3. Accurate Leveling

If the circular bubble is in adjustment the TSI will be nearly level. To get it accurately level requires using the tube bubble. Not only is tube bubble more sensitive than the circular one, it also rotates with the instrument so the level can be accurately checked in various directions. 

The process of accurately leveling the TSI is based on the principle of reversion and ABT/VA relationship described in Chapter B. Axes, Maladjustment Effects. Each step is accompanied with an illustration of a top view of the tube bubble and tribrach showing bubble orientation and leveling screws used.

Step (1)

Rotate the TSI so the ABT is parallel with the line through two leveling screws (it doesn't matter which two).

Using just those two screws, center the tube bubble.

Figure C-8
Center Tube Bubble Using Two Leveling Screws

Step (2)

Rotate the instrument 180° so the TSI's position is reversed.

If the bubble runs off center, use the same two leveling screws to bring it halfway back.

For example, if it runs two divisions, bring it back one division.

Figure C-9
Rotate TSI 180° and Check Bubble Run

Step (3)

Rotate the TSI 90° so the ABT is at a right angle to the first two screws.

Using only the third leveling screw, center the bubble.


Figure C-10
Center Tube Bubble Using Third Leveling Screw

Step (4)

Rotate the TSI 180° so the TSI's position is reversed.

If the bubble runs, bring it back halfway using the third leveling screw only.

If the bubble ran in Step (2), it should run the same amount in this step. This indicates a systematic error.

Figure C-11
Rotate TSI 180° and Check Bubble Run

Step (5)

Check the level in all directions by rotating the TSI.

If the bubble stayed centered throughout the leveling process, then it should stay centered regardless where the TSI is rotated.

If the bubble ran in Step (2) [and (4)] and was corrected back half the run, it should be off that same amount wherever the TSI is rotated.

If it doesn't meet either of these conditions, then there was either a leveling process error or the instrument set up isn't stable.

Step (6)

Check the optical plummet to see if it is centered on the ground mark.

If it isn't, loosen the instrument mount screw and slide the TSI to align the marks.

The TSI should now be accurately leveled and centered over the ground mark. If it is off the mark, loosen the instrument screw and slide the TSI (to-from, left-right) until it is over the point. Re-tighten the screw.

But wait ... the optical plummet could also be out of adjustment so it should be checked.