E. Bubbles, Axes, and Optics, Oh My!

1. Bubbles

Most surveying equipment has a bubble to facilitate being correctly oriented to gravity. Two kinds of bubbles are used: bubble tube and circular bubble. Figure E-1 shows top and side view of both.

Top View

Side View

a. Tube

b. Circular

Figure E-1


The axis of the bubble tube (ABT) is a line tangent to the center of the upper curved surface of the bubble, Figure E-2.

a. Tube

b. Circular

Figure E-2
Axis of the Bubble Tube

Technically, since a circular bubble is spherical, the ABT is actually a plane, Figure E-3, but we can refer to its edge view as the ABT.

Figure E-3
Circular Bubble Plane

Bubble sensitivity is an expression of surface flatness. It is the angle at the curvature center for a specific surface distance, usually 2 mm, Figure E-4.

Figure E-4
Bubble Sensitivity

A less curved (flatter) surface has a larger radius and smaller angle than a more curved surface. A flat surface means the bubble responds more quickly (is more sensitive) to vertical movement. Most contemporary tube bubbles have a sensitivity in the range of 20" to 30 ", a circular bubble is typically 10'.