Why does my turbidity method require a multi-line calibration?
For turbidity, the reasoning is a bit different. The MANTECH turbidity flow cell has specific optical properties which are different than the cuvettes which you would manually insert (for example the sealed standards for the 3-month calibration). The difference in optical properties between the flow cell cylinder and the cuvette can cause the relationship between actual NTU and expected NTU to be slightly non-linear. And like the fluoride, this is really only applicable in the low end of the measuring range, typically <1NTU. To ensure the lowest detection limit possible, we use multi-line to accurately capture how the optical properties of the flow cell are different than the cuvettes.