Current edition approved March 1, Published April Originally approved in DOI: Periodic weighings determine the rate of water vapor movement through the specimen into the desiccant. No further reproductions authorized.
Other units may be used in other standards. The vapor pressure difference is nominally the same in both methods except in the variation, with extremes of humidity on opposite sides.
Significance and Use 5. 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 X1. Apparatus 6. 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. The desiccant or water area shall be not less than the mouth area except if a grid is used, as provided in 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 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 1 3 see For a 3-in. An allowable ledge may be interpolated for intermediate sizes or calculated according to Joy and Wilson.
Some standard test conditions that have been useful are given in Appendix X1. The temperature chosen shall be determined according to the desired application of the material to be tested see Appendix X1. Both temperature and relative humidity shall be measured frequently4 or preferably recorded continuously.
Air shall be continuously circulated throughout the chamber, with a velocity sufficient to maintain uniform conditions at all test locations. The air velocity over the specimen shall be between 0. Suitable racks shall be provided on which to place the test dishes within the test chamber. However, it is very desirable to enter the controlled space, and a comfortable temperature is more satisfactory for that arrangement.
Temperatures of With cyclic control, the average test temperature may be obtained from a sensitive thermometer in a mass of dry sand. The temperature of the chamber walls facing a specimen over water should not be cooler than the water to avoid condensation on the test specimen. A light wire sling may be substituted for the usual pan to accommodate a larger and heavier load.
In 18 days of steady state, the transfer is 10 g. If, however, the balance has a sensitivity of 0. Materials 7. It must not affect the vapor pressure in a water-filled dish. Sealing methods are discussed in Appendix X2. Sampling 8. The sample shall be of uniform thickness. Test Specimens 9. When a product is designed for use in only one position, three specimens shall be tested by the same method with the vapor flow in the designated direction. When the sides of a product are indistinguishable, three specimens shall be tested by the same method.
When the sides of a product are different and either side may face the vapor source, four specimens shall be tested by the same method, two being tested with the vapor flow in each direction and so reported.
Alternatively, it may be sliced into two or more sheets, each being separately tested and so reported as provided in 9. When it is homogeneous, however, a thinner slice of the slab may be tested as provided in 9.
Failure to use this dummy specimen to establish modified dish weights may significantly increase the time required to complete the test. Because time to reach equilibrium of water permeance increases as the square of thickness, thick, particularly hygroscopic, materials may take as long as 60 days to reach equilibrium conditions.
Attachment of Specimen to Test Dish If necessary, mask the specimen top surface, exposed to conditioned air so that its exposure duplicates the mouth shape and size and is directly above it.
A template is recommended for locating the mask. Thoroughly seal the edges of the specimen to prevent the passage of vapor into, or out of, or around the specimen edges or any portion thereof.
The same assurance must apply to any part of the specimen faces outside their defined areas. Suggested methods of attachment are described in Appendix X2. Procedure for Desiccant Method Leave enough space so that shaking of the dish, which must be done at each weighing, will mix the desiccant. This weight may be helpful to an understanding of the initial moisture in the specimen. A data point is the weight at a particular time.
Thus, if weighings are made every hour, record the time to the nearest 30 s; if recordings are made every day, a time to the nearest 15 min would be allowed. At first the weight may change rapidly; later a steady state will be reached where the rate of change is substantially constant.
When results of water vapor transmission are expected to be less than 0. Such a dummy specimen should be attached to an empty cup in the normal manner. The environmental effects of temperature variation and buoyancy variability due to barometric pressure fluctuation can be arithmetically tared out of the weighing values. This precaution permits earlier and more reliable achievement of equilibrium conditions. Analyze the results as prescribed in The method should be selected that more nearly approaches the conditions of use.
The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard. However, derived results can be converted from one system to the other using appropriate conversion factors see Table 1.
User icon An illustration of a person's head and chest. Sign up Log in. Web icon An illustration of a computer application window Wayback Machine Texts icon An illustration of an open book. Books Video icon An illustration of two cells of a film strip. Video Audio icon An illustration of an audio speaker. Audio Software icon An illustration of a 3. Software Images icon An illustration of two photographs.
Images Donate icon An illustration of a heart shape Donate Ellipses icon An illustration of text ellipses.
EMBED for wordpress.
0コメント