Abstract: Consistency of slurries such as paper pulp stock is controlled by ultrasonically measuring the flow of the slurry (the velocity of the particles therein) as the slurry is agitated by an impeller which induces circulation thereof within a region in a tank containing the slurry. Dilutant is added under the control of the measurement and a mixed slurry of desired consistency is obtained, respectively at an inlet and an outlet in communication with the region of the tank where flow is being induced and measured. Consistency control is obtained in the containing vessel or tank with greater accuracy and at lower cost than in apparatus where consistency is measured outside of the vessel containing the slurry.