- What are your power output measurements (in microwatts, or uW)? This information will help us help you troubleshoot. Specifically, if you could let us know what the power output was on the tenth day (when the LED blinked), on the 16th day, and on the 17th day (when it dropped to 60% of the previous day), this should be very helpful.
- Was there any point when the peak power seemed to mostly stabilize, such as not changing by more than 0.5 uW to 1 uW for two or three days in a row? For example, when the values "increased" from days 10 to day 16, how large was the increase each day?
- What type of soil are you using? Some types of soil (such as potting soil) will not work well with the microbial fuel cell (as described in the Materials section).
- You have probably already checked this, but can you confirm that the hacker board is set up correctly, as described in the "Setting Up the Microbial Fuel Cell" section of the Procedure in steps 21-25? Be sure and check that all wires are securely in place (try wiggling them a little) because if any wires are not making the right connections on the board, then the LED may not light up.
- Have you added any salt yet? (I am guessing the answer is no, but I just wanted to check.)
The idea is that the microbial fuel cell's power output should stabilize after about ten to 12 days, but this is affected by different factors, such as the soil used. Changing the temperature of the fuel cell can also affect its power output, so it is best to leave it in the same location the entire time you are testing it. Once the power output is stable, salt can be added to see how this addition changes the power output of the fuel cell.
Based on the data collected so far, it looks like the fuel cell was doing well until it was moved (right after the measurements were taken on day 10?). Moving the fuel cell looks like it caused the activity of the bacteria to greatly decreased (many likely died from the change, or if it was switched to a colder temperature, this may have slowed the bacteria down). I'm not sure what conditions you are keeping the fuel cell in now, but it should ideally be kept at room temperature (around 67 to 72 F or so) and kept out of direct sunlight, as this can greatly heat up the fuel cell.
Depending on how much time you have before the science project is due, I'd recommend one of two approaches: (1) wait a few days to see if the power output comes back up a little and stabilizes or (2) start the project over.
If you take approach #1, it's quite possible that the bacteria are still recovering from a shock and the power output may increase in the next few days and then stabilize. Even if the measurements remain low and/or vary by about 1.5 uW over a period of three days, it may be worth it to go ahead and add salt at that point. (For example, the fuel cell seemed pretty stable over days 13 to 15, but I'm not sure what caused the drop after day 16.) Also, if the fuel cell seems very damp inside (you mentioned you see moisture accumulating), this could be a problem too -- you want to keep the soil moist, but it shouldn't be too wet. You can carefully pour off any extra water in the fuel cell (see step 15 in the first part of the Procedure for how to do this).
If you take approach #2, you'll need at least 35 days (ideally a bit more) to do the entire experiment, but if you are short on time you could just try to do one salt addition and see how this changes the power output over the next four or five days. Again, I'd go ahead with adding the salt as soon as the measurements vary by only about 1 to 1.5 uW over three days. It should stabilize after around 10 to 12 days, but this is a rough estimate because there can be a lot of variability in how quickly soil bacteria grow. Be sure not to move it or change the room temperature, etc., after setting up the fuel cell.
As a side note, I'm surprised you didn't see any LED blinking on day 16 (before the last drop) because you saw it blink at lower power outputs before (and typically it starts blinking somewhere between 5 to 15 uW). Since you are taking measurements with a multimeter, the LED blinking measurements are not as important, but you could try testing the LED with a battery to make sure the LED is still working -- see Figure 5 in this project idea for how to do this: http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p100.shtml#procedure
All of that said, a science project does not have to work as anticipated in order to be real, good, and good-grade-worthy science. From a grade perspective, the student needs to communicate (on their display board, report, or however they are presenting their projects) their question, hypothesis, and what they anticipated would happen, and then what they actually saw (the voltage measurements), their questions about what was happening, their attempts to trouble shoot, and then possibilities of what could be happening and how they'd test that if they had more time to do so.
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