Uncertainty reduction due to correlation effects in weighing during the preparation of primary gas standards

The accurate weighing of gas into high-pressure cylinders plays a central role in providing traceability for primary gas mixtures. The uncertainties in the masses of successive components added to such mixtures are significantly correlated. We show that a consequence is that the amount fraction of the resulting mixture is not influenced by changes in buoyancy due to the linear expansion of the cylinder. This result will provide a useful simplification to some procedures used in gas metrology