ERDC/CHL CETN-IV-24
March 2000
2. Salinity meters. Water quality data loggers measure conductivity and temperature and
compute salinity concentrations corrected to a known calibration. This type of recorder uses a
specific conductance electrode cell and a thermistor-type temperature sensor. The salinity
concentrations are calculated from the measured conductivity and temperature. The range and
accuracy of these recorders vary with individual instruments. The user must choose the one that
will be the most appropriate for their needs.
3. Suspended sediments. Water samples for analysis of suspended sediment concentrations and
total suspended solids are obtained by several methods. Niskin samplers, P61 isokinetic
samplers, and pump samplers all acquire suspended sediment samples. Each method has its own
set of optimal operating capabilities. The manually pumped water sample has a somewhat
limited range of applicability because of the biasing that this technique has to the grain-size
distribution of the sample. Typically, a pumped sample undersamples the larger grain sizes;
whereas, the P61 isokinetic sampler captures a more accurate total suspended materials sample
for a wider range of velocities. The Niskin sampler captures a finite volume by closing the two
ends of a piece of pipe. This method might also undersample the larger grain sizes at higher
flow sites. It is not been fully tested against isokinetic samplers to verify this claim.
Automatic water samplers are also available to provide unattended sampling. Water samples are
collected in plastic bottles located inside the sampler body. This type of sampler is fully
programmable to obtain any volume of sample up to the maximum size of the bottle, for
obtaining composite samples, for setting different intervals between samples, and for setting
times to begin the sampling routine.
Another method of measuring suspended sediments is by using an optical backscatter sensor.
This type of sensor is designed to measure turbidity and suspended solid concentrations by
detecting scattered infrared light from suspended matter. It consists of a high intensity infrared
emitting diode, a series of silicon photo-diodes as detectors, and a linear solid state temperature
transducer. As with other optical turbidity sensors, the response of the back-scatterance sensor
depends on the size distribution, composition, and shape of particles suspended in the medium
being monitored. For this reason, these sensors must be calibrated with suspended solids from
the waters being monitored.
4. Wave gauges. Wave data, such as height and frequency, can be recorded using water level
recorders and are similar to the tide gauges. This type of recorder uses a strain-gauge or quartz-
type pressure transducer, which records the absolute pressure of the water column above the
instrument. Some of these instruments are not vented to the atmosphere, and changes in the
barometric pressure must be corrected. Water pressure is measured for a desired sample interval,
and an average value is computed and finally stored on the data logger. These instruments
should be placed in shallow water to negate damping effects caused by the absolute pressure of
the water column. As with the tide gauges, recording interval, frequency, and sampling rate are
set according to the needs of the project.
Wave followers or buoy gauges are also available to obtain wave data. These instruments are
tethered to the bottom by means of an elastic shock cord. The buoy follows the waves by
floating on top of the water surface. Accelerometers measure the motion of the buoy as waves
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