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What is in this Water? Experiments with a Homemade Turbidity Meter

Time Required Long (2-4 weeks)
Prerequisites A familiarity with basic chemistry is required. Experience with simple electronics would be helpful, but is not absolutely required. Although the procedure provides step-by-step instructions, this is a DIY (do-it-yourself) science fair project that may call for some creative problem solving on your part.
Material Availability Electronic components are required. See the Materials & Equipment list for details.
Cost Average ($50 - $100)
Safety Use caution when working with laser pointers. Wear safety goggles when using the drill.


Light interacts with matter in a variety of ways—it can be absorbed, reflected, refracted (bent), and scattered. The scattering of light explains why the sky is blue, why milk is white, and why the Mississippi River is called "The Big Muddy." In this biochemistry science fair project, you will make an electronic device to measure the amount of scattered light in milk. You will also use the device to track the activity of protease (a type of enzyme) in pineapple juice, based on its ability to chop up the proteins in milk, which cause it to thicken and become less opaque.


Build a simple electronic device to measure light scattering in liquids, and use it to measure the effect of protease activity.


David Whyte, PhD, Science Buddies

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MLA Style

Science Buddies Staff. "What is in this Water? Experiments with a Homemade Turbidity Meter" Science Buddies. Science Buddies, 13 Oct. 2015. Web. 30 July 2016 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/BioChem_p032.shtml>

APA Style

Science Buddies Staff. (2015, October 13). What is in this Water? Experiments with a Homemade Turbidity Meter. Retrieved July 30, 2016 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/BioChem_p032.shtml

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Last edit date: 2015-10-13


Water in a mountain stream is usually murkier, or more turbid, after a rainfall because the rain washes soil and other small particles into the stream. Environmental engineers and others who track the quality of drinking water measure the turbidity of the water supply using a turbidity meter, called a turbidimeter. The turbidimeter measures light scattering. Whereas in light absorption, colored molecules in the liquid absorb the incoming light, in light scattering, the incoming light bounces off of small, suspended particles, and is thus redirected away from its initial path.

Instruments that measure light absorption or light scattering typically have different setups. For detecting light absorption, the instrument measures how much light passes directly through the liquid. For light scattering, the instrument measures how much light is redirected, or scattered, at a 90-degree angle from the direction of the incoming light. See Figure 1.

Scattered light.

Figure 1. Some of the incoming light is scattered by suspended particles, and is then detected by the light meter at an angle of 90 degrees from the initial path of the light beam. Some light is also absorbed. The remaining light is transmitted through the liquid and emerges on the opposite side.

A number of factors affect the scattering of light by suspended particles. The concentration and the size of the particles are two such factors. The physical characteristics of the particles are clearly important, too; for example, fat globules in milk will behave differently than dust in the atmosphere.

Another factor is the wavelength of the light. Blue light is scattered by the atmosphere more than red light is, which is why the sky looks blue. As sunlight passes through the atmosphere, the blue light is redirected, and we "see" it here on the ground. Sunsets are red/orange for the same reason—the sunlight has been depleted of blue colors by passing through the atmosphere.

The goal of this science fair project is to measure the amount of light scattered by the suspended particles in milk, and to determine how the amount of scattered light is related to the concentration of the particles. (Can you predict what will happen at low, medium, and high concentrations?) The Experimental Procedure describes how to make a simple light detector so that the amount of light scattered can be measured. You will also use the light detector to track the enzymatic activity of proteases found in pineapple juice, measured by the reduction in turbidity of diluted milk. Enzymes speed up the rate of chemical reactions. Proteases are a class of enzymes that chop up proteins. The action of proteases causes the proteins in milk to coagulate, which makes the milk less turbid.

Terms and Concepts

  • Turbidity
  • Suspended particles
  • Light scattering
  • Light absorption
  • Wavelength (of light)
  • Enzyme
  • Protease
  • Coagulate
  • Light transmission
  • Light intensity
  • Voltage drop
  • Photoresistor
  • Potentiometer
  • Multimeter
  • Water bath


  • Based on your research, how does particle size affect light scattering?
  • What is the scientific definition of the word turbidity?
  • What is "Rayleigh scattering"?
  • How does the wavelength of light affect light scattering?
  • Is there a linear relationship between particle concentration and intensity of scattered light?
  • What causes natural water sources to be turbid?
  • What effect does water turbidity have on its oxygen content? (Hint: Turbid water absorbs more sunlight and thus, can be warmer than clear water).


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Materials and Equipment

The Experimental Procedure describes how to make the light-sensing circuit on a breadboard. Using the breadboard gives you a lot of control over how the circuit is set up. If you are not familiar with how to use a breadboard, see the Science Buddies How to Use a Breadboard.

