Water Vapor Transmission (WVT) - ASTM E96 Plastic Test Standard

Water Vapor Transmission (WVT) - ASTM E96

Water vapor transmission is a measure of how much water vapor will pass through a material per unit area per unit time. Testing is performed by sealing a specimen to the open mouth of a test dish containing either desiccant or water and placing the assembly into a controlled atmosphere. The test unit is weighed periodically and the weight is plotted as a function of time. Water vapor transmission is taken as the slope of the curve (in the linear region) divided by the area of the dish opening.

Water Vapor Transmission is calculated using the formula:
WVT = G / t / A= (G / t) / A
where G is the weight gain, t is the time tested, and A is the area of the test area (area of dish opening).

Testing Methods

Method	Summary	Diagram
Desiccant	The test specimen is sealed to the open mouth of a test dish containing a desiccant and the assembly placed in a controlled atmosphere. Periodic weighings determine the rate of water vapor movement through the specimen from the controlled atmosphere into the desiccant.
Water	The test specimen is sealed to the open mouth of a test dish containing water and the assembly placed in a controlled atmosphere. Periodic weighings determine the rate of water vapor movement through the specimen from the water to the controlled atmosphere.
Inverted Water	Same as the water method except that the test dish is turned upside-down so the water is in contact with the specimen.

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