Engineer’s Level Adjustments
Regardless of how well an engineer’s level is manufactured, you should perform certain checks and field adjustment at regular intervals; for example, you should test the instrument every day before starting work. You also should check it for proper adjustment anytime the level is bumped or jolted. The parts of the level that you will check are the level tube and the cress hairs. For the latter, be sure that parallax is removed and that the cross hairs and objective are sharply focused. To do this, use a well-defined object at least 250 feet away. When parallax is present, the image is not exactly in the plane of the cross hairs, and the objective focusing must be refined. Since this condition can occur eachtime the objective lens is focused, you must make a parallax check each time you observe a new object. When adjusting the engineer’s level, it is important that you accomplish the tests and adjustments in a prescribed sequence. The reason for this is that one adjustment may depend upon, or alter, another adjustment. The following paragraphs describe, in proper sequence, the test and adjustment procedures that you should follow when checking and adjusting the engineer’s level.
Figure 6-1.—Adjusting the level tube.
ADJUSTING THE LEVEL TUBE.—The vertical axis of rotation of the instrument is the basis for all adjustments to the engineer’s level. When the instrument is set up and leveled the vertical axis of rotation and the longitudinal axis of the level tube should be perpendicular to one another. If they are not perpendicular, then the vertical axis cannot be made truly vertical. Adjustment of the level tube makes the axis of the level tube perpendicular to the vertical axis. To check and adjust the level tube, you should follow the procedures below:
1. Setup the instrument and approximately levelthe bubble over each pair of opposite leveling screws. Then carefully center the bubble over one pair of screws, as shown in view A, figure 6-1.
2. Rotate the instrument 180°. If the bubble remains centered, then the level tube is in proper adjustment. If the bubble does NOT remain centered note the movement of the bubble away from center (view B, fig. 6-1).
3. Bring the bubble half the distance back to thecenter of the tube by turning the capstan nuts at one end of the tube (view C, fig. 6-1).
4. Relevel with the leveling screws (view D, fig.6-1) and rotate the instrument again. Repeat Step 3 above if the bubble does not remain at the center of the tube.
5. Check the final adjustment by noting that thebubble remains in the center of the tube during the entire revolution about the vertical axis.
NOTE:When the level tube is out of adjustment, you can compensate for it by releveling the instrument before each sighting.
ADJUSTING THE HORIZONTAL CROSSHAIR.— For the horizontal cross hair to lie in a truly horizontal plane when the instrument is leveled, the horizontal cross hair must be perpendicular to the vertical axis. To make the horizontal cross hair (fig. 6-2) lie in a plane perpendicular to the vertical axis, you should perform the following steps:
1. With the instrument carefully leveled, sight oneend of the horizontal cross hair on a well-defined point at least 250 feet away. Turn the telescope slowly about the vertical axis, using the slow motion screw. If the cross hairs are in adjustment, the horizontal cross hair will stay on the point through its entire length.
2. If it does not stay on the point, loosen twoadjacent reticle capstan screws and rotate the reticle by lightly tapping two opposite screws.
3. Sight on the point again. If the horizontal crosshair does not stay on the point through its entire length, rotate the ring again.
4. Repeat this process until the condition issatisfied.
NOTE:To compensate for the above maladjustment, you should use only that part of the horizontal cross hair that is closest to the vertical hair for all sightings.
ADJUSTING THE LINE OF SIGHT.—For a perfectly adjusted level, the line of sight is parallel to the axis of the level tube. When the level meets this condition, the line of sight will generate a truly horizontal plane when the instrument is rotated. When
Figure 6-2.—Adjusting the horizontal cross hair.
Figure 6-3.—Two-peg test method.
the line of sight is not parallel to the axis of the leveltube, then you must adjust the line of sight. The method used for adjustment is known as the two-peg test (fig. 6-3). This method requires you to do the following steps:
1. Setup and level the instrument (first setup, fig.6-3). Drive stake (peg) A about 150 feet away, then drive stake B at the same distance in the opposite direction.
2. Take a rod reading a on stakeA and a rod reading b on stake B. With the instrument exactly halfway between the two stakes, b-a is the true difference in elevation between the stakes.
3. Move the instrument close to stakeA (second setup, fig. 6-3) so that the eyepiece is within a half inch from the rod. Then, by sighting through the objective-lens end of the telescope, take a rod reading c on stake A. Next, take a rod reading d on stake B in the normal manner. If the instrument is in adjustment, d-c will equal b-a.
4. If the instrument is out of adjustment, calculatewhat the correct rod reading e should be on the farther rod B (e = c + b - a). Set the rod reading e with a target for accurate reading. Move the horizontal cross hair to the correct reading (on target) by loosening the correct vertical screw and tightening the opposite screw.
5. Check the horizontal cross hair adjustmentagain. The ring may have rotated during this adjustment.
6. Rerun the peg test to check the adjustment.
NOTE:The compensation for the above maladjustment is careful balancing of your backlights and foresights.