A project kit containing most of the items needed for this science project is available for purchase from AquaPhoenix Education. Alternatively, you can gather the materials yourself using this shopping list:

  • Breadboard, 2 x 3 inches
  • 9-V battery
  • 9-V snap connectors
  • 24-inch Insulated test/jumper leads
  • Potentiometer, 1 mega-ohm,
  • Digital autoscaling multimeter; available at most hardware and auto supply stores or at Amazon.com
    1. Some multimeters require that you turn a knob to change the range of voltages the meter is able to read. Autoscaling multimeters do this automatically.
  • Photoresistor
  • Jar lid (1)
  • Black electrical tape
  • Drill with 1/4-inch drill bit to make a hole in the jar lid. A stiff knife will work, too.
  • Safety goggles
  • 2- x 4-inch board (about 8 inches long)
  • Laser pointer
  • Clear glass jars or clear plastic cups, 1-cup or greater capacity (6)
  • Permanent marker
  • Masking tape
  • Milk, fat-free
  • Tap water
  • 10-mL Graduated cylinder
  • 25-mL Graduated cylinder
  • 50-mL Graduated cylinder
  • 250-mL Graduated cylinder or beaker
  • 500-mL Graduated cylinder or beaker
  • Spoon
  • Syringe that can measure 25 milliliters (mL)
  • Dim light, such as a nightlight or LED flashlight
  • Fresh or frozen pineapple; do not substitute canned pineapple as it will not work well
  • Cheesecloth; any piece of cotton fabric (even an old cotton T-shirt) can be substituted
  • Food grater
  • Tablespoon
  • Microwave-safe container
  • Stopwatch
  • Graph paper
  • Lab notebook

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Experimental Procedure

Note Before Beginning: This science fair project requires you to hook up one or more devices in an electrical circuit. Basic help can be found in the Electronics Primer. However, if you do not have experience in putting together electrical circuits you may find it helpful to have someone who can answer questions and help you troubleshoot if your project is not working. A science teacher or parent may be a good resource. If you need to find another mentor, try to find someone who has hobbies like robotics, electronics, or building and fixing computers. You may also need to work your way up to this project by starting with an electronics project that has a lower level of difficulty.

Assembling the Light Meter

  1. To begin this science fair project, assemble the parts needed to make the digital light-sensing meter. The circuit for the meter is shown in Figure 2. You can learn how to use a breadboard in the Science Buddies How to Use a Breadboard.
  2. The circuit has a 9 V battery, a photoresistor, a potentiometer, and a digital multimeter (if you need help using a multimeter, check out the Science Buddies How to Use a Multimeter). The resistance of the photoresistor is decreased in the presence of light. The potentiometer, which has a knob that changes its resistance, is used to vary the sensitivity of the meter. In bright light, the voltage drop across the photoresistor is reduced, leading to an increase in the voltage drop across the potentiometer, which is detected by the multimeter.
Light sensor circuit diagram

Figure 2. Left: a circuit diagram for the light-sensing meter. It converts light energy (input) into voltage (output). Right: a breadboard diagram for the light-sensing meter. If you are not familiar with circuit diagrams, you can follow the breadboard diagram exactly to build your circuit.

Making Your Circuit

  1. Connect the battery terminals to the breadboard. The positive (red) lead of the snap connector should be connected to the power bus, and the negative (black) lead should be connected to the ground bus.
    1. Insert the red wire from the positive terminal into one of the holes in the power bus (marked on most breadboards as the row of holes next to the red line).
    2. Insert the black wire from the negative terminal into one of the holes in the ground bus (the row of holes next to the black or blue line).

Connect the photoresistor to the circuit with alligator clip leads. This will allow the photoresistor to be moved and positioned as needed.

  1. Connect one alligator clip lead to each lead of the photoresistor, as shown in Figure 3 (the red and green alligator clip leads).
  2. Connect the other end of each alligator clip lead to a small jumper wire, which can be pushed into the breadboard.
  3. Push the jumper wire from the red alligator clip into the positive power bus on the breadboard.
  4. Push the jumper wire from the green alligator clip into hole A1 on the breadboard (refer back to Figure 2).
photo of light sensor circuit

Figure 3. Picture of the light-sensing circuit.

Connect the potentiometer to the circuit.

  1. The potentiometer has three pins. Push them into holes E1, E2 and E3 on the breadboard, as shown in Figure 2. Notice that one pin of the potentiometer is now connected to one lead of the photoresistor, since all the pins in row 1 of the breadboard are connected.
  2. Use a small jumper wire to connect pin A2 (and thus the potentiometer's middle pin) to the ground bus, as shown in Figure 2.
  3. The third pin of the potentiometer is left "floating", or not connected to anything.

