Comparison of primary mass flow standards for calibrating MFC's & similar systems used in dynamic ref. gas mixtures preparation

Project Description

Thermal mass flowmeters and controllers (MFC's) are increasingly used as dynamic gas mixtures generation systems. Therefore, calibration facilities of these industriel devices have been developed in several national metrology laboratories.

A direct mass flow calibration with dynamic weighing technique is operated in some laboratories; in some others, the calibration is based on the measurement of a volume of pure gas at fixed pressure and temperature.

Yet, these calibration facilities have never been compared at the primary metrological level. LNE proposes to manage comparisons in the flow range 10 mg.min^-1 (10 sccm) to 10 g.min^-1 (10 slm) in order to contribute to improve the comparability of the existing primary standards in Europe.

LNE has already carried out a first intercomparison with OFMET, using a  laminar element system (molbloc TM) as transfer standard. The maximum  relative difference between the primary standards was 0.5 % and this value  should be considered as the minimum comparability level for flow standards  traceable to the SI.   
The gas flow calibration facilities being quoted to have an expanded  uncertainty of 0.2 % relative of reading, a comparability about 0.2 %  should be aimed at. A comparability better than 0.25 % has already been  achieved between LNE and NRLM (Japan) flow standards.

Final Report 2002-02-20

The project has been completed and the report can be downloaded here>>.

This project has been carried out as a series of bi-lateral comparisons between LNE and four EUROMET laboratories (OFMET, PTB, IMGC and CMI) and two outside Europe (NRLM and NIST). Some results have already been published at various conferences, the report of the two comparisons between PTB and LNE has been published in Metrologia (2001, 38. n.3, 197-202).
Gas mass flows can be measured most accurately using continuously measuring dynamic-gravimetric systems.
Two systems of this type, the mass flow calibration system at the Bureau National de Métrologie-Laboratoire National d'Essais (BNM-LNE, France) and the mass flow calibration system at the Physikalisch-Technische Bundesanstalt (PTB, Germany), were compared twice using a commercial gas flow measuring instrument as a transfer standard. Mass flow rates of nitrogen between 0.5 mg/s and 20 mg/s (approximately 25 ml/min to 1000 ml/min) and mass flow rates of carbon monoxide between 0.5 mg/s and 10 mg/s (approximately 25 ml/min to 500 ml/min) were used to check the agreement of the two calibration systems. The results of the comparison show that the two systems do not deviate by more than 0.23 % for flow rates higher than 2 mg/s. For lower flows, down to 0.5 mg/s, the differences increased to as much as 0.37 %.
Project was brought to an end of 2000, but pilot laboratory was ready to continue the comparisons on bilateral basis.