Final report of CCQM-P225a and b, international comparison on carbon dioxide in air and nitrogen at ambient levels (350 μmol/mol to 800 μmol/mol)

The pilot studies CCQM-P225a and b aimed at validating the performance of the pVT−CO₂ manometric reference facility maintained at the BIPM as a reference for the planned ongoing comparisons BIPM.QM-K2a and b for CO₂ in air and nitrogen standards, respectively, over the range 350 μmol mol⁻¹ to 800 μmol mol⁻¹. The planned launch date of the BIPM.QM-K2a and b comparisons is 2024 and will enable NMIs and DIs to have access to on-demand comparisons for their CO2 in air and nitrogen standards over the range 350 μmol mol⁻¹ to 800 μmol mol⁻¹.The pilot studies supplemented the validation activities on the BIPM reference facility already reported in CCQM-P188, and ongoing validation studies against NIST, NOAA and VSL gas standards. A description of the BIPM facility performance will be submitted for publication in a peer-reviewed journal in 2023, with a full uncertainty analysis showing that it can operate with a standard uncertainty of 0.1 μmol mol⁻¹ over the amount fraction range of the comparison (Viallon et al 2023b).The comparison was organised using a sufficient number of independent standards, to demonstrate the performance of the BIPM system, whilst limiting the duration of the pilot study, allowing the key comparison to start as planned in 2024. Following discussions at the 44th GAWG meeting it was decided that comparisons with 3 independent standards at 3 nominal amount fraction values (380, 480 and 800 μmol mol⁻¹) in the two matrices (air and nitrogen) would be sufficient.Results presented in this report show a good agreement between values assigned by participants and those measured at the BIPM, over the entire range of the comparison and for both matrices (air and nitrogen). The BIPM pVT−CO2 facility is ready to act as reference facility for the ongoing key comparison BIPM.QM−K2.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 https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).