This has the formal name compression set under constant deflection in air. The specimen is compressed to 75% of its original height for a set time and at a set temperature (sample is compressed to.75 of its original height). Compression set B is (like Compression set A) defined as the percentage of specimen deflection after it has been left in normal (uncompressed) conditions for 30 minutes. C B, the compression set B is given by C B = [(t o - t i) / (t o - t n)] * 100 where t o is the original specimen thickness, t i is the specimen thickness after testing and t n is the spacer thickness or the specimen thickness during the test.