PURPOSE:To enable accurate online measurement of density of a liquid containing massive sludge continuously, by switching the supply of a cleaning water for a filter to supply the cleaning water to a vibrator. CONSTITUTION:In the course of a conduit 1 on the upstream side from a vibrator 3, a filter 5 for trapping sludge in a liquid is provided, a cleaning water supply pipe 6 is connected to the conduit 1 between the filter 5 and the vibrator 3 and a cleaning water discharge pipe 9 to the conduit 1 on the upstream side from the filter 5. Moreover, a bypass tube 11 is connected to the conduit 1 on the upstream side from the discharge tube 9 and a liquid flowing through the conduit 1 is discharged through the bypass tube 11 at a fixed cycle, while a cleaning water is supplied to the filter 5 from the supply pipe 6 to remove sludge adhering to the filter 5. Then, the supply of the cleaning water is switched to supply the cleaning water to the vibrator 3 from the supply pipe 6 and after the removal of sludge adhering to the vibrator 3, the supply of the cleaning water to the vibrator 3 is continued upto the subsequent measurement thereby enabling accurate measurement of the density of the liquid containing the sludge continuously.
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