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ISO 16911-1:2013 specifies a method for periodic determination of the axial velocity and volume flow rate of gas within emissions ducts and stacks. It is applicable for use in circular or rectangular ducts with measurement locations meeting the requirements of EN 15259. Minimum and maximum duct sizes are driven by practical considerations of the measurement devices described within ISO 16911-1:2013. ISO 16911-1:2013 requires all flow measurements to have demonstrable metrological traceability to national or international primary standards. To be used as a standard reference method, the user is required to demonstrate that the performance characteristics of the method are equal to or better than the performance criteria defined in ISO 16911-1:2013 and that the overall uncertainty of the method, expressed with a level of confidence of 95 %, is determined and reported. The results for each method defined in ISO 16911-1:2013 have different uncertainties within a range of 1 % to 10 % at flow velocities of 20 m/s. Methods further to these can be used provided that the user can demonstrate equivalence, based on the principles of CEN/TS 14793.