Development of humidity standard in trace-moisture region: Characteristics of humidity generation of diffusion tube humidity generator

The characteristics of humidity generation of a diffusion tube humidity generator (DTG) have been studied by the real-time mass measurement of evaporating water using a magnetic suspension balance and by humidity measurement using a moisture analyzer based on cavity ring-down spectroscopy in the amount-of-substance fraction range of water between 20 nmol/mol and 600 nmol/mol. The observed mass-change rates of a diffusion cell due to water evaporation agree well with the evaporation rates calculated on the basis of Fick's law of diffusion. The stable generation of trace-moisture in nitrogen gas is realized by precisely controlling temperature and pressure. The relative standard uncertainty of the stability of trace-moisture generation of the DTG is 0.006 (0.6%). The effect of temperature on the stability of humidity generation of the DTG is smaller than that of a frost-point generator.