Concentration of a gas flowing inside a pipe was measured by using ultrasound from the exterior of a pipe without extracting the gas from the pipe. The concentration of the gas can be measured based on speed variation of the ultrasound traveling inside the pipe. Drilling a hole in the pipe to extract the gases is not necessary for the gas concentration measurement. In this paper, the concentration of hydrogen gas in the air was measured accurately while the air was flowing inside the pipe. Various airflow speeds were examined, and it was estimated that it is possible to measure the gas concentration for more than about 60 m/s of the airflow rate. The response time for measuring gas concentration was less than 0.1 s.
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