LISST-200X User’s Manual LISST-200X Introduction and Tutorial Page 2
Clean Water
Backgrounds
and Background
QC
Imperfections, such as micro-scratches on glass windows and lenses,
also scatter laser light. This is measured and stored; it constitutes a
Background (formerly termed zscat, as in scattering by zero
concentration of scatterers). This background is subtracted from the
total signal seen by ring detectors. It is also helpful in determining if the
instrument is properly aligned, and if all optics are clean and
unscratched. A factory background acts as a reference to compare
with, to identify degradation in laser output or fouling of optical
surfaces. The LISST-200X stores its factory background data in its
memory, as well as the user’s acquired background data.
The LISST-200X incorporates a major advance over our previous
LISST-100X. This is the automatic quality control of the background
data (QC). During background acquisition, after grabbing a group of
scans of all detectors, their means and variances are computed.
Comparing with the on-board factory background, these are used to
alert the user of possibilities such as laser degradation, possible 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. If no warnings are generated,
the background passes QC and the user has the option to save it, or
repeat.
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 daylight at shallow depths can still be significant.
The LISST-200X now can measure the ambient light independently of
scattered light, and subtract it to completely remove its effects. (Note
that this function requires firmware version 1.4 or later, introduced in
May, 2017). See section H for more about ambient light rejection
(ALR).
Quick Estimates
of Particle
Concentration
and Mean Size
The process of inverting laser diffraction data to produce PSD requires
too much computation to execute in real time within the instrument. But
the LISST-200X does provide real-time estimates of the total particle
volume concentration and mean particle size. These are approximate
quantities, 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 70.)
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