This part of ISO6145 specifies a method for the continuous production of calibration gas mixtures, containingtwo or more components, from pure gases or other gas mixtures by use of critical orifice systems. By selectionof appropriate combinations of orifices and with the use of pure gases, the volume fraction of the calibrationcomponent in the calibration gas mixture can be varied by a factor of . Additionally, it can be changed by afactor of by changing the initial pressures in the orifice systems. The uncertainty of the method dependsmainly upon the flow calibration method and the variations in temperature and outlet pressure. The relativeexpanded uncertainty in the volume fraction obtainable for a binary mixture (at a coverage factor of 2) is 3%.If pre-mixed gases are used instead of pure gases, much lower volume fractions can be obtained (seeAnnexA). The mass flow rates or volume flow rates, from which the mass or volume fractions are determined,can be calculated and can be independently measured by a suitable method given in ISO 6145-1.The merits of the method are that multi-component mixtures can be prepared as readily as binary mixtures if theappropriate number of orifices is utilized, and that a large quantity of calibration gas mixture can be prepared ona continuous basis. The range of flow rates can be from several millilitres per minute to approximately 10l/min.Although particularly applicable to preparation of gas mixtures at barometric pressure, the method also providesa means of preparation of calibration gas mixtures at pressures above barometric pressure.AnnexB gives practical hints on the use of the method.