Mixing Stirring Mechanism

One of the major problems that we faced is with the mixing of the solution. After we did a few run on the system, we realized that the system response is very laggy and tends to have overshoot. Hence we tested the system with colored food dye and we found out that the dyed solution takes a long time to diffuse throughout the mixing chamber. So the pH obtain from the pH probe is actually the pH at time around T – 10s. Even if the pH has already reach the set point, the system does not know it and thus keep adding more solution which cause it to overshoot. Similarly, when it tries to control it down, it will overshoot and fluctuate endlessly. Hence the system requires a good mixing mechanism. Figure below shows how we implement the mixing mechanism in the system.


In our implementation, an addition pump is employed to force the solution to mix. The solution will first be suck out of the mixing chamber by the pump and being force back into the mixing chamber from the top. The position of the re-entry of the solution is very important. The way we implement is force the solution inside the mixing chamber to stir by using the force of the solution entering the mixing chamber. Figure below shows a mixing test using food dye.


(1) The red dyed solution entering from one of the inlet.
(2) Solution being sucked out quickly by the mixing mechanism.
(3) – (4) Colored solution being reintroduced into the mixing chamber.
(5) – (8) Solution mix well in the mixing chamber.

The mixing process takes around 5 sec. We tried the mixing process for both inlet and we still obtain the same result. Initially we are worried about having the solution entering from 2 different inlets (Acid/Base). After this test, we are sure that no matter where we introduce the solution, the pH reading will not be affect because the mixing mechanism is able to mixing the solution quickly.

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