Sequoia LISST-100X, Руководство пользователя

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LISST-100X User’s Guide 1
Section 1: Introduction to the LISST-100X
Introduction
Principle of
Operation The product name LISST is derived from the term that describes its
operation:
L aser I n- S itu S cattering and T ransmissometry. LISST is
a Trademark of Sequoia Scientific, Inc.
The LISST-100X instrument uses the technique of laser diffraction
to obtain
particle size-distribution (PSD), also called volume
distribution in this manual. This section describes the principle of
operation of your instrument.
The laser diffraction method for sizing particles was invented in the
1970’s and rapidly became the most widely used optical method for
determining size distribution for the simple reason that for laser
diffraction, the composition or refractive index of the particles is not
important. This method determines size distribution of an ensemble
of particles, as opposed to counting type devices that size one
particle at a time.
The reason that laser diffraction is unaffected by composition of
particles is that the scattering of laser light is observed at multiple,
small forward angles. At these small angles, light scattering is
determined almost entirely by light diffracted by the particle. The
light transmitted through the particle makes only a weak
contribution to the measured scattering. Since only the light
transmitted through the particle would
experience the composition
of the particle, i.e. its refractive index, and since it makes only a
weak contribution to the observed scattering, the method of laser
diffraction is mostly
independent of particle composition.
In the aquatic sciences, particle refractive index is poorly known
because particles are often a mixture of mineral grains and living
and dead particles of biological origin. Thus, except for shape
effects, laser diffraction offers an excellent method for size-
distribution estimation.
By calibrating for concentration with ISO standard natural particles,
shape effects are empirically included for the determination of the
volume concentration of the suspended particles. The user can
convert to mass concentration by multiplying with an appropriate
density. If the density of the particles is not knows it is not possible
to convert the volume concentration to mass concentration.