Optical Laser diffraction instruments (LISST)
This article is a summary of sub-section 188.8.131.52 of the Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas. This article describes four types of in situ laser instruments to measure the suspended load transport.
Various Laser diffraction instruments are commercially available to measure the particle size and concentration of suspended sediments. The LISST instruments (Laser In-Situ Scattering and Transmissometery) are manufactured by Sequoia Inc, USA (Agrawal and Pottsmith, 2000; 2002). Diffraction patterns are shown in Figures 1a and 1b.
Types of instruments
The LISST-100 (see Figure 2) is the most widely used Laser diffraction instrument, which delivers the size distribution by inversion of the 32-angle scattering measurements.
The LISST-ST has been designed to obtain the settling velocity distribution of sediments of different sizes. In this case, a sample of water is trapped and particles are allowed to settle in a 30 cm tall settling column (see Figures 1c and 1d) at the end of the instrument-housing. Movable doors are present on both ends of the tube, which are programmed to open at regular intervals. Using a motorised propeller, a water sample is drawn into the tube through 8 openings of 20 mm diameter. Throughout, the size distribution is monitored near the bottom of the settling tube. After sampling, a few seconds are allowed for turbulence to break down before the doors are closed and the sample is allowed to settle for several hours. During settlement of 12 and 24 hours runs, respectively 72 and 83 Laser scans are made in logarithmically scheduled time intervals. Over time, the size distribution shows zero concentration in sizes that have settled out. The time for settling is used to estimate settling velocity. From knowledge of the size versus settling velocity, mass density can be estimated. This instrument obtains the settling velocity and particle density for 8 size classes in the 5 to 500 micron range. The assumption that all particles settle independently in a complete stagnant fluid is often violated. As a result, the calculated particle density distribution often becomes unrealistically wide to compensate for effects such as convection and particle interaction.
The LISST-25A and 25X instruments are simpler (Figure 3), less expensive versions of the LISST-100. Replacing the multi-ring detector of the LISST-100, a special shape for a focal plane detector was invented. This shape (comet-detector) is the result of solving the mathematical problem: does there exist a detector shape that would measure light scattering in a manner that it holds calibration for all sizes? Indeed, the LISST-25 holds calibration for spheres over a 200 to 1 size range, where earlier sensors would vary in calibration by a factor of 200! The LISST-25 instrument is a superior sensor to the LISST-100 when only concentration measurement is required. The LISST-100 obtains sediment concentration by first inverting the 32 multi-angle scattering data to construct the size distribution and then summing the concentrations in the 32 size classes. When small numbers of particles are present, as can happen with coarse particles, the inversion can miss them due to noise. In contrast, since the comet-detector directly estimates concentration from the weighted sum of angular scattering, it misses nothing. A second attribute of the LISST-25 is that this device obtains particle area concentration from the optical transmission. The ratio of the volume concentration and area concentration is called the Sauter Mean Diameter (SMD), first introduced in the aerodynamics-droplet combustion literature. The two types of LISST-25 refer to an analogue output only version and a second version that is fully recording and presents a coarse fraction concentration in addition to the total suspended load. The LISST-25X instrument has new comet shapes built in to separate between wash load finer than 63 micron and the sand load larger than 63 micron. The two new comet shapes deliver the total concentration and SMD in the entire size range and concentration and SMD in the coarse sand range. The comet shapes assume nothing regarding the underlying size distribution of sediments. The only requirement is spherical shape for particles. Inaccuracies of perhaps as much as 100% may occur if the particle composition changes from mineral to biogenic.
This instrument (Figures 4a and 4b) is a streamlined body that draws a sediment-laden stream into it for Laser measurements. It incorporates a Laser, optics, multi-ring detector identical to the LISST-100 and electronics for signal amplification and data scheduling and transmission. A pump is also built-in to ensure isokinetic withdrawal rates. The pump is controlled by a microprocessor, which is fed information about the river velocity by a propeller type current meter to ensure isokinetic velocity sampling. The propeller is mounted above the body itself and a sensor is employed to count the number of its rotations in a short period of time. This device includes pressure transducers to record the depth of sampling. The LISST-SL has been designed to provide real-time data on sediment concentrations and particle-size distributions. The velocity and concentration data are used to compute fluxes (on-line) for up to 32 particle size classes at points, verticals or in the entire stream cross-section (Gray, 2004).
The LISST-SL offers a very powerful instrument for on-line measuring of particle size, concentration per size class (32 classes) and hence transport using a separate sensor for velocity measurements in rivers and estuaries. A severe limitation is the relatively small concentration range (up to 500 mg/l) due to insufficient light penetration of the optical sensor in conditions with concentrations larger than 500 mg/l. For technical specifications, see Sequoiasci.
Summaries of the manual
- Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas
- Chapter 1: Introduction, problems and approaches in sediment transport measurements
- Chapter 2: Definitions, processes and models in morphology
- Chapter 3: Principles, statistics and errors of measuring sediment transport
- Chapter 4: Computation of sediment transport and presentation of results
- Chapter 5: Measuring instruments for sediment transport
- Chapter 6: Measuring instruments for particle size and fall velocity
- Chapter 7: Measuring instruments for bed material sampling
- Chapter 8: Laboratory and in situ analysis of samples
- Chapter 9: In situ measurement of wet bulk density
- Chapter 10: Instruments for bed level detection
- Chapter 11: Argus video
- Chapter 12: Measuring instruments for fluid velocity, pressure and wave height
Other internal links
- Optical remote sensing
- Light fields and optics in coastal waters
- General principles of optical and acoustical instruments
- Optical backscatter point sensor (OBS)
- Acoustic point sensors (ASTM, UHCM, ADV)
- Acoustic backscatter profiling sensors (ABS)
- ↑ Rijn, L. C. van (1986). Manual sediment transport measurements. Delft, The Netherlands: Delft Hydraulics Laboratory
- ↑ Agrawal, Y.C. and Pottsmith, H.C., 2000. Instruments for particle size and settling velocity observations in sediment transport. Marine Geology, Vol. 168, p. 89-114.
- ↑ Agrawal, Y.C. and Pottsmith, H.C. 2002. Laser Diffraction Method: two new sediment sensors. Sequoia Inc., USA .
- ↑ Gray, J.R., 2004. The LISST-SL streamlined isokinetic suspended-sediment profiler. Proc. 19th Int. Symp. on River Sedimentation, Yichang, China.
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