Agreed Proposal, 5 September 2005
In the field of gas metrology, many national metrology institutes have developed and maintained primary standard gas mixtures (PSM) which are obtained by using gravimetry as preparation technique.
The principle of this technique described in the International Standard ISO 6142 “Gas analysis-Preparation of calibration gas mixtures-Gravimetric method” (April 2001) is the following : the gas mixture “are prepared by transferring parent gases (pure gases or gravimetrically prepared mixtures of know composition) quantitatively from supply cylinders to the cylinder in which the calibration gas mixture will be contained. The amount of gaseous component added from the parent gas is determined by weighing after each successive addition”.
The mass fraction of each component in the final gas mixture is then given by the quotient of the mass of that component to the total mass of components present in the gas mixture. Gas composition is preferentially expressed as a mole fraction (mol/mol).
The accuracy of the composition of the final gas mixture depends significantly on the purity of the pure and balance gases used in the gravimetric method.
This determination of the purity can be obtained by the identification and then the quantification of the impurities in the balance and pure gases.
Consequently, the uncertainty contributions of the balance and pure gases depend on the amount of impurities present in these gases and upon the accuracy with which these impurities have been measured.
This is the reason why it seems important to work on the analysis and on the quantification of the impurities in balance and pure gases.
The objective of this comparison is to determine the concentrations of the impurities of CO, CO2, CH4 and O2 present in pure nitrogen.
This comparison could be a good way to determine the efficiency of the different facilities used to analyse very low concentrations of CO, CO2, CH4 and O2 in different laboratories.