Metrological certification is done through referable measurement of two flows : the inlet to be diluted and the diluted gas outlet.   The ratio of those flows gives the dilution factor. 

Measurements get validity reason by referencing with recognised standards : standard references are conserved and maintained by national metrological institutions and are continuously checked by exchanging the results with recognised institutions of other nations.

In Italy IMGC Institute of CNR (National Research Centre) in Turin has in charge the "mass" standards, which include the fluids flow standards.

The purpose of a gas blender is to mix two gas flows (a gas to be blended and a neutral gas) in a well defined rate, which allows the production of well defined gas concentrations.

According to the traditional approach, the dilution rate is accurate when both the fluids received by the blender are measured or generated with low uncertainty : this may be checked and assured by referring the involved flows to traceable standards only.

Perhaps a different approach does exist : it focuses the attention on the "ratio" of the flows, more than on the individual flows.  This approach gets legitimization by the fact that (in the due limits) the uncertainty of the blended gas flow is not a qualifying parameter for a gas blender : the quality of a gas blender is measured by the low dilution rate uncertainty, which leads to a low uncertainty of the mixed gas concentration.

In other terms, given a dilution rate, the diluted gas flow may be affected by significant errors, without performances decrease when an error of the same proportions affects the flow of the gas to be blended.

This approach is widely used by Be.T.A. Strumentazione in the capillaries selection process and in patented MFCs self-referring calibration : this is well described in technical papers available in this site.