Evaluation of the LISST-ST instrument for suspended particle size distribution and settling velocity measurements

The Laser In Situ Scattering Transmissometry (LISST) instruments produced by Sequoia Scientific Inc. use light scattering to measure concentration and size distributions of sediment suspensions. In this article, the capabilities of the LISST-ST instrument are studied. The instrument reports the suspended sediment concentration and the settling velocity for eight different size classes by measuring the sediment concentration at the bottom of a small settling column. The size resolution of the LISST-100 and LISST-ST instruments is studied. This study utilizes a new inversion algorithm to obtain the size distribution from the scattered light pattern. In addition, attention is directed to observed deviations in laboratory measurements that may affect the relative concentrations and the settling velocity estimations. These deviations are analyzed in terms of: sample mixing problems; deviations on the settling of a suspension from its ideal behavior; and the effect of the shape of natural particles on laser scattering compared to that of spheres. Results suggests, of these three points the difference in the light scattering by irregular particles compared to the scattering by spheres is the main source of deviation on the settling velocity estimations. Finally, practical recommendations are given in order to gain confidence in the instrument results.