This is a horizontal bubble in the middle. After rotating 180, the bubble deviates. How to correct the bubble?

The method is as follows: center in one direction and turn to 180 degrees. At this point, the center of the connecting line between the center and the bubble center position is the real center position of the leveling bubble. Adjust the two small screws beside the level gauge to the center of the connecting wire. The effect may not be obvious once, so adjust it several times.