Connect the multimeter to the circuit.

  1. The multimeter's probes cannot be pushed directly into the breadboard holes. Attach alligator clip leads to your multimeter's probes (red and black leads in Figure 3).
  2. Attach short jumper wires to the other ends of the alligator clip leads, so you can push them into the breadboard.
  3. Connect the red probe from the multimeter to hole C1, as shown in Figure 2.
  4. Connect the black probe (ground) from the multimeter to the ground bus, as shown in Figure 2.

Attach the photoresistor to a jar lid. This will allow you to block unwanted light.

  1. Cut or drill a small 1/4-inch hole in a jar lid.
  2. Cover the wires from the photoresistor with electrical tape so they can't touch any other metal, or each other. This will prevent a short circuit. Leave a small amount of wire showing at the end in order to attach the alligator clips.
  3. Place the photoresistor inside the hole in the jar and tape it in place with the black electrical tape. Make sure no light can enter through the hole.

Calibrating the Light-sensing Circuit

  1. Turn on the multimeter.
  2. Set the multimeter to read DC voltage, in the 20 V range. Remember to refer to the Science Buddies How to Use a Multimeter if you need help using a multimeter.
    1. Your goal is for the voltage to range from less than 100 millivolts (mV) in the dark to around 8 V in bright light. To accomplish this, you can use the potentiometer to tune the circuit.
    2. Keep in mind that the photoresistor will pick up light from behind if it is not shielded. Make sure the hole you drilled in the jar lid is well-sealed with electrical tape, so no light leaks through the back.
  3. Place the photoresistor in bright light (for example, directly under a lamp). Adjust the potentiometer until the multimeter reads about 8.0 V.
  4. Place the photoresistor in as dark of an area as possible, where you can still read the multimeter screen. Turn off all the lights in the room, and close the blinds if you are in a room with windows, then cover the photoresistor completely (for example, with your hand, or an opaque plastic cup). If the voltage is still over 100 mV, try adjusting the potentiometer slightly, or moving to an even darker area. The photoresistor is very sensitive to even a small amount of light.
  5. Move the photoresistor back into a bright light, and check that the multimeter still reads about 8.0 V. You may need to iterate back and forth between light and dark several times as you adjust the potentiometer.
  6. If the circuit does not work at all (i.e., your multimeter reading does not change when you move from light to dark), try the following:
    1. Check all of your connections carefully. Make sure they follow the breadboard diagram shown in Figure 2. Just one incorrectly placed wire can prevent the circuit from working properly.
    2. Make sure the battery is not dead. Use your multimeter to measure the voltage across the battery terminals directly. If it is not about 9 V, get a new battery.
    3. Make sure the potentiometer is working. Use alligator clip leads to attach your multimeter to the middle pin and one outer pin of the potentiometer, and measure the resistance (not voltage). The resistance should change as you turn the knob.
    4. Make sure the photoresistor is working. Use alligator clips to attach your multimeter directly to the leads of the photoresistor. Measure its resistance (not voltage) as you move it from light to dark or vice versa. The resistance should change considerably.

Setting Up the Laser Pointer

  1. Tape the laser pointer onto the 2- X 4-inch board so that it points over the short end of the board.
    1. This will give the laser a steady mount at a fixed height.
    2. Any solid base will do.
  2. Tape the base to the work surface to keep it stable.

Setting Up Your Samples

In this step, you will set up samples with decreasing amounts of light-scattering particles.

  1. Label six clear glass jars or clear plastic cups 1 to 6 with the masking tape and permanent marker.
  2. You will make a series of 1:10 dilutions. Use 25 mL in 250 mL.
  3. The jars should have the following contents (there are four 1:10 dilutions), further described in steps 4–9:
    1. Water
    2. 100 percent fat-free milk
    3. 1/10 dilution
    4. 1/100 dilution
    5. 1/1,000 dilution
    6. 1/10,000 dilution
  4. Put 250 mL of water in jar # 1.
  5. Put 250 mL of milk in jar #2.
  6. Put 250 mL of water into jar #3. Add 25 mL of milk. Stir with a clean spoon.
  7. Put 250 mL of water into jar #4. Add 25 mL from jar #3. Stir with a clean spoon.
  8. Put 250 mL of water into jar #5. Add 25 mL from jar #4. Stir with a clean spoon.
  9. Put 250 mL of water into jar #6. Add 25 mL from jar #5. Stir with a clean spoon.

