A new pressure, volume, temperature, and time (PVTt) primary gas flow standard for calibrating flowmeters has an expanded uncertainty k=2 of between 0.02% and 0.05%. The standard diverts a steady flow into a collection tank of known volume during a measured time interval. The standard spans the flow range of 1 slm1 to 2000 slm using two collection tanks (34 L and 677 L) and two flow diversion systems. We describe the novel features of the standard and analyze its uncertainty. The thermostatted collection tank allows determination of the average gas temperature to 7 mK (0.0023%) within an equilibration time of 20 min. We developed a mass cancellation procedure that reduced the uncertainty contributions from the inventory volume to 0.017% at the highest flow rate. Flows were independently measured throughout the overlapping flow range of the two systems and they agreed within 0.015 %. The larger collection system was evaluated at high flows by comparing single and double diversions; the maximum difference was 0.0075%.

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Design and Uncertainty Analysis for a PVTt Gas Flow Standard
J. Res. Natl. Inst. Stand. Technol.
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