Fig.2 shows the concept graphically. W is coating weight, P is pressure of air knifechamber, D is distance from air knife nozzle to strip and V is line speed. If the previous step is denoted by index k − 1, current step is indexed as k. When weld point of two coils pass through air knife, there may be large change on coating weight from ¯ Wk − 1 to ¯ Wk. This is called as long term change.