When it is impracticable to condition the whole piece in the standard atmosphere for testing, the length (and widths and the mass of the piece are determined after relaxation in the prevailing atmosphere, and the mass per unit length (mass per unit area) is calculated and corrected by application of a correction factor, determined by comparison of the length land width) and the mass of a specific portion cut from the piece after relaxation, and measured firstly in the ambient atmosphere and then in the standard atmosphere for testing.