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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 prefered 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 ° C. 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.