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LISST-SL V2.1

User’s Guide

It is important that you understand the significance of the patterns of light on the rings. The
total amount of light on rings is a combination of a ‘

Background’

and light scattered by

particles. The Background arises from imperfections and micro-scratches on optical
windows. This Background is very carefully measured to create a

Background File

. The

background is subtracted from the total signal on the rings to compute the scattering from
particles alone. The background changes due to age, or due to particle deposition on
windows. Hence, it is necessary to measure the background before every experiment.

To understand the importance of the Background, it
is  important  to  understand  the  nature  of  light
scattering  by  particles.  For  this  purpose,  we  show
the  light  scattering  patterns  on  the  rings  for

equal

concentrations

of large (94 microns), and small

particles (13 microns). Note that for a given volume
or mass concentration, small particles produce a lot
more  scattered  light  than  large  particles  (the
relationship  is  inverse  with  size).  Also,  the  main
peak  of  scattering  occurs  at  small  rings  for  large
particles,  and  vice  versa;  i.e.  here  too,  there  is  an
inverse  relationship  between  the  particle  size  and

the ring where its maximum scattering occurs. Viewed another way, light on small rings is
interpreted as the presence of large particles, and vice versa.

This  last  point  is  very  important.  It  follows  that  an  error  in  the  measurement  of  the
Background on the small rings is equivalent to a large error in the amount of large particles.
A similar error on the outer rings would amount to a smaller error of the small particles. For
this  reason,  the

strongest emphasis is placed on getting extremely good

measurements of background light on the smallest rings.

This is why we repeatedly

emphasize maintaining clean optics and low, stable values of background scattering on the
inner rings. See also Appendix D on background scattering.

General Precautions:

The instruments are precision opto-electronic devices,

avoid shock

and vibration

. While in storage, be sure to cover the pitot tube intake with a pinched tube,

so that it does not dry out.

Operational Limits:

While working in extreme conditions of water turbidity, you should

note that there are limits on both the upper and lower concentrations:

At the lower end, too few particles can lead to very weak scattering on the ring
detectors. A guide for water turbidity is the

optical transmission

measured across

the  test  cell.  The  optical  transmission  is  the  ratio  of  laser  power  transmitted
through  the  cell,  to  its  value  in  clean  particle-free  water.  This  defines  the  lower
concentration limit of operations. Although with care, you may be able to operate
and  get  meaningful  data  at  transmission  as  high  as  99.8%,  the  instrument  is
designed to produce good data at transmissions below about 98%.

In  contrast,  at  the  high  turbidity  end,  when  too  many  sediment  particles  are
present  in  the  laser  beam,  multiple  scattering  degrades  results.  Multiple
scattering grows with decreasing transmission. As a practical matter, we suggest
that  at  transmission  below  about  30%,  accuracy may  degrade.  These  limits  are
worth remembering while you interpret your field measurements. An approximate
estimate  for  operating  limits:  from  about  10mg/l  to  about  2,000  mg/l.  Above  the
highest specified concentration (i.e. transmission below 30%) the data can still be
used all the way down to a transmission of 10%. The error in concentration and
size distribution will simply increase at these low transmissions. For example, for
a  single  size  particle  measurement,  we  have  seen  an  error  of  about  10%  in
concentration  at  a  transmission  reaching  10%.  In  other  words,  the  data  at