Sequoia LISST-200X, Руководство пользователя

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LISST-200X Operation Details 72 LISST-
200X User’s Manual
Watch for
Outgassing and
Bubbles Water may contain dissolved air that develops into bubbles as the
water warms. Bubbles, especially if they cling to the windows of the
LISST-200X, will greatly modify the scattering pattern, rendering the
background useless. Be sure to remove any bubbles from the windows
before acquiring a background. Use a squirt bottle or pipette to blow
the bubbles off the window.
Ambient Light
Rejection Starting with firmware version 1.4 in May, 2017, the LISST-200X
automatically measures and subtracts light that is not produced by
scattering from its laser. This Ambient Light Rejection (ALR) prevents
sunlight or other bright sources from contaminating the scattering
measurements. Without ALR, as in earlier LISST instruments, high
ambient light levels, if not physically blocked from the LISST’s optical
aperture, could falsely elevate the apparent concentration of small
particles.
You can verify that ALR is active by connecting to the instrument with
LISST-200X, then opening the terminal window. Type DS and press
enter, and look for “Ambient light subtraction: on” in the output. If it is
not shown, you should upgrade the firmware. Contact Sequoia
Scientific for assistance.
ALR works by measuring the signal on the LISST-
200X’s 36 detectors
first with the laser on, then with the laser off. This on-off process is
repeated 3 times per second. Because of the time required for the laser
and electronics to change between the on and off states, rapidly
changing ambient light may not be completely rejected. Therefore, if
working in bright sunlight, near the water surface, and in conditions
where the sunlight is rapidly modulated by waves, ambient light could
still have an effect. ALR will reduce the average effect, but the
changing ambient conditions could increase sample-to-sample noise. In
those worst-case conditions it is still advisable to shade the LISST-
200X optics from ambient light. The same is true if operating under
intense artificial light modulated by AC power.
Optical
Alignment The background measurement (described step-by-step on page 29) is
the best source of information on the current health of the LISST-200X.
It provides information on the current functionality of all of the major
systems including laser, ring detector, data collection electronics, and
optical alignment. The LISST-200X is a sensitive optical device. The
laser must be aligned such that the focused spot is centered on the
hole in the center of the ring detector. If this alignment in not correct
the instrument will not function correctly. The LISST-200X software
automatically detects and warns of serious misalignment, during the
background measurement process.
The first indicator of an alignment problem is a severe drop in the
transmitted laser power when compared to the factory laser power.
The laser power and laser reference values will change over time but
they should track together.