Sequoia LISST-200X, Руководство пользователя

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Appendix I: Path Reduction Module 96 LISST-
200X User’s Manual
Appendix I: Path Reduction Module
Introduction
Transmission
Limit
Increasing
Limits by
Reducing Path
Concentration
Limits The LISST-200X can measure over a wide range of particle
concentrations. However, as concentration increases, an increasing
fraction of detected light is scattered more than once before reaching
the detector. This multiple scattering can distort the apparent size
distribution, specifically by skewing it toward smaller sizes. The Path
Reduction Module (PRM) reduces this effect, extending the operating
range to higher concentrations.
There is no abrupt threshold when multiple scattering suddenly
becomes excessive. Rather, it is a gradual effect: as overall
scattering increases, the proportion of multiple scattering increases
as well. The effect is also highly dependent on particle size. The
relationship is complex. However, to assess the risk of multiple
scattering it is generally sufficient to consider only the optical
transmission. When transmission is below 30%, multiple scattering is
likely to have a noticeable effect on size distribution data
, “inventing”
fine particles. Transmissions as low as 10% may yield usable data,
but would require special care in processing and evaluating data.
If transmission is too low, meaning multiple scattering is too high, for
accurate measurements, the optical path that the laser passes
through must be reduced. This reduces the number of scattering
interactions between the laser beam and particles, thereby reducing
the proportion of multiple scattering.
Transmission and maximum concentration vary inversely with path
length. Therefore, reducing the path by a factor of 2 increases the
measurable concentration by a factor of 2. Sequoia’s Path Reduction
Module (PRM) for the LISST-200X reduces the path from 25 mm to
5 mm, for a factor of 5 increase in maximum concentration.
The table below shows the concentration limits for various particle
sizes, with and without the PRM.
Mean particle
diameter
Wentworth
grades
LISST-200X Estimated Maximum concentration in mg/L
µm Phi 25mm Path (Standard) 5mm Path (with PRM installed)
1.95 9.00 Clay 83 416
3.9 8.00 Very Fine Silt 166 831
7.8 7.00 Fine Silt 332 1662
15.6 6.00 Medium Silt 665 3324
31.25 5.00 Coarse Silt 1332 6659
62.5 4.00 Very fine sand 2728 13638
125 3.00 Fine sand 5327 26637
250 2.00 Medium sand 10655 53273