Experiments with a homemade turbidity meter

[hubert]

The project can be found on this website: http://www.sciencebuddies.org/science-f ... p032.shtml
Hello,
I am having a lot of difficulty with my science project, and i have spent many hours and a lot of money trying to make it work. Do you have any ideas?

The digital multimeter does not display a measurement in volts, so I twisted the knob (it is not an auto-scaling multimeter) to kilowatts, which was the only measurement that would give me a reading and gave me 101.3 kw. When I turned off the lights before turning on the laser to measure the scattered light in the skim milk/water dilutions, the reading stayed the same. Clearly, this means something is wrong, because no matter what measurement the multimeter is on, the numbers should fluctuate when the lights turn off. The numbers on the multimeter stay the same when the photoresistor rests on top of the dilution samples. This happens when the laser shines through as well. I tested the continuity of the circuit with a continuity tester and it was a closed circuit and everything worked fine. Despite that the multimeter is not reading properly. What is wrong and what should I do?

Thanks,
hubert

[kgudger]

Hi and welcome to the forums.

You said that "the digital multimeter does not display a measurement in volts, so I twisted the knob to kilowatts." This is very confusing to me, as I believe all multimeters display voltage, and know of none that display watts. Do you have a standard multimeter, or is it a "Kill-A-Watt" meter? The Kill-A-Watt meter is just for measuring appliances running on standard house current, and will not work in this application.

I looked at the experiment, and the output of the photo-detector circuit "voltage should range from less than 100 millivolts (mV) in the dark to around 8 V in bright light." When you have a properly working multimeter, you will probably want to use the 0-2V scale (for a non-auto ranging type.)

Could you reply with the make and model number of the multimeter you're using? I can find the model on the web and give you more information about how to use it.

You could also try the multimeter out on a "wall-wart" type of power supply. Check the label on the transformer for voltage output - If it's DC and less than 20V, you can use your multimeter to measure the plug (safely) - and that should tell you whether your multimeter is working properly. HTH

Keith

[hubert]

Hi Keith,
Thanks so much for replying quickly. My multimeter is not a "kill-a-watt", but it has different ranges, and I set it on 200K, which I assumed to be 200 kilowatts. As mentioned previously, I did this because no readings appeared on volts or other settings, but the kilowatt setting gave me a number. Can you please explain to me what the different settings such as ACV, DCV, DCA,and OHM are? Do you know why the multimeter won't give me a number in volts? I got this multimeter at radioshack, part 22-810, but the actual item does not appear on the website. You can look at the manual, which is located on the site, or google it and click on a link to radioshack support, look through the part number glossary, and find this multimeter. Also, what is a "wall-wart"? What does it do and how can it help me?

I look forward to reading your reply,
hubert

[kgudger]

Hi Hubert:

The settings you mentioned are AC volts, DC volts, DC amps and Ohms. You want to measure DC volts. For the light scattering experiment, while the output of the photo detector can go up to 8V, it will probably be less than 2V, so the 2V DC scale is probably going to work. It sounds like you've been using the OHMS scale, which is powered by the multimeter.

A wall-wart is an AC adaptor - googling it gave me the Wikipedia page: http://en.wikipedia.org/wiki/AC_adapter. They are basically DC power supplies. If you have a cordless telephone or computer laptop, you probably have one of these with a two connector output. One connector is on the outside of the plug, the other on the inside. You can use this to test your multimeter. The voltage should be written on the part that plugs into the wall - make sure it's DC and less than 20V. You will measure the plug on the end of the wire from the transformer. I just suggested this as a way to make sure that your multimeter is working.

I found a number of multimeter tutorials on the web - just search for them. Are you using both terminals from the multimeter? The experiment says: "Connect the red wire (positive) from the multimeter to position E1, in the row that contains the leads from the photoresistor (position A1) and the potentiometer (C1). Connect the black wire (ground) from the multimeter to the ground bus." Are you clear what is the "ground bus"? Let me know if you need help with this.

