How to adjust the bubble in the long tube of theodolite to be centered on three sides! It is centered on three sides, not 90 degrees.

1. First, set the shelf and center it, and then adjust the height of the leg to center the circular level.

2. When rotating together, the leveling tube is parallel to the12 angle screw. Adjust the12 angle screw to center the leveling tube.

3. Turn 90 degrees, and the leveling tube is perpendicular to the 3-angle spiral. Adjust the 3-angle screw to center the leveling tube.

4. Repeat steps 2 and 3 to keep the horizontal tube horizontal at both sides.

5. Check the alignment. If it is offset, move the instrument and repeat steps 2, 3 and 4.

6. Check the alignment. It's ok if it's not cheap. If it is offset, repeat 2, 3, 4 and 5 until it is normal.

Theoretically, if there is nothing wrong with the instrument, do steps 2 and 3 again, and any surface is horizontal.