LISST - 25 User’s Guide
1
Introduction
The LISST-25X Laser Suspended Sediment Sensor provides a new level of
accuracy in measuring the size and concentration of suspended sediment.
In-situ
measuring devices such as optical transmissometers, optical
backscatter, and acoustic devices have a limitation whereby the signal is
highly dependent on the size of the material being measured. The easiest
way to emphasize this limitation is to describe a simple experiment. Start
by filling one glass container with a known mass of fine material and a
known amount of water. Place an equal mass of coarse sand into the same
amount of water in a second glass container. Both containers will have the
same mass concentration. If you stir the solutions and look through the
glass containers, you will see that light can easily pass through the
container with the larger material. The container with fine material is
much cloudier. if not opaque. This difference is due to completely different
optical transmission and optical backscatter properties. Similarly, the
acoustic properties are greatly different. The commonly used single
parameter sensors noted above will record two very different values for the
same concentration. The example just described uses a large change in
sizes to emphasize this point. Many users of sensors other than the LISST-
25X, have gone to great lengths to try to calibrate their instruments. Many
use bottom samples at the site of interest to calibrate their sensors. This
calibration will provide a constant that produces a linear response to
concentration. However, the problem with this approach is that the sample
used for calibration may or may not be what is seen in the water column.
The size distribution can change dramatically by the location, both
horizontal and vertical, or over time. Small changes in the size distribution
can cause errors up to a factor of ten in concentration measurements for
these single parameter sensors. The advantage of the LISST instruments
are that they simultaneously measure the scattered light a many angles and
thus can automatically compensate for changes in the distribution.
Sequoia’s initial laser instrument, the LISST-100 Particle Size Analyzer,
was developed to provide detailed measurements of size distribution and
concentration of particles
in-situ
. In many studies, the detailed size
distributions are not required. Only accurate measurements of the total
mass concentration are required. The LISST-25X was developed to
provide this specific measurement.
The LISST-25X is based on the same principles of laser light scattering as
the LISST-100. The Mie-theory of light scattering suggests that
collimated laser light illuminating particles will scatter most of its energy
at particular angles. Small particles scatter their energy at larger angles
and large particles at smaller angles. By recording the scattering at
different angles, the LISST-100 can mathematically invert the scattering to
find the size distribution and concentration of particles in the water. The
