Sequoia LISST-200X, Руководство пользователя

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LISST-
200X User’s Manual
71 LISST-200X Operation Details
F.
Performance Optimization
Background
Measurement
and its
Importance
As discussed in earlier sections, the LISST-200X measures light
scattered at small angles, to characterize particles in water. However, a
certain amount of scattering is also generated by pure water, and by
the LISST-
200X’s own optics and windows. We call this the
background scattering. In order to determine the scattering contributed
by particles, the background scattering must be accurately measured
and subtracted from the total scattering measured. The measurement
of background scattering is sometimes called a
“zscat”, meaning the
signal measured in water with zero
“scatterers” or particles.
The minimum concentration of particles measurable by the LISST-200X
is very sensitive to the quality of the background scatter file. The lower
the concentration in the water being measured, the more important the
quality of water and the care used when measuring the background.
When measuring in water with high particle concentrations, the relative
signal-to-background noise ratio increases, and final results will be less
sensitive to flaws the background. However, a background should
always be collected before an experiment.
Techniques for
measuring
background
Cleanliness It is essential to use clean, bubble-free water for the background
measurement. The water can be fresh or salt water. For most
applications, steam-distilled water works well. Steam-distilled water is
typically available in one-gallon containers. We have found that this
distilled water tends to be a bit cleaner than typical bottled or packaged
water. We buy steam-distilled drinking water and filter it through a
0.2µm filter.
A small volume horizontal chamber has been provided for submerging
the optics while acquiring a background. It is inserted between the
windows of the optics. The instrument should be placed horizontally on
the supplied white plastic supports. However, plastic bags, Tupperware
or clean containers of any kind can be used to hold water for a
background measurement
. The instrument’s optics end can be
submerged in them vertically, as long as the optics are completely
submerged and there is no blockage of the windows by bubbles or
other objects.
Because the area surrounding the windows is submerged, it is
important to thoroughly clean and rinse this part of the instrument
before acquiring a background.
Toothbrush, liquid soap and water works well for cleaning the optical
end of the instrument. Do not use abrasive powders, which will scratch
the windows and degrade instrument performance.