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Comparing Different Methods of Assessing Turbidity

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Updated on Feb 11, 2009


In this project three different methods of testing turbidity in water will be compared, the secchi disk, spectrophotometer, and turbidity test kit. Turbidity is caused by suspended particles that scatter light passing through water. Investigating turbidity in water is very important because SAV's (Submerged Aquatic Vegetation) need light to carry out photosynthesis, and if the turbidity is too high then the SAV's get less light and they might die.

This project has two sections. The first section is, to determine if the readings from the spectrophotometer, secchi disk, and turbidity test kit correlate. From here on out this will be referred to as "Problem 1". The second section is to determine which test is the most precise, and will give the same result if repeated by another person. From here on out this will be referred to as "Problem 2".

In Problem 1, in order to establish the degree of variation within each test, fifteen different water samples were tested for turbidity using three different methods, test kit, secchi disk, and spectrophotometer. The same person did all of the tests.

In Problem 2, fifteen different people, using the three different methods, tested the same water sample in order to establish the degree of variation within each test.

Both sets of numbers were entered into a spreadsheet (Microsoft Excel 2000). For the first set of data, correlations were found between the secchi disk and the colorimeter, secchi disk and turbidity test kit, and turbidity test kit and colorimeter. The second set of data was used to determine the standard deviation within each test, turbidity, spectrophotometer, and secchi disk. Then the mean and the percentage of the mean were calculated from the standard deviation.

In conclusion this project shows that the secchi disk has a negative correlation in comparison to the other tests because it measures clarity, the opposite of turbidity. For any correlations involving the secchi disk a type of correlation was used that is for curves called a power regression. The Colorimeter and the turbidity test kit had a close correlation. The standard deviation data show that the Secchi disk is the most precise test and the turbidity test the least precise; the Colorimeter is very close to the secchi disk although not as precise. This is expected because the test kit is very dependent on perception and varies with each person and the colorimeter measures a small amount of water.


  1. Do the readings from the spectrophotometer, secchi disk, and turbidity test kit correlate?
  2. Which test is the most precise, and will give the same result if repeated by another person?

Precision: "The level of agreement or variability among a set of repeated measurements obtained under similar conditions. Precision is usually expressed as the standard deviation in absolute or relative terms." (EPA, "Volunteer Monitor's Guide to Quality Assurance Plans")

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.








Problem 1 - Correlating Three Tests

The first set of data was done on different water, by the same person to find correlation. Correlation was found in two ways, power regression and linear regression. The perfect correlation is one or negative one. The Turbidity test kit and the secchi disk have a negative correlation of -.80 and a power regression of -.92. The Colorimeter and secchi disk also have a negative correlation of -.84 and a power regression of -.97. The test kit and Colorimeter have a positive correlation of .95 and a power regression of .89. The scatter plots comparing these tests show the same correlations.

Problem 2 - Determining the Precision of Each Test

The second set of data was done on the same water by different people to see how much each test differs. The turbidity test had the highest standard deviation at 9.3. The secchi disk had a standard deviation of 5.3 and the Colorimeter had a standard deviation of 5.1. The mean for the Colorimeter was 18.8. The mean for the secchi disk was 30. The mean for the turbidity test kit was 16. The percentage of the mean was 17.8%; for the secchi disk, 58.1%; for the Colorimeter, and 27.3%; for the test kit. Using the percentage of the mean indicates that the secchi disk is the most precise.


Problem 1 - Correlating Three Tests

Two types of correlations were used for this project, a linear regression and power regression. The first was used for straight lines and the second was used for curves. This project shows that the Turbidity Test Kit and the Smart Colorimeter correlate closely to each other with a correlation of .95. For that set a linear regression was used because the data shows up as a straight line on the scatter plot. This is logical because the test kit and Colorimeter both measure the turbidity of a sample of water. The other two were measured using power regression because they were curves. The secchi disk to Colorimeter was a -.97; this is very high. The secchi disk to test kit was -.92, which is not as high. This was expected because as seen in the second section of this project these are the two most precise. The formula was available for all the regressions so predictions were done. It is now possible, using a spreadsheet, to predict readings fairly accurately from one test form another's readings.

Problem 2 - Determining the Precision of Each Test

The second set of data was analyzed using the mean, standard deviation, and the percent of the mean. The standard deviation was highest in the turbidity test kit, at 9.3; this is logical because the test kit is subject to human error. The Colorimeter has the lowest standard deviation, 5.1, because it is not subject to human error. The secchi disk was in the middle, at 5.3, this is reasonable because when the secchi disk has disappeared that cannot be disputed. The mean for the secchi disk, colorimeter, and test kit were, 30, 18.8, and 16, in that order. The standard deviations of different sets of numbers are not comparable so the percentage of the mean was found from the standard deviation. This makes the secchi disk the most accurate test and the test kit the least precise. The Colorimeter was in the middle. The Colorimeter is a machine and should be the most accurate but it is a very small amount of water, and even the smallest change makes a large difference, such as light bouncing off suspended particles. The secchi disk is done in a larger amount of water so a little change makes very little difference. The test kit is too dependent on human perception, and will change with each person, as the data shows.


These findings are significant to many groups, including LaMotte and the Chester River Foundation. LaMotte is the company that made all of the equipment that was used for this experiment; they would like to know how to improve the tests and which to eliminate. The Chester River Association Chester Testers is a group of volunteer monitors who monitor different sites along the Chester and Corsica Rivers, and might want to make it standard that the turbidity test be repeated and that all possible sites use a secchi disk. This experiment shows that the test kit might not be as precise. There are many possible corrections to be made in this project. One might want to do more than 15 tests. Also one might consider controlling what happened to the water more, no wind, rain, or evaporation. In this project there was no way to determine if the tests were standardized, or a known standard to test each kit's accuracy.

Suggestions to further study this issue are the following, it would be useful to know definitely if one of the tests is seriously flawed and how to fix it. To supplement this experiment one might want to investigate how SAV's and turbidity are related to each other. To test this one might want to grow SAV's and see how different levels of turbidity affect them.


A recommendation, based on this data, is for all the possible sites to use the secchi disk. Although in some places the water is too shallow, these sites should purchase a Colorimeter, the test kit is not precise enough and could create many problems with misjudging the health of the body of water. The test kit is not precise enough to be used for statewide-recognized data. The Colorimeter is fairly accurate and could replace other field test kits that aren't as accurate. It would also be beneficial for the Data Center to use a specific formula to translate all the readings into one type of number. This allows the data to be compared among different sites. In addition this makes the report easier for the general public to comprehend. This is important because not all the people who read this report have a scientific background.