Science Buddies: "Ask an Expert"

We are doing the pee power project using the microbial fuel cell. We have been tracking our data using the resistors
4670, 2190, 1000, 470, 220, 100 and 47. To calculate the Power output we are using the formula
P= Voltage squared/R. Some of the results appear to be really small. For example, the voltage reading at the 47 resistor was 12. The converted to 3.06 x 10 (-12 power) after using the formula.

We took the reading of 12 divided by 1000 before using the formula. so we had .012 x .012 = .000144

P = .000144 divided by 47 =.000003064 .000003064 / 1,000,000 = 3.06 x 10 (-12)

Does this make sense?

Thank you.

Hi,

Your calculations are ok up to the point where you convert watts to microwatts (uW). There are one million uW per W so you have to multiply your answer by 10^6. The result is 3.06 uW. That's a little low but still ok. What power did you get for the other resistors?

Sybee

The procedures for the project from Science Buddies contain two major errors. See excerpt from the experiment procedures below: Under 4b below, the instructions read to divide by 1,000,000 to arrive at microwatts when in actuality you must multiply by 1,000,000. Second, the equation for calculating Power is also incorrect but this was subsequently corrected by someone in the procedure section for this experiment. P=V2R is incorrect. It should be P = V squared/R. These errors in the written procedures cost us valuable time in doing our experiment


Excerpt From Procedures Section of Pee Power Experiment:

4. Calculate the power output (in microwatts, or μW) for each resistor. You can calculate this by using a derivation of Ohm's law shown as Equation 1 below.
a. To use Equation 1, you will need to convert your voltage readings from millivolts (mV) to volts (V). To do this, divide the millivolt values by 1000 to give you volts.
i. For example, if you had a voltage reading of 45 mV, this would equal 0.045 V.
b. Using Equation 1, your answer will be in watts (W). Convert watts to microwatts by dividing your answer by 1,000,000.
Equation 1:
P=V2R
 P is the power in watts (W).
 V is the voltage (V).
 R is the resistance in ohms(Ω).

Our power output numbers have ranged from below 1uW for a 47 ohm resistor to over 100 uW for a 470 ohm resistor depending on the day. We are now in day 13 and we are waiting for the power output to stabilize. Our last 3 days peak power numbers (without urine) have been

1000 ohms 75.076 day 11
1000 ohms 81.79 day 12
1000 ohms 77.84 day 13

We have not stabilized to 0.5 uW as described in the FAQ.

Thank you.

Kerry McGill

Hi Kerry,

I checked the procedure for the fuel cell project (https://www.sciencebuddies.org/science- ... #procedure) and the equation for power is correct: P = V^2/R

Did you copy or print the instructions? I tried doing a copy/paste of the equation and it came out as P = V2R

You are correct about the conversion of W to uW being wrong. Your value in watts has to be multiplied by 10^6 because one W = one million uW. You can see this better if you set it up dimensionally: (W) x (uW/W) = uW

I calculated your power output from the mV readings you gave:
Day 11. 75.1 mV 5.64 uW
Day 12. 81.8 mV 6.69 uW
Day 13. 77.8 mV 6.05 uW

I'd say you are stable because the difference between Day 11 and 13 is 0.41 uW and that is <0.50 uW. If you take the average it is 6.13 uW and that value is within 0.5 uW of Day 11 and 13 and only off by 0.06 uW on Day 12. I'd say that was close enough.

Sybee

Dear Sybee:

Thank you so much for the great insights. I did cut and paste so that explains the in the formula.

With respect to the data I listed for the last three days 11 through 13. Those calculations were already in microwatts (uW) so they fell outside the 0.5 uW criteria for stabilization. We are now at day 14 and the peak power is 96.62 uW which is up from 77.84 uW yesterday (day 13). I am concerned that the output may die before it stabilizes. It's a great project but a little long for a 7th grader.

Thanks again for all the help.

kbmcgill

Oh, I should have realized those were powers and not voltages--that's what you said!

