Visibility Disc – According to Secchi

The visibility disc according to Secchi is a simple, means of determining the visibility depth.

The visibility disc is lowered into the water with a wire, and when the disc is no longer visible in the water a reading of the depth is made by using the marks on the rope.

The disc is then lowered another 0.5 metres and pulled up slowly. When the disc becomes visible, the second reading is done. The visibility depth is determined by taking the arithmetic mean of both measurements.