Once the Secchi disk itself sinks out of view, the user checks the markings on the line to see how far down into the water the disk went before it faded from sight. The user takes a second reading when they pull the disk back up, then compares the results.

On this Voyage of Discovery, the students brought an updated verson of the Secchi disk with them, simply called a turbidity tube. It works on similar principles as the traditional Secchi disk.