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Temperature Characterization in the Collection Tank of the NIST 26 m(3) PVTt Gas Flow Standard



Aaron N. Johnson, John D. Wright, Michael R. Moldover, Pedro I. Espina


Gas temperature gradients created during the filling stage of a PVTt (Pressure-Volume-Temperature-time) calibration cycle, and those imposed by inhomogeneous room conditions, lead to uncertainties in the average gas temperature in the collection tank. Because this temperature plays an important role in obtaining accurate flow measurements, NIST upgraded the temperature-averaging scheme used in our 26 m3 PVTt system. Instead of arithmetically averaging 10 thermistors to obtain the mean gas temperature, this value is now calculated via a volume-weighted trapezoidal integration procedure using 35 thermistors. Applying the new temperature-averaging scheme, the mean gas temperature can be determined to better than 88 mK after only 2700 seconds of fan mixing. As a result, the flow uncertainty in the NIST 26 m3 PVTt system has decreased from 0.22 % to 0.13 % (with a coverage factor of 2).
No. 5


carbon dioxide, critical nozzle, relaxation, sonic nozzle, sonic venturi, sulfur hexafluoride


Johnson, A. , Wright, J. , Moldover, M. and Espina, P. (2003), Temperature Characterization in the Collection Tank of the NIST 26 m(3) PVTt Gas Flow Standard, Metrologia, [online], (Accessed April 23, 2024)
Created September 1, 2003, Updated February 17, 2017