WHAT IS THE “BEST” TRANSFER STANDARD FOR GAS FLOW?
John D. Wright
Abstract:
National metrology institutes around the world have embarked on an effort to compare their primary gas flow standards under the supervision of the Comité International des Poids et Mesures and its Working Group on Fluid Flow. As the pilot laboratory for the key comparison of low-pressure gas flows below 1500 L/min (CCM.FF-K6), NIST is designing the transfer standard package with the assistance and advice of the other participants. A number of topics are studied as part of the transfer standard design process. An analysis of the flow ranges and types of primary standards used by the possible key comparison participants based on their Calibration and Measurement Capabilities is given. The goals of the key comparison and the desirable characteristics of the transfer standard are considered in a general way. Next, a survey of the gas flow comparison literature suggests that laminar flow elements a nd critical flow venturis are the most practical candidates as transfer standards. These two flowmeter types are compared analytically and experimentally regarding their sensitivity to the external influences of temperature and gas composition. Finally, the calibration stability for these two flowmeter types is considered.
