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 FCM. The exact specifications for those calibrations can be found in the [methods] for the ACD performance testing.

The lever arm is made of a 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 for different plant influent turbidity readings.

The chemical dosing tube is an empty tube with a T connection at the top, which allows the FCM outflow tubing to drain into a larger tube that had a pressure break.

Chemicals (or in this case, water) flow from the bottom of the chemical dosing tube to a plastic column with a pressure sensor at the bottom. This pressure sensor gives readings of water height in the column, which can be converted into flowrates from the FCM.

Design Modifications and Improvements

Balancing the T connection at the pressure break

The flow control module (FCM) outflow tubing connects to the T in the selector, which created an unbalance in the T, causing it to tip towards the FCM. This was fixed by balancing the T connectors on the FCM 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 to the lever arm were too heavy for the fishing line 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 apparatus weight.