The deviation between the strain measurement system and a calibration system is determined depending on the elongation. If the measured value is located inside the green area, it is within the valid limits. If it is located inside the red area, it is outside the valid limits. If for all measured elongations the deviations are in the green area the measuring system satisfies the accuracy class. (A usefull set of elongations is defined in the standard.)

The accuracy class 0.5 means, that the deviation has to be smaller than 0.5 % of the measured value. If the elongation is very small the relative limits (%) cannot be fulfilled by a real measuring system. So for the small elongation an absolute limitation is used. This absolute limitation is valid for elongations between 0 and 300 µm, for larger values the relative limits (%) are valid.

The figure below illustrates the limits for calibration. The area of ± 3 x 0.5 µm (the upper blue rectangular plane) and the area of ± 0.5 % (the blue triangular plane underneath) are combined and together represent the valid limits.