A simple method is described to obtain the water vapor transmission rate (WVTR) forwhole bags of polyethylene and other hydrophobic films. Candidate bags are sealed with 100 g waterand then suspended in a controlled low-humidity chamber above a saturated LiCl salt solution.Humidity inside the bag is uniformly maintained at 100 using paper wicks. Successive weighings atthree to seven day intervals give WVTRs that are 90 to 95 accurate compared to checks by ModernControls, Minneapolis, Minnesota, using ASTM F372-94. Existing ASTM methods are comparativelymore expensive and complex or slower, less flexible and less accurate. The whole bag (water)method gives the average WVTR for the entire bag surface rather than for a small sample piece andit can be determined at any desired temperature. An existing method also uses the entire bag surface,however, it requires a supply of air at constant relative humidity and temperature, making it lessadaptable for tests at various temperatures.
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