Bilateral Comparison between NIST and PTB for Flows of High Pressure Natural Gas
Aaron N. Johnson, B Mickan, H. Toebben, Tom Kegel
In 2009 NIST developed a U.S. 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 0.7 actual m3/s to 10.7 actual m3/s with an expanded uncertainty as low as 0.23 % at high flows, and increasing to almost 0.35 % at the lowest flows. Details regarding the traceability ladder and uncertainty analysis are documented in prior publications. The current manuscript verifies NISTs calibration uncertainty via a bilateral comparison with the German national metrology institute PTB. The results of this bilateral comparison are linked to the 2006 key comparison results between three Euramet national metrology institutes (i.e., PTB, VSL, and LNE). The bilateral results demonstrate NISTs natural gas flow measurements are within their specified uncertainty and are metrologically equivalent to the key comparison reference value established by the Euramet national metrology institutes.