Conference Contribution Details


RE Imhof, MEP de Jesus, P Xiao & the TEWL Calibration Consortium.
Contributed Talk, US-Regional ISBS Meeting, Orlando, October 2004.



The ASTM-96 Standard has been widely used as a basis for calibrating and comparing TEWL instruments. However, the standard was developed for measuring water vapour transmission through permeable materials and its suitability for TEWL calibration has not been verified. For this reason we have developed a mathematical model of this calibration for open chamber measurement heads, for which it has been widely used. It uses Fick’s law to calculate a calibration error as the percentage deviation between the flux density indicated by a TEWL measurement head in contact with the membrane and the gravimetrically determined flux density.

The main conclusion is that significant calibration errors are inherent in the method, mainly because the flux density passing through a measurement head in contact with the membrane is not equal to the gravimetrically determined flux density. Another difficulty is that diffusion resistances that can be ignored when testing low permeance membranes have a significant effect at the higher flux densities (>5 g/[sq.m h]) needed for TEWL calibration.


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