Environmental Engineering Reference
In-Depth Information
particle-size distribution, laser-path length, and SSC;
it ranges from tenths of a gram per liter (for small
particle sizes) to a few grams per liter (for larger
particle sizes). In addition, as is the case with all
types of
in situ
optical instruments, biofouling can
degrade measurements.
These problems can be addressed with anti-fouling
shutters or optical blocks that reduce the laser path
length (Sequoia Scientifi c, Inc. 2008). For example,
reducing the optical path in water from the standard
5 cm to 3 mm has been effective in extending meas-
urement limits to 2-3 g/L of fi ne material. For still
higher SSCs, a LISST-Infi nite was developed as part
of a research-and-development project with the
USGS. The LISST-Infi nite, a prototype of which was
tested by the USGS (Konrad
et al.
2006), pumps a
water-sediment sample to the instrument, and then
uses automated multi-stage dilution (as necessary)
before measuring PSDs and SSCs with a built-in
LISST-100. Thus, the measurable SSC limit is, in
theory, extended to the highest SSCs of material that
can be pumped to the LISST-100 (Yogesh Agrawal,
Sequoia Scientifi c, Inc., personal communication
2008). However, the process of pumping the water-
sediment sample from a point in the channel may
alter the original size distribution. Still another
version of the LISST-100, the LISST-FLOC, is
designed to measure larger particles such as fl occu-
lated estuarine marine particles.
As previously presented, laser diffraction tech-
niques historically have interpreted the light scat-
tered by natural particles as 'equivalent spheres', i.e.
an ensemble of spheres with identical angular scat-
tering properties. However, spherical particles are
rarities in nature. Angular scattering from irregularly
shaped particles is different to that from spheres. An
irregular particle scatters light similarly to that of a
spherical particle that is ¼- to ½-phi larger than the
irregular particle's median diameter (Agrawal
et al.
2008). For example, a natural particle of diameter
10
volume conversion constant, an empirical calibration
constant supplied by the manufacturer. Although
laboratory versions of laser diffraction instruments
are available from several manufacturers, the authors
are aware of only one (Sequoia Scientifi c, Inc. 2008)
that produces commercially available instruments
designed for
in situ
applications and manual
deployment.
The purchase price of one of the laser instruments
(
in situ
and manually deployed) described in this
section ranges from about two to six times that for
a fully equipped turbidimeter, depending on the
instrument of interest. The instrument-measurement
realm of the
in situ
instruments described herein is a
point in a stream. When used for measurement of
PSD or volume SSC, they do not require routine
instrument calibrations.
The LISST-100, which has been fi eld and labora-
tory tested, has been shown to successfully determine
PSDs of natural materials and the size of mono-sized
particle suspensions within about a 10% accuracy
(Traykovski
et al
. 1999; Gartner
et al
. 2001; Meral
2008). It can also be used to determine mass SSC
from volume SSC if particle density is known
(Traykovski
et al
. 1999; Gartner
et al
. 2001; Melis
et al
. 2003). Unlike single-frequency optical back-
scatter instruments, these instruments are not subject
to potential inaccuracies associated with changes in
PSDs if the particle sizes fall within the range of
instrument sensitivity (Agrawal & Pottsmith 2000).
Onboard memory and power allow high temporal
resolution sampling at intervals up to 5 Hz during
fi eld studies that range in time scales from days (or
tidal cycles) to months. In addition to analysis of
PSDs and concentrations of inorganic material,
LISST instruments are now being used increasingly
for analysis of size distribution and population con-
centration and mixing dynamics of organic material
such as phytoplankton (see, for example, Serra
et al
.
2001, 2003; Karp-Boss
et al
. 2007).
There are limitations associated with the use of
LISST instruments for determining size distribution
of suspended sediment. The scattering model (Mie
theory) requires absence of multiple light scattering;
thus, there is an upper SSC limit because of the pres-
ence of multiple scattering from particles at high
SSC. Agrawal & Pottsmith (2000) found multiple
scattering effects occurred when optical transmission
was less than 30%. The limiting SSC is a function of
m particle
using laser diffraction. Agrawal
et al
. (2008) quanti-
fi ed the multi-angle laser scattering characteristics of
natural particles. They interpreted the measured laser
light scattering as random shaped particles rather
than spheres, an interpretation that produced results
consistent with sieved samples.
An instrument somewhat similar to the LISST-
100, the LISST-25, measures mean SSC and Sauter
μ
m may be inferred as a 12- to 14-
μ
