My turbidity cuvette has no line to guide the volume added.

The instructions for the Turbidity sensor state that the sample cuvette be filled "so that the bottom of the meniscus is at the top of the white line." Some cuvettes no longer have this line. If you are using a cuvette that has no line, you can draw your own with a permanent marker such as a Sharpie. Simply place the empty cuvette next to the 100 NTU standard cuvette. Draw the line so that the levels of the liquids will be at the same height.