Photometric detection setup for measuring scattered light that eliminates the effects of absorbent substances.
When the turbidity value is measured by determining the sideward scattered light intensity, a light beam is passed through the flow cell and the scattered light produced by the solid particles (turbidity) is detected at a specific angle. This measurement method ignores the light that passes straight through the cell.
Fig. 8: Turbidity measurement without colour compensation
If the medium contains absorbent substances (such as coloured constituents), these can easily attenuate the light beam by factors ranging from 2 to 5 and thus falsify the results. So it is absolutely necessary to eliminate the effect of the medium's absorption in such cases. This is done by using two light beams, a measurement beam and a reference beam.
Fig. 9: Colour compensation in the dual-beam system with two photodetectors
The reference beam passes right through the medium and is attenuated by both the absorbent substance and scatter. Because the geometry of the flow cell ensures that both beams traverse the same path length, the measurement beam is attenuated by exactly the same amount. This eliminates the effect of absorption.
Colour compensation provides additional bonuses. For one, it compensates the cell window contamination that occurs inevitably in continuous measuring setups, because both beams pass through two windows. For another, it prevents the occurrence of the maximum, i.e. reversal of the characteristic curve in the case of high turbidities (multiple scatter).