Improvement of flow and pressure controls in diffusion-tube humidity generator: Performance evaluation of trace-moisture generation using cavity ring-down spectroscopy

A system of flow and pressure controls for a magnetic suspension balance/diffusion-tube humidity generator (MSB/DTG) has been developed to realize a humidity standard in the trace-moisture region. The system essentially consists of a pressure regulator combined with a piezo valve, and adopts a simple stepwise change in the total flow rate. It has been demonstrated that the uncertainty of evaporation rates is greatly reduced by applying buoyancy correction to mass data measured with the MSB. Using the system and buoyancy-corrected data, the negative effect on the MSB/DTG observed upon changing the total flow rate become negligible. The relative standard uncertainty of total flow rates is evaluated using mass flow meters composed of critical flow Venturi nozzles to be less than or equal to 0.22%. The uncertainty of humidity generation has been significantly reduced by improvements introduced in this work. A comparison between humidity generated with the DTG and humidity measured with a moisture analyzer (MA) based on cavity ring-down spectroscopy is presented in the amount-of-substance fraction range between 10 nmol/mol and 250 nmol/mol. The results of the comparison confirm that the DTG is capable of producing a trace-moisture gas down to approximately 10 nmol/mol, and of easily varying and controlling the amount-of-substance fraction of water using the system. The sources of bias observed in the MA reading are attributable mainly to systematic errors in the temperature of the absorption cell and absorption cross section used to calculate the water concentration with the MA.