Sequoia LISST-SL, Руководство по эксплуатации

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LISST-SL V2.1
User’s Guide
4
Quick Reference Sheet: The touch-screen panel is intuitive and easy to use. A two-sided,
laminated Quick Reference Sheet is attached to the inside cover of the TCB. Once you
understand the operations, this sheet will be sufficient to remind you of the procedures for
use.
Pen: For note writing, or for selecting very small fonts (as in Delete Files), a pen is
provided. Please keep this attached to the clip on the inside of the cover.
Laser Diffraction Principles : Because some of the terminology in this manual refers to
principles and components of laser diffraction, a brief explanation is offered here. The
measurement of sediment size distribution and concentration is based on the principle of
laser diffraction. The idea is as follows. Inside the
–SL, the river water passes between a
pair of parallel glass windows. A laser beam crosses these windows. Light scattered by
particles in this laser beam is collected by a receive lens, and is sensed by 32 ring-shaped
detectors, each one covering a small, distinct angle sub-range. From these 32
Ring Detector
measurements, 32 solutions are obtained using methods of linear algebra. These 32
solutions are the
concentration of sediments in 32 size classes. The sum of all these
represents the total suspended sediment concentration . A full description of this method is
available in several publications. For more detail, we invite the reader to review the
Application Note ‘How LISST instruments measure the size distribution and concentration
of particles
’ on our website (
http://sequoiasci.com/Articles/ArticlePage.aspx?pageId=119 ).
1.4 Terminology
Background Files, and
“
VIEW RINGS
”
. In this manual, you will frequently read about ring
detectors
(sometimes just ‘rings’). It is important to understand what this means. We
therefore offer a brief description of instrument optics. In the figure above, a laser beam
illuminates particles in water, flowing between two water-tight glass windows. [For a cut-out
cross-section of the instrument, see figure at end of this chapter]. The particles scatter light
(one example ray is shown as a thin line). A receive lens collects this light. On the focal
plane of this lens, a special detector is placed. It consists of 32 concentric rings of silicon
(hence the term ‘
rings
’ or ‘ ring detector’
). At the center of the rings is a 75 m diameter
hole. The laser beam passes through this hole and is detected with a silicon photodiode
(transmission sensor)
. In this manner, the ‘rings’ measure light scattered out of the laser
beam, while the power of the beam that passes through the central hole is the beam
‘transmission’ measurement.
This is the
attenuation suffered by the laser as well as by the scattered light as it passes
through water. This measurement of attenuation is used to de-attenuate the measured
scattered light on the ring detectors. The attenuation is not used to estimate sediment
concentration.