This International Standard specifies a method for the preparation of calibration gas mixtures by a static volumetric method and provides a procedure for calculating the volumetric composition of the mixture. It can be used either with binary gas mixtures (containing one calibration component in a complementary gas, which is usually nitrogen or air [1,2]) or with mixtures containing more than one component in the complementary gas. This International Standard also specifies how the expanded uncertainty in the volume fraction of each calibration component in the mixture is determined by a rigorous evaluation of all the measurement uncertainties involved, including those associated with the apparatus used for the preparation of the gas mixture and those associated with the experimental procedure itself.
NOTE 1 This International Standard is generally applicable to the preparation of calibration gas mixtures containing calibration components in the concentration range 10 x 10^-9 (10 ppb - parts per billion) to 50 x 10^-6 (50 ppm - parts per million) by volume. However, gas mixtures may be prepared at larger or smaller volume fractions, provided that the components used in the static dilution process are selected appropriately.
NOTE 2 A relative expanded uncertainty of not greater than+/- 1% at a level of confidence of 95% may be achievable at these concentrations, provided that;
-the purities of the parent gases have been determined by analysis and any significant impurities and the uncertainties in their measured concentrations have been taken into account;
-no significant adsorption effects or chemical reactions occur between the gaseous constituents and the internal surfaces of the apparatus, and there are no reactions between any of the gaseous components, i.e. between the calibration component and complementary gas or between the calibration components themselves;
- all the relevant apparatus used in the preparation of a calibration gas mixture have been calibrated with assigned measurement uncertainties which are appropriate to calculating the final expanded uncertainty for the calibration gas mixture prepared.