b. Circular Bubble

(1) Description

The circular bubble is used to roughly level the instrument. On most instruments, the circular bubble is tribrach-mounted and does not rotate with the instrument so the Reversion Principle cannot be applied.

(2) Check

Level the instrument using the plate bubble and check the circular bubble. If it is centered, no adjustment is needed.

(3) Compensation
(a) Procedural

Use the plate bubble to level the instrument.

(b) Adjustment

Because the instrument was leveled with the plate bubble the amount the circular bubble is off is the actual amount of maladjustment error.
Use the circular bubble adjusting screws to center the bubble. The screws may be located around the top or bottom of the bubble.