Trace-moisture generator designed for performance tests of trace-moisture analyzers

Abstract

The National Metrology Institute of Japan (NMIJ) developed a trace-moisture generator, named the “simplified trace-moisture generator (STMG),” which was designed particularly for effectively testing the performance of trace-moisture analyzers. The STMG produces trace moisture in nitrogen gas from approximately 10nmol/mol (ppb) to 10μmol/mol (ppm) in amount fraction (mole fraction) of water, using diffusion-tube and two-stage dilution methods. In contrast to the magnetic suspension balance/diffusion-tube humidity generator (MSB/DTG) developed at NMIJ in 2007, the STMG does not require an MSB to measure the evaporation rate of water from the diffusion cell (hence, the name “simplified”). Instead, a moisture analyzer based on cavity ring-down spectroscopy (CRDS) calibrated in a manner traceable to the International System of Units (SI) measures the amount fraction of water in the generated gas. By introducing a sonic nozzle placed at the outlet of the generation chamber, the pressure stability in the chamber is maintained even after the rapid change in the flow rate of the diluent nitrogen gas. This enabled smooth moisture-concentration switching within 1min for 90 % response to the step change between 11nmol/mol and 8.8μmol/mol. The controllability and short-term stability of the moisture content in the generated gas were demonstrated successfully. The long-term stability of moisture generation was evaluated by estimating evaporation rate from the flow rate of nitrogen and the amount fraction of water measured by the CRDS trace-moisture analyzer. It was confirmed that the STMG can generate trace moisture in nitrogen with good stability and repeatability for five consecutive months.