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THE SECCHI DISK

Angelo Secchi was an astronomer (1818-1878) who classified stars by their spectra. For some reason unknown to this brilliant writer, one of the most widely used instruments for measuring water transparency, called the Secchi disk, is named after him. Such an instrument can be made in at least two ways.

a. Bind the edge of a metal coffee can lid with adhesive tape. Spray white paint on one side of the lid and black paint on the other (or half of each side of the lid can be sprayed white and the other half black, or the lid can be divided into quarters, with two opposite quarters painted white and two black). Now tie a string to a horseshoe magnet, and hold the lid with the magnet.

b. Insert a weighted eyebolt through a metal disk about 8 inches in diameter. Suspend the disk from a chain or calibrated line.

Prepare two or three aquariums; fill one aquarium with tap water; fill the others with different amounts of powdered milk to make them cloudy. Lower the disk into one aquarium, then another.

When the disk disappears from view, record the depth by marking a line at the water surface and measuring the vertical distance to the disk.

Order the cloudiness of the water in the three aquariums from your measurements. The device you used is called a Secchi disk. It is used to determine the cloudiness or turbidity of water.

Use the Secchi disk to test other liquids (e.g., baking soda and water, adobe clay and water).

(Note: Rinse out the aquariums when you are finished -- some materials such as powdered milk, decay quickly and become difficult to remove.)

Here's more:

Secchi disks can be used to measure the transparency of water, expressed as the depth or limit of visibility of lakes, ponds, estuaries, oceans, or other bodies of water.

Lower a Secchi disk on a line marked every 3 inches into a body of water until the disk disappears from view. (Be sure the disk remains horizontal with the water surface at all times.) Take a line reading -- the distance from the surface of the water to the disk -- when the disk disappears from view. Raise the disk until it reappears, and take another reading. The limit of transparency can be determined by finding the arithmetic ("ar-rith-METT-ick") mean between the depth at which the disk disappeared and the depth at which it reappeared.

Now take several more measurements. You may find that many variables can influence the readings: the distance of the observer's eye from the water surface, the roughness of the water, the time of day, etc. Standardize your procedures to make your readings consistent. (Consistency of the data can be improved if the disk is viewed with the observer's eye at a fixed distance from the water surface and if the obser- vations are made in the shade during the middle of the day.)

Keep other records such as the date, time, and place.

You can use the Secchi disk in other ways:

1) to determine to what extent the water transparency of the same body of water differs from day to day and season to season;

2) to investigate differences in the water transparency of local lakes, estuaries, and/or other bodies of water;

3) to find out if water transparency varies significantly with distance from industrial areas or sewage treatment plants.

The various bodies of water can also be ordered by their degrees of transparency.

Why do we need to know this stuff?

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