Turbidity

Turbidity is a natural phenomenon when light passes through a solution, which presents itself as light scattering and absorption. It is caused by small particles which can differentiate with regard to size, surface area and colour and thus can have an influence on the resulting measurement. Turbidity measurements are taken by recording the scattered light (reflection, sideways scattering, or via the reduction in the light beam transmission). Turbid substances have an influence on the determination of in vivo chlorophyll fluorescence since they absorb incidental light which is not available for fluorescence excitation. Moreover, the blank value is altered by the turbid substances, which should be taken into account particularly at low chlorophyll levels. In general, at low turbidity levels (transmission > 75 %) and measurements of fluorescence emission below an angle of 90°, the chlorophyll deviations due to the particles present are below 5 % and are mostly neglected. For fluorescence instruments which measure the reflection below an angle of 150 – 180°, higher deviations in the chlorophyll measurements are produced by the particles. These deviations then have to be corrected.

For this, the AlgaeTorch uses an IR LED which does not excites the chlorophyll.  The same photodetector used to measure the chlorophyll volume is used to determine the measurement values. A turbidity measurement is taken for each chlorophyll measurement. The calibration of the turbidity measurement is carried out using formazine as standard (at bbe) so that the display is given in FTU units (formazine turbidity units). The automatic turbidity correction during the chlorophyll measurement makes subsequent correction unnecessary, improves the accuracy of the chlorophyll measurement and enables evaluation of the finished measurement in the field at site.