A solid concentration is an important factor for supervising operating conditions of hydraulic dredgers.
　Some experiments on measurement of solid concentration by differential pressures were conducted and discussed those characteristics to practical applications.
　Results are summarized as follows
　(1)In a case of fine sand or coarse sand, the delivered solid concentratinn and both measured soild concentration which are obtained by differential pressures in U-pipe method (using up and down flow) and Shingle-pipe method (using up or down flow only) are strongly interrelated.
　(2)An accuracy of solid concencentration obtained by Bend-tube method (using differential pressures at three points in an are shape tube) is unsatisfactory beca
use of a centrifugal force and secondary flow in a tube.
　(3)Even though a case may not be ignored the effect of settling velocity of particles, the solid concentration measured by Single-pipe method represents exactly with the solid concentration in a pipe. Especially the accuracy for the use of the up flow is better than that of the down flow.