Water Vapor Transmission, Water Vapor Permeance, Water Vapor Permeability
Water vapor transmission (WVT) is determined by dividing the rate of weight change by the area of the test specimen. Water vapor permeance is in general the degree to which a material admits water vapor. Water vapor permeability is related with the thickness of the material.
Calculation of Water Vapor Transmission, Water Vapor Permeance, Water Vapor Permeability
The permeance and permeability are calculated from the WVT. The permeance is calculated by dividing the WVT by the vapor pressure across the material under test. The permeability is calculated by multiplying the permeance by the thickness of the material.
ASTM D1653 standard and Test Method
ASTM D1653 standard determines the water vapor transmission rate of films of paint,lacquer,vanish, and other organic coatings. The films may be free films or be applied to porous substrates. Under ASTM D1653 standard, there are two test methods: dry cup method and wet cup method.
ASTM D1653 Dry Cup Method
By sealing a film of the test material to the top of a vessel that is filled with water or desiccant, WVT is determined. It is called wet cup method when the vessel is filled with water. The test determines how much water vapor is lost through the film. The dish loses water over the course of the test so that the plot tends downward. The wet cup method is the preferred one for obtaining values that is related to conditions of use where is high relative humid.
ASTM D1653 Wet Cup Method
When the vessel is completed with desiccant, it is called the dry cup method. Through the test method, it is obviously known how much water vapor the desiccant attracts through the film. Meanwhile, the plot points upward as the dish gets water over the course of the test. The dry cup method is the preferred one for obtaining values that is related with conventional dwellings where is lower relative humid.
Rules of ASTM D1653 Test Method
At the beginning of test, the dish may gains or loses weight quickly, and then goes into a constant rate of weight change. Certainly, the process depends on the material being tested.
Attention: By the wet cup method, the values obtained are higher than ones by the dry cup method under the same conditions of air temperature and humidity.
ASTM E96 standard and Test Method
ASTM E96 standard is used to test paper, plastic films, other sheet materials, fiber-boards, gypsum and plaster products, wood products, and plastics. ASTM E96 standard introduces two basic methods: the desiccant method and the water method.
ASTM E96 Desiccant Method
The desiccant method: the test dish contains a desiccant. Leave enough space so that the shaking of the dish to mix the desiccant, which have to be done at each weighing. The test specimen is sealed to the open mouth of the dish. The rate of water vapor movement is determined by periodic weighing of the test dish.
ASTM E96 Water Method
The water method: the dish contains distilled water. Water vapor permeates through specimen into the controlled atmosphere. Through the periodic weighing of the test dish, the values can be obtained.
ASTM E96 Test Method Attention
Under different test conditions, the valued obtained may not be equivalent. As this reason, select the test conditions which are the closest with the conditions of use.
The difference of ASTM E96 standard and ASTM D1653 standard
The testing scope of ASTM E96 standard is more wider, which can evaluate many materials, including coatings which is mainly tested under ASTM D1653 standard.
ASTM D1653 includes directions for preparation of a free film, whereas ASTM E96 does not.
ASTM E96 requires that permeance values exceeding 2 perm are corrected for resistance due to still air and specimen surface. ASTM D1653 does not specify the correction.
Saicheng' WPT-301B Water Vapor Permeability Tester which adopts gravimetric method complies with ASTM E96 standard and ASTM D1653 standard.
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