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LISST-100

(Laser In-Situ Scattering and Transmissometry)

The LISST-100 is an optical instrument for in-situ measurement of particle size spectra in waters with sediment loads exceeding 200µ/l. Based on the principle of laser diffraction, the LISST-100 records the small-angle intensity distribution of light scattered by particles suspended in water. This distribution, which is proportional to the volume scattering function, is inverted to obtain the particle concentration and size spectrum. In addition, the optical transmission is sensed for this inversion, so the instrument is also a transmissometer. By avoiding water sampling, particle aggregates remain undisturbed.

An on-board computer permits use of the instrument over periods up to six months in a user-programmed data-acquisition schedule. Two supporting parameters are included in the measurement; pressure and temperature. One external analog input port is available on an underwater connector for storage of data from other instruments. Additionally, two external digital I/O ports are available for receiving/transmitting timing or trigger signals. The emphasis in the design is on reliability, flexibility and low power consumption. Alkaline battery packs provide up to six months of operational life. An external RS-232C port permits easy programming and data off-loading. Software is provided for conversion of data to MATLAB or ASCII format and subsequent inversion to obtain size distribution.

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