A Secchi disk is a 20 cm black and white disk that disappears when lowered into water and again when raised. Before using the Secchi disk, students should find a place in the shade to take the measurement of transparency of the water sample. Students should then lower the Secchi disk into the water until it just disappears. At this point a student should make a mark on the rope. Then students can raise the Secchi disk until it just reappears into view and again a student should mark this point on the rope.
Students can then measure the distance between the two points on the rope.
Since different people see the Secchi disk disappear at different depths this observation should be made by three different students. Turbidity tube A Turbidity tube is a tube with a pattern similar to that of the Secchi disk on the bottom. Students should again find a place in the shade before taking the measurement of transparency of the water sample. Students can then pour sample water into the tube until the image at the bottom of the tube is no longer visible when looking directly through the water column at the image. Students can then record the depth of water in the tube when the pattern disappears.