WVTR barrier WVTR Testing Equipment for Barrier Food Packaging W401 covers a procedure for determining the rate of water vapor transmission through flexible barrier materials using a Modulated Infrared Sensor . The method is applicable to sheets and films up to 3 mm (0.1 in.) in thickness, consisting of single or multilayer synthetic or natural polymers and foils, including coated materials. It provides for the determination of (1) water vapor transmission rate (WVTR), (2) the permeance of the film to water vapor, and (3) for homogeneous materials, water vapor permeability coefficient.