How LISST instruments measure the size distribution and concentration of particles
How LISST instruments measure the size distribution and concentration of particles
[Sequoia Scientific, April 18, 2008; article identical to Application Note L001 Rev. 10/2006]
LISST instruments represent a new generation of systems for measuring the size distribution and concentration of suspended sediments in unprecedented detail. These systems replace old technology optical backscatter sensors or single frequency acoustic sensors which suffer from calibration changes with sediment grain size and composition. The detailed size distribution of the LISST-100 far exceeds in information content the information offered by multi-frequency acoustics as well. Sequoia’s LISST-100 systems are based on small-forward angle scattering technology, formerly called laser diffraction. Original research at the company has incorporated shape effects of natural sediment grains in its processing as well – not available elsewhere.
Figure 1: Schematic of the optics shows laser (L), receiving lens (R), multi-ring detector (D) and optical transmission sensor (P).
LISST sensors observe scattering of light at multiple angles, 32 angles to be precise. Inversion of this data leads to a size distribution, i.e. concentration of sediments in 32 size classes. The size classes span a 200:1 range of particles (1.25 to 250; 2.5 to 500; or 7.5 to 1500 microns). The range of sizes within a size class covers a 1.18 ratio of sizes [i.e. 1.1832 = 200]. In addition, these systems also measure beam attenuation, which is used to de-attenuate measured multi-angle scattering. Thus, LISST’s have the transmissometer function built in.
Each ring detector measures scattering into a sub-range of angles. The ring diameters increase logarithmically from the center. The diameters of the ring detectors and the focal length of the receiving lens determine the range of angles
over which multi-angle scattering is measured. This directly determines the range of sizes which can be observed. Thus, for example, the Type-B version of the LISST-100 instrument covers the range of angles from 0.1 to 20°, which corresponds to sizes from 1.25 to 250 microns. The Type-C measures over a range from 0.05 to 10°, and this corresponds to 2.5 to 500 microns.
Figure 2: Scattering signatures of 2 distinct sizes of spheres for equal volume concentrations. Note, magnitude proportional to 1/diameter.
The total scattering seen by the ring detectors is the sum of many curves such as in Figure 2, each weighted by its corresponding concentration. Inverting the total scattering recovers the size distribution. Its sum is Concentration. The standard path length of the laser beam in water is 5 cm. For higher concentration applications, path-lengths are typically shortened to avoid errors due to multiple scattering of light. The lower end of measurable concentrations is set by noise. As usual, this limit can be continuously extended by reducing noise through averaging of data over many views (scans) of scattering. Versions of these systems are available for deep ocean applications, and elsewhere as in storm runoff monitoring, or sediment monitoring for hydropower.
