In this part of the procedure, we will test our artificial pancreas model. We will do this by first normalizing it to a neutralized solution to make sure the pump will turn off once our solution is neutralized. We will then put the conductivity sensor in an acidic solution (i.e., pure vinegar), which will make the pump move a basic solution (i.e., a solution of baking soda) into the bowl of acidic solution until the solution is neutralized and turns off the pump.
2.5.1 Take three mixing bowls and label them "Neutralized," "Vinegar," and "Baking Soda."
For labeling, we can use masking tape and a permanent marker or small sticky notes and a pen or pencil.
On a scale, place a measuring cup or other small container to weigh baking soda on the scale. Zero out the scale and then weigh out (g) of baking soda.
Put the 28.6 g of baking soda into the mixing bowl labeled "Baking Soda."
Use the graduated cylinder, or a metric measuring cup, to measure out 400 milliliters (mL) of distilled water. Add the 400 mL of distilled water to the baking soda in the mixing bowl.
2.5.2 Mix the water and baking soda until the baking soda is completely dissolved.
Measure out 100 mL of the baking soda solution and add this to the mixing bowl labeled "Neutralized."
Tip: We would want to use a small cup with a spout to transfer the baking soda solution.
Measure out 100 mL of distilled white vinegar and very slowly add it to the "Neutralized" bowl.
Caution: Mixing an acidic solution with a basic solution can cause a powerful chemical reaction. We must pour the vinegar into the bowl very slowly to give the two solutions time to slowly react, otherwise we may end up with a big mess and will need to make up fresh solutions!
What happens as we pour the vinegar into the baking soda solution?
Once the reaction has slowed, slowly mix the solution to make sure the vinegar and baking soda have completely reacted.
Note: The amounts of vinegar and baking soda we are using are the same. Because of this, the acid and base should react and neutralize the solution.
Optional: If we want to see what the pH of the neutralized solution is, we can measure it now using pH test strips and record it in our lab notebook. Note that the pH will not be 7, but may be closer to 6, because of buffering effects of the solutions.
Measure out 200 mL of distilled white vinegar and carefully pour it into the "Vinegar" bowl. Take the tubing from the pump and place both ends in the "Vinegar" bowl.
Optional: If we want to see what the pH of the vinegar is, we can measure it now using pH test strips.
Once the vinegar-baking soda solution stops reacting, the solution should be neutralized. Carefully place our conductivity sensor in the "Neutralized" bowl, letting the straw part be submerged and the Styrofoam piece float on the surface, as shown in Figure 22, below. our overall setup should now look similar to the one in Figure 23, below.
If the Styrofoam piece is not floating evenly, we can try taping the test leads onto the rim of the mixing bowl to keep things in place.
Figure 22. Place the conductivity sensor in the neutralized solution so that the Styrofoam piece floats and the straw part with wrapped wire is submerged.
Figure 23. When we are equilibrating the artificial pancreas circuit in a neutralized solution, our setup should look like this one.
- The pump may start running as soon as we put the conductivity sensor in the neutralized solution, but do not worry if the pump is not running yet. (When the pump runs, vinegar should simply be pumped out of, and then back into, the "Vinegar" bowl.) In this step, we will normalize our artificial pancreas model so that the pump does not run in a neutralized solution, but still runs in a solution that is slightly more acidic (which will be more conductive). We will do this by adjusting the potentiometer (the blue component with the white knob that we put in holes B24, B25, and B26).
- Remember that a potentiometer is a variable resistor; we can change its resistance by turning the white knob. When we change the resistance of the potentiometer, this affects how much voltage is sent to the transistor, which controls whether the pump is turned on or not. If we want to find out more about how this works (it involves forming a voltage divider with the conductivity sensor).
- If the pump is not running, slowly and gently turn the white knob on the potentiometer until the pump turns on. Try turning it all the way clockwise and all the way counter-clockwise find out which way turns the pump on (which way we need to turn it will depend on which way we put the potentiometer into the breadboard).
- Once the pump is running, very slowly turn the potentiometer's knob in the opposite direction to turn the pump off. Stop turning the knob when it is just reaches the point that makes the pump turn off. we can play around with adjusting the knob until we are satisfied that the pump does not run in the neutralized solution (but will still run if turned slightly).
- While we are adjusting the potentiometer, identify which pump tube has liquid flowing out of it. When the pump is not running, dry the end of this tube and mark it with a small dot using a permanent marker. This will help we in the next step when we need to pump a liquid into a different bowl.
- Safety note: Do not leave the pump running unattended, and do not let the circuit run for more than about 15 minutes at one time. Note that the transistor may become warm while the pump is running, but it should not become dangerously hot. If it is very hot, or if we notice any smoke or a burning smell, this probably means that we have a short circuit. Immediately disconnect the battery pack from the breadboard, and make sure that everything else is connected correctly by referring to the diagrams above. Just one misplaced wire can prevent the circuit from working, or create a short circuit! Remember to make sure that the exposed metal parts of different components, like the resistors and alligator clips, are not bumping into each other, as this will also create a short circuit.
- Note: If the pump does not turn on, no matter how we turn the potentiometer's knob, check the following:
- Make sure all of the jumper wires and components are pushed firmly into the breadboard's holes. A single loose wire can prevent the circuit from not working.
- Make sure no exposed metal parts (like the leads of the resistors) are touching each other, as this will create a short circuit.
