No single set of test conditions can represent all climatic and use conditions, so this WVTR test method serves more to compare different materials at a stated set of conditions than to predict their actual performance in the field under any conditions.

The water vapor transmission rate, under known and carefully controlled conditions, may be used to evaluate the vapor barrier qualities of a sheet. Direct correlation of values obtained under different conditions of test temperature and relative humidity will be valid provided the barrier material under test does not undergo changes in solid state (such as a crystalline transition or melting point) at or between the conditions of test.

1.1 This test method covers dynamic evaluation of the rate of transfer of water vapor through a flexible barrier material and allows conversion to the generally recognized units of water vapor transmission (WVT) as obtained by various other test methods including the gravimetric method described in Test Methods E 96/E 96M.

1.2 Limitations—This test method is limited to flexible barrier sheet materials composed of either completely hydrophobic materials, or combinations of hydrophobic and hydrophilic materials having at least one surface that is hydrophobic.

1.3 The minimum test value obtained by this test method is limited by the leakage of water vapor past the clamping seals of the test instrument. A reasonable value may be approximately 0.01 g/24 h·m² for any WVTR method including the desiccant procedure of Test Methods E 96/E 96M at 37.8°C, and 90 % relative humidity. This limit can be checked for each instrument with an impervious specimen such as aluminum foil. Calibration procedures can compensate for the leakage rate if so stated.

1.4 This test method is not suitable for referee testing at this time, but is suitable for control testing and material comparison.

1.5 Several other ASTM test methods are available to test a similar property. This test method is unique in that it closely duplicates typical product storage where a transfer of moisture from a package into the environment is allowed to proceed without constantly sweeping the environmental side with dry gas. Methods with constantly swept dry sides include Test Methods F 1249, F 372, and F 1770.

1.6 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.