Flow rate measurement for hydrogen gas is very difficult because the density of this gas is very low and the sound velocity is very high. This difficulty was overcome by improving the structure of the conventional ultrasonic flowmeter, and the ultrasonic flowmeter for hydrogen gas developed herein was evaluated for both steady and unsteady flows. Although the error for the flow rate measurement was slightly negative, the scatter was within +- 1percent for the range from 30 to 240 L/min at a pressure of 100 kPa. It is thought that the flow rate can be measured to within +- 1percent by calibration.
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