Development and Its Application of Flow Velocity Anemometer Using Ultrasonic Waves in Underground Airways

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The anemometer using with two 40 kHz ultrasonic transducers set near wall of underground airways (hereinafter called an ultrasonic anemometer) has been developed. The anemometer measures the difference in travel time between two ultrasonic pulsewaves along and against the airflow. One transducer is located upstream of the other and transmitting of the waves are synchronized. The travel times of the ultrasonic waves in the upstream and downstream directions are measured over the same path. The difference in travel time is proportional to the airflow velocity. Analogue output and recording system using a simple composition circuit was used. And a digital counter circuit to measure the time differences was utilized to calibrate its analogue output as against airflow velocity.Some experiments with ultrasonic anemometer related to airflow velocity were executed compared with a thermal anemometer at a laboratory. From the results, it is presumed that its measurement error is within 3% and its resolution is minimum 0.03 m/s.The ultrasonic anemometer has been investigated and examined at underground mine. The anemometer doesn't become an obstacle for passing vehicles or workers going through its setting area. The measurement error to estimate average velocity is relatively small and can detect flow direction. When vehicles or workers pass though the measurement point, sudden increase or decrease of output voltage occurs. Using with these output characteristics, a method is presented to recognize a passing. The ultrasonic transducer did overcome disturbances of dusts by self/cleaning of its vibrating surface. In case of the examination in dusty airflow of the underground mine, it has been operated for few months without a maintenance.