The readings shown by turbidimeters are not scaled in the measured light intensity, but in concentration of a reference suspension. Because the accuracy of this calibration solution determines the reliability of subsequent turbidity measurements, it is of crucial importance.
The internationally established turbidity standard for calibration is formazine. This standard can be reproduced at any time with the recipe taken from standard ISO 7027 (Water quality - Determination of turbidity).
It is extremely important to observe the prescribed preparation temperature, because it
affects the particle size of the formazine particles
perceptibly. The following Figure illustrates this. Errors caused by temperature
variations are on the order of 1..2% per °C. Consequently, Sigrist keeps the preparation
temperature constant to within ± 1°C.
Particle size distributions of formazine at various preparation temperatures
Another critical variable is the purity of the water used. Experience has shown that water filtered as directed will have a residual scatter of about 0.02 FTU = 20 mFTU (inherent brightening effect). This should be taken into account during calibration and in the detection of very low turbidity levels.