I don't know what the lifetime of the mud bacteria is under artificial conditions. Bacteria grow and divide when there's enough food, but if the nutrients become limiting then they will start to die. 75-82 uW is a pretty good power output for the 1000 ohm resistor, so your bacteria seem to be healthy. I would make a note that the fluctuation is 4-6 uW and not the recommended 0.5 uW and then go ahead and add the urine. If you looked in the FAQ section of the project under the question about power not stabilizing, it says that this can happen for several reasons--differences in temperature, for example. Bacteria generally do better when it is warm. Is it possible that the cell was cooler or warmer on Day 11-13? It would be a good idea to use a digital thermometer to record the temperature in the room several times during the day and in the evening to see if there's a substantial fluctuation. If this were being done in a lab the cells would be held in a temperature-controlled incubator at a constant T.

Good luck and please keep updating us on the progress. We really like to know how a project works out especially if something unexpected happens so we can use the information to help the next student.

Sybee

Sybee:

The peak power for the last three days continue to increas 96uW, 112uW and 118uW so we still have not stabilized asfter 16 days. If the peak power was within a few microwatts I would say add the urine but we are still increasing. Any thoughts?

Thank you.

Kbmcgill

Well, I have two questions:

1. Where did you get the mud?

2. Does the temperature fluctuate much where the cells are located?

Since the power is increasing and the bacteria are the source of the electrons, I would say that the bacteria must be growing and dividing so that their numbers are increasing. I don't know any other explanation for your results. The only way the bacteria can grow is if they have plenty of nutrients and if the temperature is warm. If your pond is especially rich in organic matter and the cells are located in a warm place then you may just have an especially good microbial fuel cell.

You wouldn't happen to have two cells going at the same time would you? That would have been the best way to do this experiment because then you could keep one as a control while you added urine to the other.

Have you plotted the power vs time? If the power is increasing at a steady rate then this should be apparent on the graph. If you are running out of time, I would go ahead and add the urine and plot the power outputs. If the urine stimulates the bacteria you will see an increase in power--the slope of the curve will increase compared to what it was before you added the urine. Have you tried measuring the power a couple of times a day just to see how much it fluctuates?

Let me know what you think.

Sybee

The mud is actually from Scotts Humus and Manure soil from Home Depot. This was one of the sources listed in the procedure section from Science Buddies. I don't think that the temperature has fluctuated that much. Your suggestion that we record temperature is an excellent idea and should probably be added to the procedures for the experiment. I have graphed the peak power output vs time and will send that to you. I purchased an extra MFC but did not add the mud to that one. If we do it again, I would run two MFCs. I will try to measure the peak power a couple of times per day and see what that reveals.

I can't thank you enough for your help. It is nice to be able to communicate with someone who understands the project.

The attached peak power chart is up through day 13. I need to add the last three days which were 96uW, 112uW and 116uW.

Kbmcgill

Peak power chart updated through day 16.

Kerry,

There was no link to the file you attached at the bottom of your post. Are you sure it was less than 256 kB? If it is too large you could upload it to Google docs and then send me the link for that.

That is very interesting that you are getting such good results with a commercial soil blend. Sometimes these are sterilized and of course then they would not work at all because the bacteria would be dead.

When do you have to finish up the experiments and write up the project? As you said, it would have been good if you had started a couple of fuel cells at the same time. In the lab we do the same experiment at least three times so we can average the results and do some statistical tests. Multiple readings allow you to calculate the mean and standard error and then you can do a statistical comparison to prove that your test differs from the control at a confidence level of 95%.

Keep plotting your data and monitoring the output. It has to stop increasing eventually so you can add the urine--hopefully at least a few days before the project is due!

I'm glad you find my suggestions useful. That's our goal and we like to know that it is achieved sometimes.

Good luck!

Sybee

peak power graph through day 16.

peak power through day 16

Kerry,

I'm still not getting your attachments. What do you see when you do a preview of your post? There should be a link at the bottom to the attachment. If not then it is getting blocked somehow. I checked the FAQ and found this:

Why can’t I add attachments?
Attachment permissions are granted on a per forum, per group, or per user basis. The board administrator may not have allowed attachments to be added for the specific forum you are posting in, or perhaps only certain groups can post attachments. Contact the board administrator if you are unsure about why you are unable to add attachments.

