To facilitate practical interpretation of the turbidity value, turbidimeters are calibrated with a standard suspension. Hence the reading is shown not in the form of the measured light intensity, but as a concentration of the calibration suspension. When a given suspension is measured, therefore, the reading shown means that the liquid in question scatters the same amount of light as the standard suspension of the concentration indicated.
The internationally accepted turbidity standard is formazine. The most common units are:
Conversion table: turbidity units based on formazine:
An older calibration suspension that has now fallen into disuse is that of diatomaceous earth (unit "ppm SiO2"). The reproducibility of its light scattering properties does not meet the requirements expected of standard suspensions, because the particle size distribution is not standardized. On the other hand, diatomaceous earth has the advantage that its optical properties are very similar to those of the natural turbidity substances found in water.
The readings are not directly comparable with those of formazine. Taking the measurement angle into account, however, the conversion factors listed below constitute a close approximation.