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SUMMARY
General design aspects of settling tube systems for fall velocity analysis of sand are reviewed, with emphasize being placed on detection methods for particle arrival at the lower end of the settling column and the various sources of errors. This is followed by a detailed discussion on the development of an improved settling tube system employing an underwater balance.
In order to achieve a high degree of accuracy, stability and damping, the weighing system includes a feedback mechanism which consists of a solenoid-magnet unit and a differentiator; this yields a resolution of 10 ~N. The overall accuracy of fall velocity is better than 3% (which takes into consideration concentration effect) and the precision is better than 4% (at a confidence level of 95%). Data acquisition and processing are performed by means of a microcomputer with a l2-bit AID converter, sampling in the velocity domain.
