Metrological performances of mass flow controllers for dynamic gas dilution

Abstract

The method of dynamically generating reference mixtures of volatile organic compounds at trace levels requires the uninterrupted blending of the component flows during the generation time. The target flow rate uncertainty depends on the application and the range of the concentration generated. Accurate air flow rates used for dilution can be realized by mass flow controllers (MFCs). The short-term stability of MFCs has been measured in order to assess their usefulness in dynamic gas dilutors. As shown here, MFCs are appropriate for the dynamic preparation of diluted gas mixtures provided they are calibrated prior to each use, because their calibration is not stable over longer time periods. The calibration results of the low pressure drop MFC proved its applicability in a wide range of pressure drop. The uncertainty of the dilution flow rate resulted mainly from the limited short-term stability of the MFC at low dilution flow rate or at short generation time. Consequently, MFCs can provide highly accurate dilution air flow rates with a relative uncertainty of 0.2 %, for flow rates larger than 70 % of the MFC’s maximum flow rate and generation times longer than 5 min.