Standard Test Methods for WATER VAPOR TRANSMISSION OF MATERIALS IN SHEET FORM (Procedure E only)

Purpose: To determine the rate of water vapor transmission (WVT) through a jacket or facing material (perm rating).

General Information: There are six test procedures described in E 96, each having different test conditions. Method E specifically measures WVT in test conditions of high temperature, low humidity on one side of the sheet and high humidity on the other side. Rate of water vapor movement is calculated by weight gain in this method.

Apparatus: An open-mouth dish is needed to hold a desiccant. A test chamber must have controlled temperature, humidity and continuous air circulation at a velocity sufficient to maintain uniform conditions in the chamber. Weights and an analytical balance are used for measurement purposes inside the test chamber.

Method: Place the desiccant in the dish and attach the test specimen across the opening. A seal should be made with a substance which does not transmit water vapor such as wax (Fig. 12). Weigh the assembly and put it into the test chamber maintaining 37.8C and 90% relative humidity. Weighings are taken successively until a constant rate of weight gain is observed.

Plot the weights against time intervals to find the rate of water vapor transmission. Record vapor pressure differences. As in previous test methods, several specimens should be tested, and the perm rating calculated.