Gas Flowmeter Calibrations With the 26 m3 PVTt Standard
Aaron N. Johnson, John D. Wright
This document describes NIST s 26 m3 pressure, volume, temperature, and time (PVTt) primary flow standard. This standard is used to calibrate gas flow meters over a range extending from 200 standard L/min to 77,000 standard L/min where the reference temperature and pressure conditions are 293.15 K and 101.325 kPa respectively, and the working fluid is dry air. This standard measures flow by collecting a steady stream of gas into a tank of known volume during a measured time interval. The ratio of the mass of gas collected to the collection time gives the mass flow. The PVTt standard measures mass flow with an expanded uncertainty of 0.12 % at the ninety-five percent confidence interval (i.e., k = 2) over the full flow range. This document presents a detailed uncertainty analysis evaluating and explaining the various components that comprise the expanded uncertainty. In addition to the uncertainty analysis, we specify the various components of the 26 m3 PVTt system, describe its theoretical basis of flow measurement, document the calibration procedure used for the standard, provide information about the calibration service (i.e., meters that we commonly test, available pipe sizes, sample calibration report, etc.), and give details regarding the unique aspects of NIST s PVTt systems.