defr Glossar

Multiple scatter

Source of error in the measurement of turbidity value by detecting the scattered light intensity.

At high turbidity levels, straightforward measurement of scattered light intensity without colour compensation first displays reduced sensitivity. After a maximum reading has been reached, the reading will decline even though the concentration is rising (Fig. 29, red curve). This is caused by the phenomenon of multiple scatter: the particles closer to the light source tend to screen off the incident light, and the scattered light is unable to propagate freely.

Scattered light behaviour with and without compensation
Fig. 29: Scattered light behaviour with and without compensation

With a dual beam system, the reversal of the characteristic curve can be prevented. The measuring system (Fig. 9) is identical with colour compensation. Parallel to the standard turbidity measurement, a reference beam passes right through the medium. As a result of the formation of the quotient of both readings, the characteristic curve (Fig. 29, blue curve) becomes clear.

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