This has the formal name compression set under constant force in air. In compression set A a force of 1.8 kN is applied to the specimen for a set time at a set temperature. Compression set A is defined as the percentage of original specimen thickness after the specimen has been left in normal (uncompressed) conditions for 30 minutes. C A, the compression set A is given by C A = [(t o - t i) / t o ] * 100 where t o is the original specimen thickness and t i is the specimen thickness after testing.