The construction of a public sewer line begins with the excavation of trenches. Then manholes are set and finally pipe is laid between the manholes. During the installation of sewer lines, a pipe laser is used to maintain a constant rate of fall. A laser ensures the pipe leading out of a manhole falls at the same rate as the pipe flowing into the next manhole and that the rate of fall for each pipe in between the two manholes is equal to their rate of fall as well. This prevents crests and valleys in the pipeline that cause backups and clogs.
Set up the laser level next to the upper manhole -- the manhole whose base is at a higher elevation. Spread the legs of the tripod into a triangle and drive the legs into the ground by stepping on the foot tabs. Put the laser level on the tripod and adjust the bubble level knobs until the bubbles indicate that the laser is shooting on a level plane. Put the receiver on the grade rod and turn on both the laser and the receiver.
Place the bottom of the grade rod in the trough of the outflow hole at the bottom of the manhole. Adjust the receiver up and down on the grade rod until it makes a constant beep indicating grade. Record the number the grade rod indicates. Walk to the next manhole with the grade rod -- do not move the laser level -- and shoot the grade of the intake hole trough. The second recorded elevation is a larger number than the first, even though it is at a lower elevation. Subtract the first number from the second and record the result.
Measure the distance from the outflow hole of the first manhole to the intake hole of the second. Divide the sum of the elevation difference by the length of the run between the two manholes. This calculation is the amount of fall per foot each pipe between the two manholes requires.
Place the pipe laser in the outflow hole of the upper manhole. Turn it on. Adjust the fall-per-foot calibration with the "Plus" and "Minus" buttons on the pipe laser to match your calculation. Remember, for fall, the number the laser indicates has a negative symbol in front of it (-). For rise, the number is positive (+). Once the correct value is entered into the laser, it self-adjusts and sends a laser beam down to the intake hole of the second manhole from its position at the first.
Place the laser target -- it is simply a piece of Plexiglas with a pair of legs -- in the intake hole of the second manhole. Press the horizontal adjustment buttons -- right and left -- until the laser is centered in the bullseye of the target.