The cup top surface, as well as both surfaces of the clamp ring are smoothly finished to provide a tight seal against the product under test. Two gaskets with the same dimensions as the clamp ring, except for thickness, are furnished for use with rough or uneven materials to insure complete sealing. Extending upward from the face of the cup are two stainless steel pins which closely engage the clamp ring, and any gaskets used, to prevent movement with respect to the test material as the cap is tightened against the cup. The cap design permits use of longer pins than possible with earlier models to accommodate thicker sample and gasket combinations.

Either a test liquid or a desiccant is placed in the cup, depending on the specified conditions of the test. A sample of the test material, precut to the outside dimensions of the clamp ring, is placed on the top surface of the cup, using gaskets if required, and sealed against the cup surface by tightening the cap ( for specimen preparation see ASTM D-823 & D- 4708). The assembly is then accurately weighed and placed in a specified controlled atmosphere. Periodic weighing determines weight loss or gain from which rate of vapor transmission is calculated. Small 10 cm² cup allows the use of a smaller specimen and less desiccant.