Comparing Different Methods of Assessing Turbidity (page 3)

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Author: Selby, Grade 7








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.

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