NIST has reduced the flow uncertainty of its 34 L and 677 L Pressure-Volume-Temperature-time (PVTt) primary flow standards from 0.05 % to 0.025 % (k = 2) for air in the flow range from 0.01 slm* to 2000 slm. Over the restricted flow range 0.1 slm to 1000 slm, the uncertainty is 0.015 %.The reduction in uncertainty is primarily the result of accurately accounting for the water vapor present in the flowing air. Additional uncertainty reductions result from improved measurements of the pressure and of the collection tank volumes. The latter two improvements slightly reduced flow uncertainties for dry gases (e.g., N2, Ar, He, and CO2) from 0.03 % to 0.025 % in the wider range.
Measurement Science Conference Proceedings
March 16-18, 2009
Critical Nozzle, Flow, Gas, Uncertainty, PVTt.