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4.2 This guide emphasizes general principles for one par- 6.2.1 The Alliance for Coastal Technologies (ACT) did an
ticular application of optical measurements in water. This in-situ evaluation of different nephelometer technologist. Users
guide covers the use of nephelometers and backscatter type of this guide may be interested in the results of this study which
turbidity meters to record data that can be correlated with SSC. can be found online.4
6.3 Before selecting the type of meter to be used, the
5. Significance and Use operator needs to review the site requirements in order to
5.1 This guide is general and intended as a planning guide. ensure selection of the proper instrumentation. Things to
To satisfactorily monitor a specific site, an investigator must consider, but are not limited to, are: the instruments ability to
sometimes design specific installation structures or modify survive the study site environment, the degree of fouling that
those given in this guide to meet the requirements of the site in may take place, and the range of readings likely to be
question. Because of the dynamic nature of the sediment encountered at the site.
transport process, the extent to which characteristics such as 6.4 If a flow-through or in-situ monitoring device is used, a
mass concentration and particle-size distribution are accurately recording system must be installed. The recording system must
represented in the monitoring program depends on the type of have enough storage capacity to store all data recorded
equipment used and method of collection of the SSC samples between site service visits. See manufacturer’s advice on
used to calibrate the optical readings. Sediment concentration which recording devices will work best for the type of monitor
is highly variable in both time and space. Numerous samples being used.
must be collected and analyzed with proper equipment and
6.5 Remote access and near real-time transmission of data
standardized methods for the rating of the optical equipment at
from the site to the office can be very important in meeting the
a particular site (see Guide D4411 and Practice D3977).
objectives of the monitoring station. Remote access and the
5.2 All optical equipment have an upper limit for valid near real-time transmittal of the recorded data take other
readings, beyond which the meter will not read properly, equipment (such as a data collection platform (DCP), and
commonly referred to as “blacking out.” If upper range of SSC transmittal antenna) in addition to the optical sensor. A DCP
are expected to cause optical instrument black out, then some performs the same fundamental function as a basic data
other means should be devised, such as automatic pumping recorder (BDR). They both collect data from attached sensors
samplers, to collect samples during this period. See Edwards on a timed interval and store the results. The difference is the
and Glysson (1)3 and Glysson (2) for information on collection BDR retains the data until it is retrieved manually, while the
of suspended sediment samples using pumping samplers. It DCP has the ability to transmit the collected data to another
should be noted that other technologies, such as lasers and location. Since data is transferred elsewhere for storage shortly
acoustic dopplers, are also being used to monitor SSC continu- after collection, a DCP may have less memory than a BDR.
ously. The data may be transmitted via telephone modem, line-of-site
radio link or satellite. It is beyond the scope of this guide to
5.3 The user of this guide should realize that because
discuss how to instrument a site for remote data transmission
different technologies and different models of the same tech-
and no single reference on how to do this is available for
nology of turbidity meters can produce significantly different
reference here. The user is encouraged to visit the U.S.
outputs for the same environmental sample, only one manu-
Geological Survey’s Hydrologic Instrumentation Facility web-
facturer and model of the turbidity meter can be used to
site5 for information on equipment needs and used by the
develop the relationship between the SSC and turbidity read-
USGS.
ings at a site. If a different manufacturer or a different model
type of turbidity meter is used, a new relationship will need to 6.6 The installation of an automatic pumping sampler,
be develop for the site. especially in remote areas, will allow samples to be collected
that can be used to relate the optical reading to the suspended-
6. Apparatus sediment concentration in the stream and also address the
blackout periods discussed in 5.2. Detailed information con-
6.1 In general, three types of configurations of installations cerning the installation and operation of pumping samplers is
of monitors can be used: (1) the flow-through monitoring beyond the scope of this guide. See Edwards and Glysson (1)
system, (2) the in-situ monitoring system, and (3) the self and Glysson (2) for more information on the use of automatic
contained, combined sensor and recording system. pumping samplers.
6.2 Optical instruments such as photoelectric nephelometer
(best used for lower levels of SSC) and backscatter sensors 7. Site Selection
(best used for higher levels of SSC) provide the basis for this 7.1 The procedure for establishing a sampling location
method. For more information concerning the advantages and should emphasize the quest for a stream-data site. A stream-
disadvantages of each, see Test Method D7315. As they data site is as a cross section displaying relatively stable
become available, other sensors may be used. hydrologic characteristics and uniform depths over a wide

3 4
The boldface numbers in parentheses refer to a list of references at the end of Available from http://www.act-us.info/evaluation.php.
5
this standard. Available from http://water.usgs.gov/hif.