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This document describes the techniques for the determination of the leakage rates across the boundary of an isolated object, subjected to a pressure difference. The techniques are based on the evaluation of the change of the mass of gas within the test object. The ideal gas equation states the relationship between mass, pressure, temperature, volume of the gas contained in the internal free volume of the object. In some circumstances one or more variables may be considered constant and so the mass change can, for example, be assessed by monitoring the pressure or pressure-temperature changes within the test object. The volume change of the object (due to pressure and temperature changes during the test) should be taken into account. In most industrial applications, however, this change is so small that can be neglected. Therefore the main part of this standard considers the volume as a constant during the test. The deformability of the object under test is normally be considered in drafting of the test specification and, if test precision can be affected by a volume change, alternative formulas or some techniques of keeping the pressure constant (flow measurement) can be used.