Interaction between the dosing and weighing system is of great importance to obtain consistent net weights. The weighing system immediately observes the dosed weight and controls the dosing system.
Control of the dosing system consists of starting and stopping the coarse and fine dosing. With screw dosing systems there is also the opening and closing of valves on the coarse and fine dosing screw to allow the sudden interruption of the product flow.
During the dosing cycle the weight increases until the front exit is reached. The weight then continues to increase because there is still product arriving. The product that has left the dosing device but not yet arrived on the weigher is called after-run.
Pressure also originates on the weigher due to the impact caused by the product on the weigher after the drop. Although this pressure practically fully falls away after changing from the coarse to fine dosing cycle, it shows a visible temporary spike in the weight.
The interaction between the dosing and the weighing systems accordingly requires optimisation, whereby the weighing system must anticipate what further drops from the dosing system. This is why the weigher will cut off a certain weight before the final weight.
The after-run value is automatically calculated in an electronic weighing module. The weighing module does this after a weighing operation has ended. The module then waits until a stop and compares the weighing with the set value. The module will correct the after-run value with a percentage of the measured difference.