3 Validation of the gravimetric values and uncertainties of independently prepared primary standard gas mixtures

The uncertainty of primary standard gas mixtures is limited by the uncertainty of their gravimetric preparation. Although this fact is widely accepted, it has never been demonstrated directly because the uncertainty involved in the analysis of gas mixtures is much larger than that from the gravimetric preparation alone. We report the first-ever analysis by a single comparison process of a set of independently prepared gravimetric gas mixtures. The analysis of the data has used a method that calculates the parameters of a regression relationship that minimizes the random contributions from both the model and the gravimetric component. The standards studied here are comparable with a standard deviation of the residuals of seven measurements (after the exclusion of two outliers) of 0.002% (relative to value), or an expanded uncertainty of 0.004% relative. This is a factor of between 2 and 30 less than the estimated uncertainties for the gravimetric preparation of each individual standard and serves to confirm the state of the art in this field.