- Be especially careful to avoid creating a short circuit by having wires from the red and blue bus strips touch each other. This can make the circuit get dangerously hot and can even melt some of the plastic components.
- Once we have normalized our artificial pancreas model so that the pump does not run when the conductivity sensor is in a neutralized solution, carefully remove the conductivity sensor from the neutralized solution (leaving the pump's tubes in the "Vinegar" bowl), and rinse the sensor briefly with some vinegar (over a sink or a different bowl). This will help remove the neutralized solution from the sensor.
- Now leave the pump tube that we marked in step 10.c.i. (the outlet tube) in the "Vinegar" bowl. Take the other pump tube (the unmarked, inlet tube), wipe the outside down with a paper towel or clean rag, and then place it in the "Baking Soda" bowl.
- Next, place the conductivity sensor in the "Vinegar" bowl. our setup should look like the one in Figure 24, below. The pump should start running, pumping baking soda solution (a drop or a few drops at a time) into the bowl of vinegar, and we should see bubbles being made as the acid-base reaction takes place.
- Note: Make sure the sensor is floating the same way that it was in the neutralized solution. If needed, tape the alligator clip test leads to the side of the bowl to hold them in place so that the Styrofoam piece is floating evenly. It is very important to make sure that the sensor is submerged in the liquid to the same depth that it was in the neutralized solution or our results may be inaccurate.
Figure 24. When we are neutralizing the vinegar solution with baking soda, this is what the setup should look like.
- While the pump is running, carefully and continually move the end of the pump tube in the "Vinegar" bowl so that the baking soda mixes well with the vinegar throughout the bowl (including under and around the sensor). It is very important to have all of the baking soda and vinegar mixed together to neutralize the vinegar solution.
- Eventually, the pump should slow down and then stop running. It might turn on and off as we mix the last bits of baking soda that is pumped in; if it does this, wait until it stops running for at least 10 seconds before moving on to the next step.
- Optional: If we want to see what the pH of the solution in the "Vinegar" bowl is now (it should be neutralized), we can measure it using pH test strips and record our results in our lab notebook.
- When the pump stops, measure how much baking soda solution is left in the "Baking Soda" bowl by carefully pouring it into a metric measuring cup or a graduated cylinder using a funnel. In our lab notebook, record how much baking soda solution is remaining.
- Note: There may be more liquid in the bowl than can fit in the measuring cup or graduated cylinder, so we may need to fill it up (and empty it out) multiple times to measure the total amount of baking soda.
- Analyze our results and determine how accurate this artificial pancreas model is.
- Since the baking soda solution we prepared is at the same concentration as the vinegar, they should make a neutralized solution when the same amount of each have been mixed together. This means that 200 mL of vinegar should be neutralized with 200 mL of baking soda solution. Because the "Vinegar" bowl had 200 mL of vinegar (from step 8) and the "Baking Soda" bowl had 300 mL of baking soda solution (since we prepared 400 mL in step 4, but removed 100 mL in step 6), the pump should have ideally stopped when 100 mL of baking soda was remaining in its bowl. How close were the results (from step 16) to 100 mL baking soda solution? Was there too much or too little baking soda remaining?
- There are many reasons why it may not have taken exactly 200 mL of baking soda solution to neutralize the 200 mL vinegar solution. Here are some possible sources of error, but we may think of additional ones:
- The circuit may not have been accurately normalized (in step 10).
- The potentiometer might have been bumped during testing.
- Some extra vinegar may have been added to the "Vinegar" bowl from rinsing the conductivity sensor.
- Liquid from either bowl may have been spilled during testing.
- The baking soda solution was not stirred enough while it was being added to the "Vinegar" bowl. If this happens, the conductivity sensor may still detect an acidic solution, even though parts of the solution in the bowl have been completely neutralized (or may even be basic).
- Mistakes in preparing the baking soda solution should not actually be a source of error.
- Note: Because pH reactions occur on a logarithmic scale, the error measurements can be on a logarithmic scale, too. This means that a margin of error that may seem large (such as having 150 mL baking soda leftover instead of 100 mL) is actually not that big.
- Think about how each possible source of error may have affected our results.
- Plan how we could change our model and/or our testing procedure to make the model more accurate. Whatever we decide to change, be sure to record our plans in our lab notebook. Specifically, think about:
- What could we physically change about our circuit, conductivity sensor, or experimental setup? For example, could we improve the stability of our sensor if it was moving around, or build a new sensor with some changes to the design?
- What could we change about the procedure we used when doing the experiment? For example, could we somehow stir the solution in the "Vinegar" bowl more evenly while the pump is on and pumping in baking soda solution?
- Clean and dry the mixing bowls and repeat steps 1–17 to test our model again, with the changes we decided on in step 18, and analyze its results.
- Note that this model may not necessarily be more accurate than the original model. This is part of the challenge of the engineering design process.
- If we want, we can change our model and/or testing procedure even more by repeating steps 18–19 one or more times.
- We can make a bar graph of our results, with a bar for each time we tested the model (labeled on the x-axis) that shows how much vinegar remained when testing each time (labeled on the y-axis in mL).We can draw a horizontal line across the graph at the "100 mL" point to show the ideal amount of baking soda left.
Then we took the pH level of the vinegar and baking soda.Vinegar was 4 and baking soda was 9.