Development of a primary measurement standard for trace moisture in Ar

Abstract

We have developed a primary measurement standard for trace moisture in Ar in the range of 10 nmol mol−1 to 1 μmol mol−1 in the amount-of-substance fraction (mole fraction) using the previously developed multi-gas trace-moisture generator. The relative expanded uncertainty (k = 2) of the primary standard in Ar was between 2.7 % and 1.5 %. The performance of a moisture analyzer (MA) based on cavity ring-down spectroscopy (CRDS), which is considered to be the best existing device thus far for trace-moisture measurement, was tested through a comparison with the primary standard in Ar. The readings of the CRDS-based MA were approximately 10 % lower than the numerical values of the primary standard. The relative standard deviation of the readings became approximately 15 %, which was 15 times as large as that for N2. These were caused by a failure in the default setting of the MA for Ar and could be improved by optimizing the setting.