Sequoia LISST-200X, Руководство пользователя

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LISST-200X Introduction & Quick Start 2 LISST-
200X User’s Manual
Clean Water
Backgrounds Scattering originates not only from particles in the water, but also from
the water itself, and from the windows and internal optics of the LISST-
200X. This non-particle scattering constitutes a
Background. (Formerly
this was called zscat, meaning scattering by zero concentration of
scatterers). Accurate measurements require subtracting the
background scattering from the total signal seen by ring detectors.
The background scattering typically changes gradually throughout the
life of the instrument, due to small changes in the laser and optics, and
wear on the exposed window surfaces. It is also very sensitive to
cleanliness of the windows. Therefore, high-quality measurements
require careful background measurements by the user. A
factory
background acts as a reference for comparison. The LISST-200X
stores its factory background data in its memory, as well as the user’s
acquired background data.
Background
Quality Control Because the background measurement is so important, the LISST-
200X firmware and software include procedures for measuring and
evaluating the background scattering. The software checks for signs of
problems such as laser degradation, optics misalignment,
contamination of data by thermal microstructure in the water used for
background, bubbles or particles in the supposedly clean water,
scratches on windows etc.
For details on the background measurement procedure, see Saving
and Evaluating Clean Water Backgrounds on page 29.
Ambient Light
Rejection The laser diffraction method requires that the light arriving on the
detector be due entirely to scattering of light that originates with the
laser beam. Light from ambient sunlight can distort the results. The
LISST physically shields the detector from much ambient light, but not
all, so daytime measurements at shallow depths can include some
sunlight. However, the LISST-200X (starting with firmware version 1.4,
introduced in May, 2017) can measure the ambient light independently
of scattered light, and remove its effects. This Ambient Light Rejection
(ALR) function is enabled by default. Also see section 0, page 71.
Quick Estimates
of Total
Concentration
and Mean Size Converting the LISST-
200X’s measurements of scattering to PSD
requires extensive computations that are time-consuming for a small
microprocessor. However, a much simpler computation, easily done in
real time, provides estimates of the total particle volume concentration
and mean particle size. These quantities are not as accurate as the
fully-processed PSD, but are useful for quick characterization of water
conditions. For example, if used in conjunction with a CTD package in
profiling applications, the CTD software could display the concentration
and mean size as a depth profile, revealing any vertical structure in the
particle distribution. (Note that real-time interface to a CTD requires
proper configuration of the LISST-
200X’s auxiliary connector; see
Configuring the LISST-200X as a Sensor for a CTD on page 56.)