Be sure that you're connecting both leads!
Keith

[hubert]

Hey Keith,
Thanks for all the information you've given me. I do have an AC adaptor, but how can I tell if it's DC? How do I use the adaptor and what will it help me do? On the wall-wart it had a little diagram of putting the positive wire from the multimeter into the wire of the adaptor (is this wire the transformer?). I did this and nothing happened. What is supposed to happen? If the multimeter only reads Ohm, does this mean that something is wrong with my circuit? Could the problem be the photoresistor? The wires of the multimeter are thick and did not fit into the breadboard sockets, so I used the jumper wires with alligator clips to connect the multimeter wires to the breadboard. I attached a small wire lead to the other end of the jumper wire and into the the breadboard socket. It seemed to work, but is there something wrong with this? I have checked numerous times to see if I followed the steps perfectly, and an electrician told me I had a closed circuit and everything seemed to be working except for the multimeter. I will read some tutorials and see if they can help me, but I am not sure they will because I followed everything perfectly and it just won't work. Do you think this problem is my fault or the multimeter's?

Thanks again,
Hubert

[Craig_Bridge]

I would do some testing on your multi-meter as the next step. Read your meter manual on where to put the leads to make various readings. Some meters require you to move the leads into different places on the meter for different scales. Take this into account when switching scales. Always disconnect the leads from any external circuit before changing FUNCTIONS (ohms, AC volts, DC volts). Switching ranges within the same function is usually ok to leave the leads connected; however, read your manual.

With the leads disconnected from everything, put the meter on the Ohms scale. Meter should indicate an open circuit (extremely high resistance). Short the leads together, meter should read 0 Ohms. If these are the results you get, then at least the Ohms scale and the meter leads are functional. If you don't get a 0 Ohms reading, then you may have a problem with your meter leads or the meter itself.

If you have an Ohms scale in the 200 Meg Ohms, then you should be able to measure your skin/body resistance by grabbing a lead with each hand and you should see the numbers jump around.

Put the meter on a DC volt scale and hook the leads to a 9v or 1.5v battery. The meter should read something close to the voltage rating of the battery. Reverse the leads and it should indicate an opposite polarity. If this does not happen, then the DC volt scale on your meter is NOT working (note the leads were checked by the Ohms scale when they were shorted).

If all of the above checks out, the meter is sufficiently functional to test out your circuit and perform your experiment.

Take the "photoresistor" out of the circuit. With the lights on, measure its resistance, switch the polarity of the leads and measure again. If it is a photodiode (and not a photoresistor), you should see a significant difference in the resistance measured between the two lead orientations. If you find a significant difference, then see which way you need to put the leads to see the most difference between light and dark. Instead of turning off the room lights, I'd just put cover the diode with something insulated (opaque plastic container/cup). With most DVM's, the common (use the black lead in this meter connection) will be the cathode and the other will be the annode of the photodiode. You will want to connect the photo diode annode to the positive battery lead in your circuit. Hopefully, you have a photoresistor and it doesn't matter which way it is in the circuit.

Try seeing what the resistance change of your photoresistor/photodiode is when illuminated with your laser pointer. Again, I'd do the experiment with the lights on and the diode and laser pointer under an opaque container. If you don't see any change, then your photoresistor/photodiode is NOT sensitive to the light wavelength/frequency of your laser pointer.

Post back your findings on the dark and light resistance readings of your photoresistor/photodiode and whether the lead orientation matters and we can help you figure out what is happening with the rest of the circuit. Without breaking it to just evaluating one piece at a time, it is way too easy to miss something simple.

[hubert]

Hello,
I have been having major problems with my science fair project. I asked an electrician and my science teacher, I've worked on this for more than two months, and I spent a great sum of money; I have come to a conclusion that only the creator of this demonstration can help me solve my issues. Do you know who Dr. David Whyte is? He is a staff scientist at Science Buddies and is the author of my project, "What is in this Water? Experiments with a Homemade Turbidity Meter" and numerous other projects such as " I love ice cream but it doesn't love me" and "How Do Roots Grow When the Direction of Gravity Changes?" He has a PhD in molecular biology and is the founder of Argus Biosciences LLC in Belmont, California. Is there a way I can contact him so I can ask him some advice?

I look forward to reading your reply. Thanks!
-hubert

[amyc]

Hi Hubert - I am sorry that you've run into trouble with the project, and I know that many of our experts have offered advice for helping to test and troubleshoot. While I would encourage you to continue to post your questions here to see if our team of experts can help, I am contacting the author of the project to see if he can review your questions here in the AAE Forums and offer some advice.

Amy
Science Buddies

[hubert]

Hey Amy,
Thanks for all of your help. The problem is that I don't know what to ask. I did everything right and made sure the circuit was perfect, so I can't post a question about anything. I was hoping to talk to Dr. Whyte so I could go through everything and know where I went wrong. Hopefully it will all work out.

