Abstract: Apparatus and a method for the controlled drying of polymer plastic pellets within a dryer hopper are provided. A sensor tree having a plurality of temperature sensors spaced vertically on the tree is positioned vertically within the dryer hopper so that the individual sensors are each located at varying vertical distances from the bottom to the top of the dryer hopper. A target temperature for a particular type of polymer plastic pellet, when maintained for a specified residence time, indicates that the pellet is thoroughly dry for use in a manufacturing process. By determining the vertical level at which the target temperature has been achieved for the desired residence time, the amount of dry material within the hopper that is ready to be input to the manufacturing process is determined. A signal processing unit operating a software program automatically controls the throughput of the dryer hopper so that only dry polymer plastic pellets leave the hopper.