3. Calibration and turbidity units

3.1 Calibration principle

For turbidity measurements to be comparable, they need to be referenced to an international standard or reference material, and formazine was chosen.

It was proposed as a standard in 1926, but not recognized until 1960. It has the advantage of being reproducible using pure chemicals as reference material, thus enabling certified uncertainty to be obtained.

Before the adoption of formazine, other "standards" existed, such as silica, Fuller's earth, kaolin, diatomaceous earth and acid-purified sediments. The results were not very satisfactory, but they enabled turbidimeters to be "calibrated" for specific applications. What's more, these standards were not international.

In the water sector,...