Measuring Scattered Light

  1. Now you are ready to measure scattered light. Move the circuit and the jars to a work area with dim light. There should be just enough light for you to work and to read the multimeter. You could use a nightlight or a red LED flashlight as a work light.
homemade turbidity meter

Figure 4. Top left: the photoresistor is attached to a jar lid using electrical tape. Top right: this allows you to use your light measuring circuit as a turbidity meter. Bottom left: The laser light enters the turbid liquid and scatters in all directions. Bottom right: To measure the scattered light, the photoresistor (attached to the jar lid) is placed on top of the container, over the liquid.

  1. Place jar #2 in front of the laser pointer.
    1. Use jar #2 first, since it will have a high level of scattered light.
    2. You will later measure the scattered light from jar #1 (water), and the other jars.
  2. The laser should be pointing into the liquid. If it is not, then adjust your setup.
  3. Place the photoresistor on top of the jar, so that it will collect light scattered upward by the milk.
  4. Turn off the room light and the laser light.
  5. Read the voltage on the multimeter and record it in your lab notebook. It should be less than 0.5 V. Adjust the potentiometer, as needed.
  6. Shine the laser into the milk.
  7. Read the voltage on the multimeter.
  8. Repeat steps 2–8 with the remaining jars.
    1. Record the voltage prior to turning on the laser for each sample.
    2. Record the voltage after turning on the laser for each sample.
    3. Keep the placement of the jars, the laser, and the sensor the same for all measurements.
  9. Graph the voltage (scattered light) vs. the dilution of the milk.
  10. Graph the voltage (scattered light) vs. the log of the dilution.

Tracking Enzyme Activity

Now that you have a working turbidity meter, you will use it to track an enzyme-dependent process. The process involves the clearing of milk by proteases found in fresh pineapple juice. The proteases cause the light-scattering particles in the milk to coagulate, so the milk becomes more transparent.

  1. Label three clean jars 1 to 3 with your masking tape and permanent marker.
  2. Make 500 mL of 10 percent milk by adding 50 mL of milk to 450 mL of water.
  3. Add 150 mL of the 10 percent milk to each jar.
    1. The three jars should have the following contents when you've completed the rest of the steps:
      • 10 percent milk, with no additional ingredients
      • 10 percent milk, with inactivated proteases
      • 10 percent milk, with active proteases
  4. Squeeze 100 mL of fresh pineapple juice into a clean container.
    1. To get the pineapple juice, take a fresh pineapple, cut off the rind, and grate the flesh. Place the grated fruit in a piece of cheesecloth and squeeze over a clean container.
    2. Any fresh pineapple fruit will work as will frozen pineapple that has been thawed at room temperature. Canned pineapple or refrigerated pineapple juice does not work as they are heat treated and heat destroys enzymes.
  5. Transfer 50 mL of the juice into a small, microwave-safe container.
  6. Microwave the pineapple juice for about 45 seconds, or as long as necessary to get the juice to boil for 3–5 seconds. Heat will destroy enzymes, but won't affect most chemicals. The point is to show that enzymatic activity, which is heat-sensitive, is what causes the milk to clear.
  7. Start the stopwatch. Record the times at which the pineapple juice is added.
    1. Add 2 mL of heated pineapple juice to jar #2.
    2. Add 2 mL of fresh, unheated pineapple juice to jar #3.
    3. You may need to change the amount of juice you add, depending on how fast the reaction occurs under your experimental conditions. Add less pineapple juice to get a slower reaction, and more pineapple juice for a faster one.
  8. Measure the amount of scattered light from each jar every 2 minutes for 20 minutes.
  9. Graph the voltage (scattered light) vs. time for all three jars.
  10. Repeat steps 1–8 of this section at least two more times.

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  • Measure the amount of scattered light at different angles. Set the photoresistor at the same level as the laser and measure the amount of light as you move in a circle around a jar. Use a concentration of milk that provides good scatter. Graph scattered light vs. angle.
  • Use a green laser, RadioShack Catalog # 63-132, instead of the red laser. Graph Scattered light vs. Angle for red and green lasers.
  • Determine how temperature affects the rate of milk clearing by proteases. Use 10 percent milk with active proteases, but place the milk and proteases in a water bath (such as warm or cold water in a cooler) at different temperatures.
  • Change the amount of proteases added to the milk in the last section.
  • Measure scattered light in other liquids with suspended particles, such as stomach antacid in water, or water from a local pond.
  • When phosphate (PO43-) and calcium (Ca2+) are mixed in a solution, the insoluble mineral calcium phosphate forms as a white substance that scatters light. Make solutions from antacid tablets (Ca2+) and phosphate-containing fertilizer, and use the sensor to follow the formation of calcium phosphate.

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