Hubert

[Craig_Bridge]

Hubert,

Did you try the things I suggested earlier in terms of trouble shooting your DVM and test leads? What were the results? Post back your results and I'll try to help.

With electrical circuits, we usually can't see with our eyes what isn't behaving as expected. Sometimes others can, but that is ususally based on years of experience and something really simple and obvious that the first person missed. Electrical engineers and technicians often work in pairs and often ask the other person to look for something obvious or suggestions on what to try.

This is why we have to rely on test equipment which may or may not be working as expected, so often the first step is test out whether the test equipment behaves as expected on its own. Trouble shooting is time consuming and frustrating but you learn a lot by doing it. An old fashion way of stating this is, It often feels like you are trying to pull yourself up by your own boot straps (the pieces of leather attached to the top of the backs of boots to give you something to grab onto to pull them on your feet). This is what test equipment is, boot straps.

-Craig

[amyc]

Hi Hubert - Dave took a look at the thread here at Ask an Expert and gave me some suggestions to pass along to you.

Since the circuit seems to be causing trouble, let's work around it.

Disconnect the photoresistor from the circuit. The circuit in the project has a voltage output, which is standard for most detectors, but resistance will work just fine for this project.

Attach the photoresistor to the multimeter probes. Use alligator clips if needed. Polarity does not matter.

Set the multimeter to read Resistance.

Check the resistance in various levels of light. Point the photoresistor toward or away from light, cover it with your hand, etc. You should see dramatic changes in the resistance in various levels of light. The resistance goes down in brighter light.

Once the photoresitor and multimeter are working, use the photoresistor as the detector.

Try clear water and a turbid solution. If there is any scattering you should see a signal (decreased resistance). You will need to protect the photoresistor from surrounding light.

If you have a signal from the turbid solution, make a standard curve, and proceed with the rest of the steps.
The signal will decrease with increasing light scatter (opposite of voltage output) but the data can still be graphed and used as a standard curve.

Dave also said that if you do the pineapple part, make sure you use fresh (not canned or frozen).

I hope that these tips, in addition to the ones Craig and the other experts have provided, can help.

Amy
Science Buddies

[hubert]

Hey
According to Dr. Whyte, I am supposed to set the multimeter to read Resistance. How do I do this? Is there a certain button I press? I do not have an autoscaling multimeter; does a regular digital multimeter not have this feature? Which measurement should the multimeter be on? I disconnected the photoresistor and attached the multimeter probes to it. Because I didn't know how to set it to Resistance, I turned the dial to all of the different measurements and the only one that gave me a reading was OHM- I set it to 20 K. When in bright light, the multimeter gave me a number around .40 K, and when I covered the photoresistor, the meter read around 5.6 or so. Keith told me that OHM is just the multimeter giving random numbers, not anything else. So why does OHM react to light and DCV not read anything? Also, how come the numbers decreased in bright light and increased when I covered the resistor? This experiment is just not working. Can Dr. Whyte please explain this?

Hubert

[rmarz]

Hubert - A quick read through your efforts leads me to one possible error. A multimeter is a very useful tool, but you should understand how it works or you may introduce errors. Your circuit has a light sensor that changes resistance depending on the light it receives. You are powering it with a 9 volt battery with a potentiometer (variable resistor) in the circuit for adjustment. According to the circuit in the experiment you are measuring the voltage drop across the potentiometer. You can ONLY use a DC voltage range on the multimeter for this measurement. The resistance measuring circuit internal to the meter provides its own reference current to use when measuring a passive resistive element. To use a resistance range in a circuit that already has a current flow through will not only give a false reading, it may damage your multimeter.

Rick Marz

[rmarz]

Hubert - Another comment. You said you cannot read any voltage measurement on the DC scales. The diagram in the article only shows 'readout' as connected between the potentiometer wiper and the photocell. This would be the positive lead (red) of the multimeter. The negative lead (black) would be connected to ground. Although not shown in the diagram, the negative terminal of the battery is also tied to ground. You should certainly see a DC voltage except when the potentiometer is adjusted to the point that the wiper is at ground potential.

Rick Marz

[kgudger]

Hi Hubert:

You can find many tutorials on the web about how to use a multimeter (e.g., http://www.ladyada.net/learn/multimeter/, and search YouTube for a video.

Resistance is measured in OHMs. When a multimeter is in OHMs or resistance mode, it supplies a voltage / current through its leads, just like the original circuit.

I've included a picture of a DVM with several of the options noted.

Keith