This part of ISO 6145 specifies a method for the continuous production of calibration gas mixtures, containing two or more components, from pure gases or other gas mixtures by use of commercially available multi-piston, volumetric pumps. Pumps of this type may be employed to prepare gas mixtures in which the volume fractions of the gaseous components in the complementary gas range from 0,1 % upwards; the relative expanded uncertainty of measurement, U, obtained by multiplying the relative combined standard uncertainty by a coverage factor k = 2 is not greater than 0,5 %. Lower concentrations may be obtained by cascading volumetric pumps, and the lowest volume fraction achievable by means of a two-pump cascade is 10—6. Moreover, if pre-mixed gases are used instead of pure gases, much lower concentrations can be obtained. The merits of this method are that a large quantity of the gas mixture can be prepared on a continuous basis, and that multicomponent mixtures can also be readily prepared.