Method of monitoring and controlling yield stress in rheologically plastic fluid

The method comprises (a) flowing the material in a forward direction through a chamber having an exit through a restricted passageway so as to produce a positive pressure in the chamber, (b) stopping the forward flow of material into the chamber and allowing the residual positive pressure in the chamber to stabilize and reach a constant value, (c) recording the residual positive pressure in the chamber under no flow conditions at the constant value reached in step (b), (d) withdrawing material from the chamber so as to produce a negative pressure and initiate a reverse flow of material into the chamber through the restricted passageway, (e) stopping the withdrawal of material from the chamber and allowing the negative pressure to stabilize and reach a constant value, (f) recording the residual negative pressure in the chamber under no flow conditions at the constant value reached in step (e), and (h) determining half the difference between the residual positive pressure and the residual negative pressure as a pressure drop proportional to the yield stress of the material. The pressure drop is determined on-line while the material is part of a process stream. The restricted passageway is a capillary passageway, a rectangular channel, a slit channel, a square channel or a tapered passageway.