A patented metal capsule small enough to fit inside a commercial TGA was fashioned with an opening that was more than 200 times smaller in area than used in the standard ASTM test method for water vapor transmission of materials. Unfortunately, in the latter method the weight determinations are not made in the controlled environment and the size of the opening is recommended to be at least 3000 mm~2 (4.65 in~2) or greater. In an attempt to gain water vapor transmission rates in-situ and rapidly as a function of temperature and humidity a new thermogravimetric approach was created. 'Wet' and dry gas streams were mixed to control the humidity inside the TGA chamber to any value between 0 and 90%. In this manner a number of measurements could be made reproducibly and quickly at a variety of temperatures and humidities. Water vapor transmission measurements, WVT, were made on a variety of polymer films ranging from polyethylene terephthalate to a permeable, Goretex® membrane.
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