The depth at which it can no longer be seen is the depth recorded in meters as the test result. The secchi disk works best in deep waters such as lakes, ponds, and rivers. For shallow water, a turbidity test kit is used. In this test kit, there is a clear tube in which to put the water. This tube will have an image on the bottom.
Water is poured into the tube until the image can no longer be seen.
The column height at which the image can no longer be seen is recorded in the information marked on the tube. NTU indicates Nessler Turbidity Units. JTU indicates Jackson Turbidity Units. A sample of water is slowly poured into a long tube that is held over top of a lighted candle. The water is poured until the candle flame as viewed from above can no longer be seen.