Sequoia LISST-200X, Руководство пользователя

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LISST-
200X User’s Manual
89 Appendix F: Particle Shape Models
Appendix F: Particle Shape Models
Particles of different shapes scatter light differently, so the mathematical model used for
processing must be appropriate to the particles under observation. Two models are offered
by Sequoia: the spherical model, and the random or irregular shape model. The resulting
inversion of data will differ slightly for the two models.
The spherical particle model performs the mathematical inversion of scattering data under
the assumption that the particles that scattered light are all spheres. Light scattering by
spheres of any size and refractive index is modeled by Mie theory. According to Mie theory,
the angular scattering depends on the size of the spherical particle, and its refractive index
relative to water. Sequoia employs the full Mie scattering model, without simplifications, for
inversion of LISST measurement as a distribution of spheres. The Mie solution is a
generalized solution to the scattering of light from spheres and is commonly used as the
standard model by all laboratory laser diffraction instrument manufacturers. However, it is
not completely accurate for non-spherical particles found in nature.
Sequoia provides an alternate model that is derived empirically from natural, irregularly-
shaped mineral particles, rather than assuming spheres. The exact details of how this
scattering model was established are described in a paper by Agrawal et al. [Light scattering
by random shaped particles and consequences on measuring suspended sediments by
laser diffraction. Journal of Geophysical Research, Vol. 113, C04023, doi:10.10-
29/2007JC004403.], which
can be downloaded from the library section on Sequoia’s
website ( www.SequoiaSci.com/library ). The method is also briefly described in this article:
http://www.sequoiasci.com/article/random-shaped-particles-lissts/ . It is noteworthy that no
other instrument manufacturer has a scattering model for irregular particles. Instead, they
often use a Mie model with large imaginary refractive index, in effect, assuming the particles
to be highly absorbing (i.e. black). This is an obvious fudge with consequences!
Which model should you use? Sequoia’s software allows you to use both, then compare the
results. But in general, the random model is best for any in-situ application.
Spherical Random (Irregular)
Best use
Particles known to be
predominantly spherical, e.g.
analytical microspheres
Appropriate for natural mineral
particles
Effect of
misapplication
If used with natural irregular
particles, tends to
invent fine
particles.
If used with spherical
particles, tends to
underestimate their size.
Relation to other
instruments and
methods
Usually produces the closest
match to other laser diffraction
particle sizers (although they
may still differ due to index of
refraction)
Usually produces the best
match to analysis by sieving.