Science Buddies: "Ask an Expert"

Hello again,
I decided to do a mini experiment and created my own device for measuring the amount of light passing through turbid samples. I guess it is an Ohm-meter because I connected the probes from the multimeter, which was set on Ohm to the photoresistor leads. I worked with what I had since I couldn't get it to read on DC in volts. I covered the photoresistor with a jar lid and attached it to three jars with different samples;one with orange juice, one with a mixture of orange juice and water, and one with water. I recorded the number displayed on the multimeter screen before I shined the laser through and then with the laser light shining through for each jar sample. I placed all of the jars in the same spot, under a light source with minimal shadow. I got some interesting results, that could be complete flukes, but I want to know if this experiment is logical. Do you think I am on to something?

I graphed my results: If you noticed, in every sample when the laser shined through the number decreased. Also, the sample with half orange juice and half water had the same average as the sample with just orange juice. There were many oddities in my graph. Do you think I'm wasting my time?
-Hubert

Did you learn anything? If so, you aren't wasting your time.
What I learned from your "preliminary" experiment is that you have validated that you have a viable measurement tool.

I do have concerns about the scientific validity of the measurements in terms of being able to draw any conculsions about your samples.

1) Unquantified Ambient Light interacting with sample produces reading X.
2) Unquantified Ambient Light + laser light interacting with sample produces reading X-Y.

Was the photoresistor immersed in the sample? If so, then the light interaction involved transmittance only. You have a chance at a valid result because the light interactions are controlled.

Was the photoresistor outside the liquid? If so, then the light interactions might involve a combination of:
1) Direct light source to photoresistor, plus
2) Reflected light source of sample to photoresistor, plus
3) Transmitted light through air + sample + air to photoresistor, plus
4) Refracted light on multiple sample surfaces to photoresistor
and possibly others that I've missed.

To have scientific validity to make statements about light interactions with a sample, you MUST control the light interactions. Multiple light sources complicates understanding/analysing the experiment. Multiple light paths complicates understanding/analyzing the experiment.

You have a great start in terms of having a working measurement tool that you have proved works. Now you need to come up with a good experimental design that can make use of your measurement tool to prove or disprove some hypothesis.

-Craig

Hi Craig,
If I were to place the photoresistor in water, which is attached to the wire, wouldn't I short out the meter or even electrocute my self? And even if the photoresistor was submerged, wouldn't the water still benefit from the ambient light?
I tried turning the lights off but there was not enough light, even when I turned my laser on, for the multimeter to give a proper reading; it just stayed at 1 Ohm. Do you think that if I submerge the photoresistor and therefore don't use the ambient light there won't be enough light to give me a reading?

Thanks,
Hubert

If I were to place the photoresistor in water, which is attached to the wire, wouldn't I short out the meter or even electrocute my self?

The only source of power in the DVM is an internal 9 volt battery so there is very little danger of getting even a mild shock from it unless you involve your tongue or other internal parts.

Most photo resistors are sealed units that have the sensor pointed one direction and the leads in the opposite direction so it should be possible to put the sensor into the liquid without getting the leads wet.
With sealed units, the worst that is going to happen is any conductivity of the liquid will affect your resistance reading.

even if the photoresistor was submerged, wouldn't the water still benefit from the ambient light?

Yes it will; however, the interaction will be entirely transmittance. By only having one mode of light interaction, the additive effects of multiple light sources is predictable and can be dealt with mathmatically.

I tried turning the lights off but there was not enough light, even when I turned my laser on, for the multimeter to give a proper reading; it just stayed at 1 Ohm.

Now you have discovered a fundamental problem with your setup. Enough of the light intensity provided by your laser pointer is not arriving at your photoresistor and/or your photoresistor is not sensitive enough to detect it and provide a reading.

You can try experimenting with just the laser pointer and photodiode/ohm meter in the dark without anything else (light, liquid, containers, etc) to see if there is some positioning that will cause a usable reading. If not, then your photodiode and laser pointer aren't very useful together.

I don't think there is any reason why you need to use a laser pointer in a homemade turbidity meter. The properties it provides are a consistent amount of light in a limited spectrum. An adjustable high intensity lamp will do just as well. You just need a light source whose output is adjusted so that the photoresistor stays in its linear region (not saturated with too much light or not enough light to register) for all of the samples you want to test.

Turbidity is typically measured with light transmittance and not reflectance or refraction or other light interaction modes so coming up with a way to put the sample between the light source and the photoresistor in a way that prevents other light coupling modes is important.