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In 2009 NIST developed a US national flow standard to provide traceability for flow meters used for custody transfer of pipeline quality natural gas. NIST disseminates the SI unit of flow by calibrating a customer flow meter against a parallel array of turbine meter working standards, which in turn are traceable to a pressure–volume–temperature–time (PVTt) primary standard. The calibration flow range extends from 0.125 actual m3/s to 9 actual m3/s with an expanded uncertainty as low as 0.22% at high flows, and increasing to almost 0.40% at the lowest flows. Details regarding the traceability chain and uncertainty analysis are documented in prior publications. The current manuscript verifies NIST's calibration uncertainty via a bilateral comparison with the German National Metrology Institute PTB. The results of the bilateral are linked to the 2006 key comparison results between three EURAMET national metrology institutes (i.e., PTB, VSL and LNE). Linkage is accomplished in spite of using a different transfer standard in the bilateral versus the key comparison. A mathematical proof is included that demonstrates that the relative difference between a laboratory's measured flow and the key comparison reference value is independent of the transfer package for most flow measurement applications. The bilateral results demonstrate that NIST's natural gas flow measurements are within their specified uncertainties and are equivalent to those of the EURAMET National Metrology Institutes. 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 CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).
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