Development of a new KRISS low frost-point generator with improved uncertainty from 7 nmol·mol−1 to 1000 nmol·mol−1

Abstract

A new saturator-based low frost-point generator (LFPG 2) is developed at the Korea Research Institute of Standards and Science (KRISS), and its uncertainty is evaluated from 7 nmol·mol−1 to 1000 nmol·mol−1 in nitrogen. The humidity generator operates in the two-temperature two-pressure mode to achieve target water vapor concentrations. First, the apparatus of the generator and the operation procedures, including purging mode and generation mode, are introduced in depth. Then, all uncertainty factors that affect trace-moisture generations are analyzed using a hygrometer based on cavity-ring down spectroscopy. Consequently, the expanded relative uncertainties in the range of (7–1000) nmol·mol−1 are 5.0%–1.0% at a coverage factor of k = 2, respectively. It is found that the effect of water adsorption/desorption is significantly reduced with compact design and sufficient purging. Thus, the uncertainty of the generator is enhanced compared with trace-moisture standards in other national metrology institutes including the KRISS LFPG 1. Finally, the time scales of humidity generations for various flow rates are investigated for efficient calibration of the hygrometers.