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In clarifier tanks in water plant or treatment plant it is necessary to locate and control the sludge level in order for efficient production and environment protection. In tanks, where sedimentary reaction take place, the sludge concentration vary gradually in vertical, normal abrupt- change based measurement techniques will fail. In this paper a concentration-dependent level measurement technique is described. The concentration-related signal is obtained through the measurement of attenuation of ultrasonic wave in water. The level is then resolved according to either the sludge concentration itself or the vertical concentration gradient. An ultrasonic sensor probe is designed. It operates at 10 MHz frequency in order to obtain required sensitivity. The probe is of immersed, and is non- interceptive to sediment thus will not disturb the concentration contribution. As temperature and contaminant will affect sensor response, an in-situ auto correction technique is developed to eliminate such response change. In the system three sensors are incorporated in, an ultrasonic sensor, a temperature sensor and a position sensor. A micro control unit controls the whole system. The attenuation property of sludge water is presented. The system has found its usage in water plant and is proved to be successful.
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