The LISST-SL (for StreamLine) is a laser diffraction system for measuring sediment properties in rivers and streams. The streamlined body is a minimum drag shape. Twin fins below the body orient the LISST-SL facing upstream before insertion. A pitot static tube on top of the body measures river velocity.
The system employs active control of the intake pump that matches river velocity to intake velocity making the sample acquisition isokinetic. Diffracted light is measured in an enclosed electro-optics section.
A major difference between the LISST-SL and other LISST instruments is that the LISST-SL is only a sensor. No data are stored on the instrument itself. The LISST-SL must be constantly connected to a topside control box, which contains a rechargeable battery, and which receives the raw scattering data from the LISST-SL itself.
Depth, velocity, and temperature are stored with each sample. With the included depth data, the system is capable of delivering sediment flux in the river. Not only is it possible to measure the total flux; since the LISST-SL measures the concentration in each of 32 size bins it is possible to measure the size-dependent flux of suspended sediment across the entire transect.
Data are transmitted up the USGS B-reel to the ‘topside’ Control Box, equipped with a touch panel display showing depth, temperature, river current velocity, mean grain size and total concentration in real-time. These data are stored together with date and time in an ASCII file, which can be offloaded and immediately displayed in EXCEL or any other spreadsheet.
In order to obtain the detailed size distribution and volume concentration in 32 size classes, the raw scattering data are also stored on the topside box. The raw data can subsequently be offloaded and processed into the standard 32-size class volume distribution.
he USGS B-reel is seen in the background. The B-reel is a NECESSARY ACCESSORY to the LISST-SL, which serves multiple purposes:
- It is used for suspending the LISST-SL in the river from the deployment location.
- It supplies power from the topside box to the LISST-SL.
- It transmits data from the LISST-SL to the topside box for real-time processing on the topside box.
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