ASTM E398 Standard Test Method for Water Vapor Transmission Rate (WVTR) of Sheet Materials Using Dynamic Relative Humidity Measurement

Introduction

ASTM E398 is a standard test technique for dynamically determining the water vapor transfer rate through a flexible barrier material and converting the results to commonly used water vapor transmission (WVT) units. The values measured in this method are expressed in SI units.

Scope

ASTM E389 is a standard test method that uses dynamic relative humidity measurement to determine sheet materials’ water vapor transfer rate. This test method is unique because it closely resembles actual product storage, in which moisture from a package can escape into the environment without being regularly swept with dry gas. Because no single set of test parameters can accurately replicate all climatic and use conditions, the WVTR test method is more helpful in comparing materials under specific conditions than for predicting their actual performance in the field under any circumstances. The water vapor transmission rate can also evaluate a sheet’s vapor barrier properties under known and carefully controlled conditions. Thus, the test method helps in material comparison and control testing.

Key Components of the ASTM E398 Test Apparatus

The ASTM E398 test method apparatus should have the following elements.

Test Procedure

First, the specimen must be cut to the required dimensions to fit in the test cell. After that, orient the specimen properly within the test setup. In dynamic testing, however, if a water-sensitive surface exists in the dry chamber, a reproducible but inaccurate reading results due to edge effects, and such a result cannot be relied on using this configuration. Once the specimen is placed in the correct position, purge the dry chamber with pre-dried air until the test cell, and the exposed surface of the specimen stabilize to a lower humidity level than what is used in the actual test cycle. Once purging is complete, turn off the purge air and seal the sensor chamber from the surrounding atmosphere. Let the sample and the chamber stabilize with moisture trickling in slowly until the required starting humidity for the test is attained.

Specimen Size

During the ASTM E389 test process, a specimen with dimensions of 108 x 108 mm and a thickness of ≤ 3 mm is used.

Result

The ASTM E398 test provides the material’s WVTR as g/m²/day. This indicates the material’s performance as a vapor barrier under controlled conditions. Leakage at the test seal may affect accuracy, so it is recommended to minimize the effects of leakage to obtain reliable results.

Conclusion

The ASTM E398 test method is reliable and dynamic for determining the WVTR of flexible barrier materials. The relative humidity measurements more closely simulate natural environments where moisture can readily diffuse from packaging or material without using dry gas. It is instrumental in comparing different materials and checking their vapor barrier property under controlled conditions. Reliable and accurate results depend on precise specimen preparation, equipment calibration, and minimizing potential leaks around the test. Generally speaking, ASTM E398 proves necessary for assessing the capability of materials to prevent moisture flow-through in packaging, construction, and related applications.

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