ASTM E96-95 pdf free download.Standard Test Methods for Water Vapor Transmission of Materials.
4.2 In the water Method: the dish contains distilled water. and the weighings determine the rate of vapor movement through the specimen from the water to the controlled atmosphere. The vapor pressure difference is nominally the same in both methods except in the variation, with extremes of humidity on opposite sides.
5. Significance and Use
5.1 The purpose of these tests is to obtain, by means of simple apparatus, reliable values of water vapor transfer through permeable and semipermeable materials, expressed in suitable units. These values are for use in design. manufacture, and marketing. A permeance value obtained under one set of test conditions may not indicate the value under a different set of conditions. For this reason, the test conditions should be selected that most closely approach the conditions of use. While any set of conditions may be used and those conditions reported, standard conditions that have been useful are shown in Appendix XI.
6. Apparatus
6.1 Test Dish—The test dish shall be of any noncorroding material, impermeable to water or water vapor. It may be of any shape. Light weight is desirable. A large. shallow dish is preferred, but its size and weight are limited when an analytical balance is chosen to detect small weight changes. The mouth of the dish shall be as large as practical and at least 4.65 in.2 (3000 mm2). The desiccant or water area shall be not less than the mouth area except if a grid is used, as provided in 12.1, its effective area shall not exceed 10 % of the mouth area. An external flange or ledge around the mouth, to which the specimen may be attached, is useful when shrinking or warping occurs. When the specimen area is larger than the mouth area, this overlay upon the ledge is a source of error, particularly for thick specimens. This overlay material should be masked as described in 10.1 so that the mouth area defines the test area. The overlay material results in a positive error, indicating excessive water vapor transmission. The magnitude of the error is a complex function of the thickness, ledge width, mouth area, and possibly the permeability. This error is discussed by Joy and Wilson.6 This type of error should be limited to about 10 to 12 %. For a thick specimen the ledge should not exceed 3/4 in. (19 mm) for a 10-in. (254-mm) or larger mouth (square or circular) or ¼ in. (3 mm) for a 5-in. (127-mm) mouth (square or circular). For a 3-in. (76-mm) mouth (square or circular) the ledge should not exceed 0.11 in. (2.8 mm) wide. An allowable ledge may be interpolated for intermediate sizes or calculated according to Joy and Wilson.b A rim around the ledge (Fig. X2. 1) may be useful. If a rim is provided, it shall be not more than ¼ in. (6 mm) higher than the specimen as attached. Different depths may be used for the Desiccant Method and Water Method, but a 3/4-Ifl. (19-mm) depth (below the mouth) is satisfactory for either method.ASTM E96 pdf download.