An improved regulator is provided for varying the rotational speed of a screw conveyor arranged to supply material to a variable-volume chamber having a piston slidably mounted in a cylinder. The regulator produces a sum signal proportional to the algebraic sum of positive input signals reflecting the desired degree of chamber filling and the desired discharge volume, and negative input signals reflecting the actual filling volume and the actual degree of chamber filling. This sum signal is converted to a proportional time-dependent signal which is used to adjust the speed of the conveyor. The conveyor speed is increased or decreased by an amount depending on the magnitude and polarity of the sum signal. In use, the regulator practices an improved method of regulating the speed of a conveyor.