THE EFFECT OF FINE BUBBLES ON MEASUREMENTS OF CONCENTRATION AND PARTICLE SIZE DISTRIBUTION OF SUSPENDED SEDIMENT USING ULTRASONIC WAVE

Ultrasonic attenuation spectrometer has attracted attention as a new measuring device for suspended sediment concentration and particle size distribution. It is necessary either to degass the samples nor to make measurement in a high frequency band where bubbles do not significantly scatter the ultrasonic waves since ultrasonic waves are greatly attenuated by fine bubbles. The ultrasonic attenuation spectrometer developed by the authors has a measurement frequency 1 to 10 MHz and fine bubbles may affect attenuation of ultra-sonic wave. Also, in the mountain rivers the water depth is shallow and the flow speed is high, so bubbles are likely to occur. In this paper we made measurement the attenuation characteristics of ultrasonic waves due to fine bubbles with diameters below 0.1 mm and verfied the effects of these bubbles on measurement of suspended sand concentration and particle size distribution in the measurement of mountain rivers.