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Uncertainty analysis for trace-moisture standard realized using a magnetic suspension balance/diffusion-tube humidity generator developed at NMIJ

A method of estimating the uncertainty for a primary trace-moisture standard realized using a magnetic suspension balance/diffusion-tube humidity generator (MSB/DTG) developed at the National Metrology Institute of Japan (NMIJ) is reported in full detail. The results of uncertainty analysis showed that the relative expanded uncertainties of the trace-moisture standard with a coverage factor of k = 2 are 6.9 % to 0.75 % for the amount-of-substance fractions (mole fractions) of water in nitrogen in the range of 12 nmol mol−1 to 1400 nmol mol−1. NMIJ participated in an international comparison of primary trace-moisture standards, and the results of the comparison demonstrated that the method of estimating uncertainty developed in this study is reasonable. The dominant uncertainty was attributable to the performance of the trace-moisture analyzer used to evaluate the uncertainty. We conclude that the development of a trace-moisture analyzer based on cavity ring-down spectroscopy with higher performance is needed to reduce the uncertainty of trace-moisture standards based on an MSB/DTG.