Comparing Different Methods of Assessing Turbidity (page 2)

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Author: Selby, Grade 7

Background Reading


Turbidity is caused by suspended solid matter that scatters light passing throughout the water. Too much turbidity in the water causes gill damage in fish and makes it difficult for them to find food. There are three different ways to measure turbidity. The secchi disk, a 20cm circle that is divided into quarters, which are alternately black and white. It is attached to a rope and lowered into the water and the units of measurement are centimeters, which are marked on the rope. The spectrophotometer or Colorimeter is a machine that measures the amount of light that can pass through a sample of water in a clear 10 mL tube. The turbidity test kit involves two 50 ml granulated cylinders, distilled water, Standard Turbidity Reagent, and stirring rod.


Turbidity is very significant to life in the bay or any other body of water. SAV's (Submerged Aquatic Vegetation) similar to land plants need light to carryout photosynthesis, the process through which plants make food. If there is no light, there is no photosynthesis, leading to a decline in SAV's. SAV's are the start of many food chains. They are eaten by small fish, which are in turn eaten by larger fish, and these eaten by bigger fish, some of which are caught and eaten by humans. Light can reach twice as far as a secchi disk measures. For example if the secchi disk measurement is 30cm, the light can reach a depth of 60cm.

Clarity vs. Turbidity

It is thought that clarity and turbidity are too different and cannot be compared. In Water Quality Monitoring they can be used interchangeably, provided that the method used is indicated. The secchi disk is used if the bottom of the body of water is not visible, and the turbidity test or Colorimeter used if the bottom of the body of water is visible. The secchi disk measures clarity, how far down an object is visible. The Colorimeter and test kit measure turbidity, the cloudiness of the water.

Secchi Disk

The secchi disk is a 20cm black and white disk that measures clarity. It is attached to a rope that is marked every 5cm alternating black and yellow markings. It is lowered into the water until it just disappears. Note the water surface level on the rope, then pull the disk up, as this is being done count the number of marks below the surface by "fives". This is your reading, in centimeters. For best results take your reading in the middle of the day and on the shady side of a pier to avoid glare. The secchi disk can only be used in the field.


The Colorimeter, also known as the spectrophotometer, is a device that measures the amount of light that can pass horizontally through a clear test tube with a water sample in it. To use the spectrophotometer to measure turbidity you first clean and wash a colorimeter tube then you fill it with distilled water, place this in the colorimeter and read it as "Scan Blank", to establish a standard. Then rinse and dry the tube. Now add the sample water, place it in the spectrophotometer and measure it as "Scan Sample". This is the reading in FTU's, Fuller Turbidity Units. The term Colorimeter can be used interchangeably with spectrophotometer; Colorimeter is a brand name, whereas spectrophotometer is a general term. The Colorimeter can only be used in the lab.

Turbidity Test Kit

The LaMotte Turbidity Test Kit #7519 is another way of measuring turbidity. The test kit consists of a bottle of standard turbidity reagent, two 50 mL graduated cylinders, distilled water, and a stirring rod. To start this test pour 50 mL of sample water in one cylinder and 50 mL of distilled water in the other cylinder. If the black dot at the bottom of the tube is not visible reduce the amount of water to 25 mL in both tubes. The next step is to add the standard turbidity reagent, add 0.5 mL at a time to the distilled water tube. After adding each increment stir then look at the bottom or the tube. If the black dots appear to be the same "fuzziness" the test is done, if not continue to add reagent until they look the same. Consult the chart included in each kit for readings in JTU's, Jackson Turbidity Units. The turbidity test kit can be used in the field or the lab.


  1. LaMotte Turbidity test kit #7519
  2. LaMotte Secchi disk #1062
  3. SmartColorimeter/spectrophotometer
  4. Colorimeter tube #0967
  5. Data Sheet
  6. 1gal Distilled water
  7. Sample water
  8. Pencil
  9. Kim wipes
  10. 30 gallon steel drum
  11. Paddle for stirring
  12. TI-83 Plus calculator



  1. Add water to a 30-gallon steel drum
  2. Add clay to make the water cloudy enough so you can't see the bottom.
  3. Stir the clay to break up the large pieces and evenly distribute the particles.
  4. Clean all sample containers out with distilled water before to using

Secchi Disk

  1. To prepare the secchi disk knot the rope as near to the top of the secchi disk so that it stays at the bottom of the rope.
  2. Using a ruler, starting at the disk, measure and mark every five centimeters alternating yellow and black marks. Graduations are sewn into the rope using needle and thread.
  3. Gradually lower the secchi disk into the water until it just disappears; take a reading in centimeters at the water's surface on the graduated rope.
  4. Raise the disk until you can just begin to see it and take another reading. Average the two readings.

Turbidity Test Kit

  1. Fill one Turbidity Column #0835 to the 50 mL line with the sample water. If the black dot on the bottom of the tube is not visible when looking down through the column of the liquid, pour out a sufficient amount of test sample so that the tube is filled to the 25 mL line.
  2. Fill the second Turbidity Column #0835 with an amount of distilled water that is equal to the amount of sample being measured. This is the clear water tube.
  3. Place the two tubes side by side and note the difference in clarity. If the black dot is equally clear in both tubes the turbidity is zero. If the black dot in the sample tube is less clear, begin turbidity test.
  4. Shake the Standard Turbidity Reagent #7520 vigorously. Add 0.5 mL to the clear water tube. Use the stirring rod #1114 to stir contents of both tubes to equally distribute turbid particles. Check for amount of turbidity by looking down through the solution at the black dot. If the turbidity of the sample water is greater than that of the clear water continue to add Standard Turbidity Reagent in 0.5 mL increments to the clear water tube, mixing after each addition until the turbidity equals that of the sample. Record the total amount of Turbidity Reagent added in JTU's on data sheet.
  5. Each 0.5 mL addition to the 50 mL size sample is equal to 5 Jackson Turbidity Units (JTU's). If a 25 mL sample size is used each 0.5 mL addition of the Standard Turbidity Reagent is equal to 10 JTU's. Rinse both tubes carefully after each determination.


  1. Rinse a clean colorimeter tube #0967 with distilled water, dry completely with a wipe then fill to the 10 mL line with distilled water.
  2. Turn on the Colorimeter, select "All Tests" then select "Turbidity".
  3. Wipe the outside of the tube then insert the tube with distilled water and select "Scan Blank".
  4. Fill a clean tube to the 10 mL line with sample to be tested.
  5. Wipe the outside of the tube, then insert the tube with sample water into the Colorimeter and select "Scan Sample". The number on the screen is in FTU's.

Problem 1 - Correlating Three Tests

  1. Repeat all three tests 15 times each on different water samples.
  2. To make the water samples different add some clay to the steel drum.
  3. When the readings become extreme, remove the water and start with fresh water and clay.
  4. Record the readings.
  5. Organize these numbers in a spreadsheet, to make scatter plots for each variable.
  6. Find the correlation for the Test Kit/Colorimeter set from the spreadsheet.
  7. Turn the diagnostics off on the calculator.
    Enter the data as two lists in the calculator statistics.
    Find the power regression for any groups involving the secchi disk.

Problem 2 - Determining the Precision of Each Test

  • Conduct each test on the same water with different people recording the results.
  • Explain the test procedures to the people.
  • Help people, but don't influence the answers.
  • Enter the data on a simple table.
  • Organize the data into columns, by test, and find the mean and standard deviation for each column.
  • To determine precision in relative terms, calculate the percentage of the mean for each standard deviation.

Clean Up

* After using all the equipment should be washed out with distilled water and dried with a paper towel. *
Place the test kits away appropriately as well as carefully.

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