veggie power

[thstage]

Hi,
I am doing the Veggie Power experiment for my 6th gr science project
I ordered the veggie kit and I want to measure the voltage in different fruits and veggies.
I know I have to set the meter to DC voltage but there are 5 settings on the DC side.
Which setting do I set the meter to and why?
The meter is an Elenco M-1000D
Thanks

[rmarz]

thstage - The voltage you will be measuring is quite low, so with this meter you will probably start with the 20 volt setting. If the voltages is really low, (below 2 volts) you could go to the 2 volt (2000 mV) setting, but 20 V is your most likely used setting for this experiment.

Rick Marz

[Craig_Bridge]

I read Rick's answer and he is correct; however, it didn't include some of the whys...

When measuring a DC voltage, best practice is to start on a voltage scale higher than the voltage to be measured. In this experiment for your meter, that would be the 20v scale. I would expect that you will see a reading less than 2 volts for a single cell battery (the maximum for your next lower scale), at which point you can safely switch down to the 2v scale because you have confirmed that it is safe (for your meter) to do so. If you then see a consistent reading above 0.05v and below 0.2v, you could then switch down to the 200mv scale to get a more accurate reading, but USE CAUTION:

If the reading is jumping around or staying less than 0.05v on the 2v scale, you probably have a connection issue, either with your test leads or with the electrodes in your fruit/vegatables. This means that switching down to the 200mv scale is a bad idea. As you play with trying to correct the connections, you could easily go way above 200mv and cause the DVM to go into a self preservation mode. If that happens, you will have to read the user manual to see what it says about its over voltage protection and what steps you have to take if any to reset your meter.

With old analog DC volt meters, the danger was "pegging the meter", driving the meter needle into a mechanical stop and permanently bending the needle. With DVM's, the danger is driving the internal circuitry into saturation. Most meters have some protection against permanent damage; however, if the overvoltage is too high, some integrated circuits can have their substrate reverse biased and stop working. Sometimes you can disconnect the power (internal 9v battery) and short the test leads and leave it that way for a few minutes to several hours to restore its functionality.

[thstage]

Hi
I have done my experiment.
I have measured the voltage on a potatoe,carrot,onion,apple,kiwi and lemon.
I measured the voltage with the electrodes at 1" and 2" apart.
I thought the lemon would have the highest voltage because of the acid in the lemon ,but the apple did.
Can you tell me what causes one fruit or vegetable to have a higher voltage?
I also thought the farther apart the electrodes, the higher the voltage, but that was'nt
the case.In some cases it was higher,but it was lower in some other.
Do you have an explanation for this?

[John Dreher]

These are very good questions -- the sort of questions that typify someone with scientific talent.

I don't really know the answers. I would expect different fruits to produce different voltages because 1) the juices that act as the electrolyte for the battery will have different compositions and 2) the flesh of the fruits would present different amounts of resistance to current flow, even if the electrolytes were the same. I suspect the detailed electro-chemistry is pretty complex. If I wanted to study this more I would start by making batteries with just the juices of the fruits, to see how well they act as electrolytes. I am not surprised that different electrode spacings produce different voltages because the farther apart the electrodes are the higher the internal resistance will be. I would hypothesize that the difference would be least when the veggie batteries are not driving a load resistor. I could be wrong, however; experiment is what counts, not my guesses.

I hope your project went well.

[Craig_Bridge]

I also thought the farther apart the electrodes, the higher the voltage, but that was'nt
the case.

There is more geometry involved than just the distance between the electrodes. The volume where current can flow between electrodes also involves the surface area of the electrodes. This even involves the "back" of the electrode (side facing away from opposite electrode). The "current density" within this volume is NOT constant and will be influenced by any non-uniformity in the fruit or vegetable (section membranes in oranges and lemons, "eyes" in potato, any root or stem, etc). This means making simple predictions is not possible.

[thstage]

Hi,
I have completed my project and finished my report with final analysis.
The only thing I have left to do is my display board for my presentation
on the day of judging. For my display I would like to power a small string of
Christmas lights using an apple or apples if needed. How do hook up the apple to the string of lights?
Thanks

[rmarz]

thstage - It appears that your experimentation to date has been focused on the voltage generated by fruits and vegetables, using dissimilar metals as electrodes. You have apparently not looked into the current generated by the experiments. There was a similar post last week about the "lemon battery" and powering a small LED. I've attached the thread for you. Bottom line, the power demand of a small string of incandescent Christmas lights will overwhelm the power generated by your apple battery. Just for some comparative numbers, the lemon battery generated about 1 volt DC, and produced a current of less than 1 milliamp using small copper and zinc electrodes. An individual incandescent Christmas bulb requires about 2.5 volts (possible with multi apple cells) but about 180 milliamps of current, about 400 times what the lemon cell generated.

Perhaps you could find a particularly sensitive red LED that would operate as low as 5 milliamps. Alternatively, you could use your multimeter as part of the display board to show how much voltage you are generating. Another thought is to use a small compass with insulated wire wound around it as a galvanometer. The compass needle will deflect when voltage is applied to the coil. Just out of curiosity, you could use your multimeter set to a sensitive current range to see what kind of power you are producing. Use the 2000 microamp range or something close to that.

Rick Marz

viewtopic.php?f=31&t=9146

[thstage]

Thanks for your reply.
Could I use more more than one apple or lemon to power the lights?

[rmarz]

thstage - If you want to pursue the light idea, I think you must use a very low current device like an LED, probably a red one. An incandescent bulb, even a very small, low power device (look up 'grain of wheat' light that miniature model-makers use) require 5-16 volts and 25-75 milliamps to work. As I mentioned before, an individual Christmas bulb will require 2.5 volts and about 180 milliamps. This is far beyond my estimation of the capability of the veggie battery. And yes, you will probably need more than one veggie cell connected in series to produce sufficient voltage to overcome the threshold voltage, even for the LED to conduct and be visible. You never mentioned what the voltage was that you measured in your experiment. By increasing the surface area of the electrodes in your battery you will be able to obtain a higher current.

Rick Marz