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.
If the medium contains absorbent substances (such as colored 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.
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.
From the standpoint of the detection setup, this configuration has the drawback that two photodetectors are required; as they age differently, the readings will be falsified (photometer). This problem is solved by using an alternating dual-beam configuration in which the two beams reach one and the same photodetector alternately.
All Sigrist photometers employ the alternating dual-beam method.
Color 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).
Color compensation has one snag, however. Because the scattering medium acts like a color filter (wavelength), the color of the light used affects the scattering process. If the light source's spectrum changes, a p error will occur that cannot be eliminated by the color compensation system. The problem is eliminated by using narrowband (or, strictly speaking, monochromatic) light. The EBC and ISO standards call for equipping turbidimeters with color filters for this very reason.