The standard method for measuring the compression set of vulcanized elastomers (ASTM Methods D 395 - 55) covers two test procedures using different types of load applications. The first, method A, uses a constant load. The second, method B, which uses a constant deflection, is the more popular method and the one under consideration in this paper. The device for measuring compression set in accordance with method B consists of two or more flat steel plates. It is between the parallel faces of these plates that the specimens are compressed. The four levels of compression set employed in method B are dependent on the hardness of the material being tested according to the following table: The compression set, C, expressed as a percentage of the original deflection is calculated as follows: C=to−tlto−ts×100 where: to = original thickness of the test specimen, tl = thickness of the test specimen 30 min after removal from the test fixture, and ts = thickness of the spacer bar.