I will check with the admin person and see if i can find out why there's no attachments. As I suggested, you can upload your files to Google docs and send me the link along with permission to view the files. If you want you could just post the power values for each day and i can plot them myself using Excel.

Sorry for the problem.

Sybee

22-Dec 9.078
23-Dec 22.707
24-Dec 34.780
25-Dec 34.530
26-Dec 35.790
27-Dec 26.520
28-Dec 132.980
29-Dec 76.730
30-Dec 70.750
31-Dec 67.680
1-Jan 75.076
2-Jan 81.790
3-Jan 77.840
4-Jan 96.620
5-Jan 112.250
6-Jan 118.330

https://docs.google.com/spreadsheets/d/ ... =629846551

link to google docs

Thanks, Kerry. I'll plot them and monitor the trend. Post the new readings as you get them.

When is the project due date?

Actually the due date is now but the teacher understands the nature of the project. My guess is that we need to wrap it up in the next week or two.

Thanks.

Kerry

Since time is short, I would go ahead and add some urine to the mud and then measure the power every every 2 or 3 hours. You have a pretty good idea how fast the power is changing in the cell now, so whatever difference you see will be due to the urine. A constant increase in power is just a different form of stability. Superimposed on that will be the effect of adding nutrients to the mud. I will predict that the slope of the power vs time curve will increase after you add the urine--at least for a few days. You need to try it now, otherwise you will never know what might happen.

Good luck!

Sybee

Just a quick update. Measurements for 1/7 and 1/8 were 114 and 132 respectively.

Guess we need to add the utine but I hate to do it before it stabilizes.

Kerry

As long as you have the graph of the readings from before you added the urine, any change you see afterwards is most likely due to the addition. I know it isn't ideal but often scientists have to make compromises in an experiment. I often need to take samples at specific time points but I try to set up the experiment so I don't have to come in at 2 am to get the sample!

In your experiment, it would have been useful to have constructed three fuel cells and had them all going together. It would have been interesting to know if your increasing power held true in multiple cells assuming you used the same mud in each. Then you could have added urine to two of them and kept one as a control. Doing replicates costs more in time and money but if you are going to spend the time to do a project it is worth it to get good data.

Have you started measuring the temperature of the room where you have the cell and of the mud at the time you take the reading? This is important as heat increases the microbial activity. There should be a correlation between temperature and power output. Let the urine come to room temp before you add it. I am curious to know how quickly the urine would affect these bacteria, so I would check the power 2 hours after adding it and then maybe at 2 or 3 hour intervals that day. If the change is very gradual then there's no point in taking lots of readings but you won't know that unless you check it.

Good luck!

Sybee

Thank you again for all of your counsel with the project. We have time before the actual science fair so we may run the experiment with 3 MFCs as you suggested.

What is the purpose of the resistors and what does peak power output at different resistor levels mean?

i.e) if peak power is at 470 versus 1000 what does that mean?

Thank you.

Kerry

Sybee:

I was going to run scenario my son's teacher before adding the urine. Today's reading is 155.106 at 470 ohms.

If we add urine without the uW stabilized, how will we estimate the impact of adding urine?

Thanks.

Kerry

Hi Kerry,

You asked a question about the resistors that are used when you measure the power of the cell. A resistor restricts the flow of electrons through the circuit by a specific amount determined by its rating in ohms. The higher the ohms the greater the resistance. A resistor reduces the current and the voltage and I think the reason that you have to use a resistor in the cell is so you can calculate the power according to the formula P = V^2/R

You could measure the V by just attaching the meter leads to the anode and cathode, but how would you determine the power? What I am not clear on is why you have to use resistors of different ohms. If the cell is producing a constant V and current then the power output should be the same regardless of the R. Maybe another expert who knows more about electricity can answer that question.

I guess I did not explain my reasoning well enough about how to see the effect of the urine. It is based on a graph of the power vs time and the assumption that the power is increasing at a constant rate. The power increases because the bacteria in the mud are growing and dividing so there are more of them. If the growth rate is constant then I assume the power increase will also be constant.

