We discovered that the "pinching"-the-tube technique to change the flow
rate of caustic into the main solution did not work. The motors just did
not have the power necessary to pinch the tubing together to stop the
flow. We therefore decided to let gravity determine the rate of flow.
To do this, first a hole was cut into the bottom of a cup, tubing was
placed through the hole, and the entire thing was sealed so it was
water-tight. Then water was placed in the cup. A device was made to
raise and lower the end of the tubing (a tubing-mover). When the END of the
tubing was above the level of the water in the cup, the flow out of the
tube stopped. When it was below the level in the cup, liquid flowed out
of the tubing. This worked because the pressure was the same in the tubing
and the top of the water in the cup. The speed of the flow was changed by
placing rubber bands and clamps on the tubing. Another cup with tubing
at the bottom was set up for the acid solution, though this will be a
constant flow and will not have to be shut on and off like the
caustic.
A stir rod was made to ensure thorough mixing of the acid and base so the
measured pH will be accurate.
The motors driving the stir rod and tubing-mover still have to be
converted from MIT motors to Lego motors. More importantly, the pH meter
still needs to be hooked up to the computer. Acid and base flow rates
will need to be calibrated in relation to each other (to keep the pH
fairly consistent around 7).
