Natural gas is an important energy vector. The determination of its composition is often used as the basis for the calculation of the calorific value. The calorific value in turn is one of the two key parameters used in natural gas trade. In the first series of key comparisons (CCQM-K1e-g), natural gas was already included with three different compositions. These mixtures contained carbon dioxide, nitrogen, ethane, propane and n-butane in methane (matrix) and were only to a limited extent representative of real natural gas. In the past years, national metrology institutes have broadened the range of components by including, e.g., i-butane, neo-pentane, n-pentane, i-pentane and n-hexane. Based on this extended components list, two new mixtures have been defined, one characteristic for a low calorific mixture (type IV) and the other for a high calorific mixture (type V). In the low calorific mixture, helium was also present. Due to presence of the butane and pentane isomers, the mixtures of type IV and V are more demanding with respect to the separation technique than the mixtures used in CCQM-K1e-g.

The measurements in this key comparison took place in 2001. There were eight participants and two coordinating laboratories. The key comparison reference value (KCRV) was based on the gravimetric preparation for all components. Even for the heavier hydrocarbons (pentanes and n-hexane) the effects of, e.g., adsorption can be controlled to such an extent that this approach is still valid. The uncertainty evaluation of the KCRVs reflected also the extent to which the preparation data could be demonstrated to be valid. The validity of the preparation data was demonstrated by comparing the composition of the mixtures prepared for this comparison with measurement standards maintained by the coordinating laboratories.

The key comparisons demonstrated that the results of the laboratories agreed within 1% relative to the reference value for most components. Even better agreement was obtained for nitrogen in the low calorific mixture (0.5%), carbon dioxide (0.5%), ethane (0,5%), propane (0.5%) and methane (0.1%). In some cases, larger differences were observed, which then also exceeded the associated expanded uncertainty

Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.

The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the Mutual Recognition Arrangement (MRA).