If you draw a straight line through the points in such a way that it is the best fit then you can determine the slope of the line. If you plot the data using MS-Excel the program can generate a best-fit line through a give set of points. Now when you add the urine I am assuming that the nutrients will stimulate bacterial growth to a higher rate than without the urine so that a NEW constant rate will be achieved. This means that the P will be increasing faster and if you run a straight line through these new points, the slope will be significantly greater.

The only reason I suggested this comparison of slopes is because I thought you were almost out of time for the project and I wanted you to test the urine while you had a little time left. Now you say you have sufficient time to do the experiment over, so next time you can have multiple cells running at the same time and you will be able to do a direct comparison between cells with and without urine.

One question you might be asking is why your power output did not stabilize. Well, the simple answer is because the bacteria are continuing to grow in the soil. The commercial soil must have lots of nutrients. The idea behind adding urine is that boosting the nutrient level should make the bacteria grow faster and produce more power--but if the soil already has a lot of nutrients then you may have to wait a long time for the bacterial growth to stabilize so the power is constant and not changing.

Let us know what you decide to do.

Sybee

I put in 1 ml of urine per the instructions. It seems like a very small quantity given the amount of mud in the MFC. We will start doing the measurements tonight. In a previous post you asked about the resistors. The instructions provide that we are to do the measurements with 4670, 2190, 1000, 470, 220, 100 & 47 ohms.

Thanks again for all your help and advice.

Kerry

I had no idea of a typical composition of normal urine, so i looked it up: http://chemistry.about.com/od/biochemis ... -Urine.htm

water 95%
urea 9.3 g/l
chloride 1.87 g/l
sodium 1.17 g/l
potassium 0.750 g/l
creatinine 0.670 g/l
with lesser amounts of other ions and compounds

Reference: NASA Contractor Report No. NASA CR-1802, D. F. Putnam, July 1971

So, the main bacterial nutrient in urine is nitrogen in the form of urea. In terms of percent, 9.3 g/L would be about 1 % nitrogen--pretty low on the fertilizer scale. I use 10-10-10 on my vegie garden, but that's for plants not bacteria. It would be interesting to know the reasoning that the project developer used to come up with 1 mL of urine per MFC. Maybe it was by trial and error. Too much urine could inhibit bacterial growth or activity. Let me know what happens.

Sybee

So 24 hours after adding the urine the power output is significantly decreased. Peak power yesterday was about 155. Today only about 16 uW. Not sure if we should add more urine.

Wow! That's a big drop in power. Didn't you take any readings less than 24 hours like we discussed? If you had measured the power several times then you might be able to prove that this decrease was due to an effect on the bacteria and not a technical problem. The kinetics of a response--the change with time--is a very important part of scientific research. If you omit the early time points then you may miss something. The power may have dropped to 16 uW immediately after you added the urine which would indicate that it was probably not an effect on the bacteria since they are on the anode which is at the bottom of the mud.

What I suspect is that the urine salts have interrupted the flow of electrons from the anode to the cathode and that is why the power decreased so much. Maybe the urine did not sink into the mud as quickly as it should have so that it formed a layer near the cathode which altered the electrical conductivity of the mud. I don't know if this is the reason and I hope someone else reading this will offer additional comments and suggestions.

Try taking readings at 12 and 24 hours. Hopefully the urine will have diffused throughout the mud and the power should increase.

Sybee

We added the 1 ml of urine late last night and we were both gone all day today so we did not take frequent measurements. The procedure states wait 5 minutes after adding the urine before putting the cathode back to make sure the urine has been absorbed into the mud I waited a bit longer to make sure that the urine was not pooled on top and began sinking into the mud. When we replaced the LED light after taking the resistor measurements the MFC appeared to be dead but subsequently came back to life. The blinks have been reduced to about every 7 seconds at this point. So do you see any value in adding more urine at this point or would that just distort the results. I guess it is possible that the urine has had no effect and the MFC is just losing power?

Thanks so much.

Kerry