Environmental Engineering Reference
In-Depth Information
(a)
(b)
Fig. 1.10
Laser
in situ
scattering and transmissometers. (a) a LISST-100
in situ
instrument; (b) an in-development LISST-SL
(streamlined) manually deployable instrument (photographs courtesy of Sequoia Scientifi c, Inc.).
1 : 200) of particle sizes. Scattering by spheres (larger
than the wavelength of light) at small angles is equal
to diffraction by apertures of the same diameter
(Swithenbank
et al.
1977; Agrawal
et al
. 1991;
Agrawal & Pottsmith 1994). In addition, scattering
is determined almost completely by light diffracted
by the particle; any light transmitted through the
particle does not affect the small angle measurement,
thus, this method of determining size distributions is
mostly insensitive to changes in particle color or
composition (Agrawal & Pottsmith 2000). However,
departure from spherical shape produces changes in
estimated PSDs and SSCs; laser diffraction instru-
ments provide the equivalent sphere-size distribution
(Agrawal
et al
. 2008).
Commercially available instruments to measure
PSD using laser diffraction have been available for
laboratory use since the early 1980s, for example
instruments made by Malvern Instruments and
Coulter Corporation to name two manufacturers.
The fi rst attempt to apply the technology for
in situ
application used a commercial laboratory instrument
adapted for ocean use (Bale & Morris 1987). A self-
contained version of a laser diffraction instrument
that could be deployed in an autonomous mode and
determined PSD in eight size classes is described by
Agrawal & Pottsmith (1994).
A more advanced and commercially available
version of the instrument (Agrawal
et al
. 1996;
Agrawal & Pottsmith 2000) capable of providing
time series of PSDs and volume SSC values is the
Laser In Situ Scattering and Transmissometry
(LISST)-100 (Sequoia Scientifi c, Inc. 2008). The
LISST-100 (Fig. 1.10a), with an overall length (minus
cable) of 87 cm and diameter of 13 cm, measures
optical transmission, water temperature, and hydro-
static pressure in addition to PSD and volume SSC.
The LISST uses a 670-nm wavelength solid-state
laser. The standard sample path of this device is a
cylindrical volume with a diameter of approximately
6 mm and a length of 50 mm, although versions with
shorter laser-path lengths are available for highly
turbid environments. The instrument uses a 32-ring
detector with logarithmically increasing radii to
measure scattering intensity at 32 small forward
angles that correspond to 1.25-250
μ
m (LISST-
m
(LISST_FLOC). The inner radius (smallest-scattering
angle) of the ring detector corresponds with the
largest measured particles and the outer radius (larg-
est-scattering angle) corresponds with the smallest
measured particles. The measured scattering inten-
sity distribution is also referred to as the volume
scattering function (VSF) (Pottsmith and Bhogal
1995; Agrawal and Pottsmith 2000). In practice, to
determine PSDs and volume SSCs, the measured VSF
is fi rst corrected with a background scattering distri-
bution. The corrected VSF is mathematically inverted
to determine a PSD that would produce the multi-
angle scattering that fi ts the measured observation in
the 32-ring detector. Details of the inversion process
can be found in Agrawal & Pottsmith (2000).
Volume SSC is calculated from the inverse of the
corrected scattering distribution divided by the
100B), 2.5-500
μ
m (LISST-100C), or 7.5-1500
μ
