Abstract
A novel flow measurement/control system was developed to improve the capability of measuring and controlling the flow rate of the dry gas in the magnetic suspension balance/diffusion-tube humidity generator at the National Metrology Institute of Japan. The flow rate was measured using a mass flow meter composed of multiple critical flow Venturi nozzles and was adjusted to a set value by controlling the pressure of the gas upstream of the nozzles using a feedback loop with the flow rate measured using the mass flow meter. Using this system, the uncertainty due to the instability of the flow rate was reduced to less than 0.0023 %, causing its contribution to the uncertainty of the standard value of the trace moisture generated to be insignificant. The improved capability to rapidly and stably control the flow rate of the dry gas allows the rapid change in trace moisture that is necessary to evaluate the performance of conventional and commonly used trace-moisture analyzers.
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