This International Standard specifies two methods for determining the coefficient of linear thermal expansion of rigid cellular plastics at sub-ambient temperatures. Method A is the preferred method since it gives a value on a large Sample at a Chosen temperature which is more representative because of anisotropy, etc., of the material or product. In practice, difficulties may be encountered in accurately controlling the uniformity of the low temperature inside the test chamber at temperatures below -70 OC. Should this be the case, Method B should be employed to obtain an “average” coefficient for the temperature interval between the cryogenic temperature and the ambient temperature. The values obtained in these tests are valid measurements only for the specific Sample tested, at the time tested. They cannot be applied generally to a product and cannot be used to predict Performance of the same material at a future date. Care should also be taken in applying the value obtained in this test to calculate the Overall coefficient of expansion of composite products of which the material under test is part. Many rigid cellular products are not isotropic. This is usually ascribed to the fact that the cells in the material are elongated in a particular direction. This direction is referred to in these methods as the “direction of anisotropy”. Tests carried out in this direction normally give results differing from those obtained from other test directions. The direction of anisotropy may vary within a product and thus the number of test specimens required for product specification will generally exceed those given in this International Standard.