC and 50% RH:
Metric Units: WVP is approximately 1/12th WVT
English Units: WVP is approximately 2 times WVT
At 25ÂșC and 50% RH with a 6 mil dry film:
Metric Units: Permeability is approximately 1/3rd WVP and 1/36th WVT
English Units: Permeability is WVP times the dry film thickness and approximately 2 times the dry film thickness larger than WVT

Water vapor transmission is a fairly common product specification and a fairly simple, but time consuming, test to run. However, the number of ways to report the data as well as the allowable variability in the test methods makes interpreting data from an outside source (competitors’ product specification, outside lab, etc) difficult, at best. Keeping this in mind, some knowledge of how the test is run, how the calculations are performed, and what data is presented goes a long way in helping to demystify this subject.

Many thanks to Jason Godlaski

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