This part of ISO 6145 specifies a method for the dynamic preparation of calibration gas mixtures containing at least one component gas and one complementary gas from pure gases or gas pre-mixtures using critical flow orifices systems. The method applies principally to the preparation of mixtures of non-reactive gases that do not react with any of the materials forming the gas circuit inside the critical flow orifices system or auxiliary equipment. It has the merit of allowing multi-component mixtures to be prepared as readily as binary mixtures if an appropriate number of critical flow orifices are used. By selecting appropriate combinations of critical flow orifices and using pure gases, the volume fraction of the calibration component in the calibration gas mixture can be varied by a factor of 1 x 104. If gas pre-mixtures are used in place of pure gases, smaller volume fractions can be obtained. Although it is more particularly applicable to the preparation of gas mixtures at atmospheric pressure, the method also offers the possibility of preparing calibration gas mixtures at pressures greater than atmospheric. The upstream pressure will need to be at least two times higher than downstream pressure. The range of flow rates covered by this standard extends from 1 ml/min to 10 l/min.