...
The float design parameters can be determined using a moment balance around the pivot of the lever arm. As seen in Figure 1 above, the major forces acting on the lever arm are the center of masses of the lever arm on either side of the pivot, the weight of the dosing slide, the alum dosing tube, and the components of the float. A moment balance was performed in order to determine what mass of float would be required in order to balance the lever arm at the lever arm angle corresponding to maximum plant flow. The forces due to the masses of the lever arm to the left and right of the lever arm cancel out. All that remains then acting on the lever arm is the force due to the alum tube, the weight of the sliding scale, and the tension caused by the float. The weight for the float is then changed until the moment about the pivot point becomes zero. The weight found makes the lever arm perfectly balanced at a maximum dosing rate of 100 mg/l at the maximum angle the doser arm will experience (max flow rate).
Figure 2
...
Figure 2 shows the orientation the doser arm experiences through min/max plant flow rate and how the float level changes accordingly.
Figure 2: Lever arm/float orientation
The formula for the moment balance can be seen below.
...