CD Apparatus Design September-October 2008

Fishing line is tied to the top of the float, from where it runs directly up and over two pulleys in series. The fishing line is then tied to the end of the lever arm at a length that zeros the float and the lever arm, while ensuring that the lever arm is at the correct height to zero the FC. The exact specifications for those calibrations can be found in the methods for the CD performance testing.

The lever arm is made of a 35 cm long flat metal bar that marked every centimeter like a ruler. It has a hole at the end, allowing it to pivot around a clamp on a support. The centimeter marks on the arm make it possible to select a lever arm position to attach the chemical dosing tube. As the tube is attached farther from the pivot, it experiences more driving head with each change in float height. This will allow us to calibrate each location along the arm for different alum dosages.

The chemical dosing tube is a tube with an open T connection at the top, which allows the FC outflow tubing to drain through a pressure break T. The tubing exiting the pressure break T has a larger diameter than the tubing entering it. The pressure break and larger outflow tubing allow a free surface at the same height as the discharge to the rapid mix (minus head loss) to form inside the larger tube while air can also flow through the tube. Currently ¼ inch outer diameter tubing is used to connect the flow controller to the pressure break T, but it is not known what size tubing is necessary to allow air to flow after the T. Experience has indicated that ½ inch outer diameter tubing is large enough, but it is possible that tubing with an outer diameter as small as 3/8 inch could work as well. The larger tube is then connected to 1/4 in outer diameter tubing, which connects the CD to the rapid mix. It is unclear if this final connection is needed, though, and it may be removed so that the larger tubing connects the T to the rapid mix directly.

Design Modifications and Improvements

These modifications were temporary fixes to increase the precision of the lab data collection, not attempts at design modification for field use

Balancing the T connection at the pressure break

The flow controller (FC) outflow tubing connects to the T through two plastic fittings, which created an unbalance in the T, causing it to tip towards the FC. This was fixed by balancing the T connectors on the FC side with two weighted nuts on the other side.

Reducing erroneous variation in lever arm-induced driving head

The chemical dosing tube and its connection (a metal rod and clamp) to the lever arm were too heavy for the float used. The combined weight was causing the lever arm to sag down and cause many more centimeters of driving head at the farther distances down the arm than closer to the pivot. This makes logical sense based on the principle of moments around a pivot. The whole connection was taken off of the dosing tube and replaced with a simple string that tied around the lever arm instead of clamping to it. The string was taped in place to prevent slipping, and it appeared to greatly reduce the problem of changing driving head from the moment induced by the weight of the doser. This is not a robust solution for implementation in Honduras, but it was a good solution to increase precision of data collection in the lab setting.