Campbell OBS-3A Especificaciones descargar pdf (Pagina 48)

Section 8. Factors Affecting OBS Response

8.2 Suspensions with Mud and Sand

As mentioned earlier, backscattering from particles is inversely related to

particle size on a mass concentration basis (see FIGURE 8.2-1). This can lead

to serious difficulties in flow regimes where particle size varies with time. For

example, when sandy mud goes through a cycle of suspension and deposition

during a storm, the ratio of sand to mud in suspension will change. An OBS

sensor calibrated for a fixed ratio of sand to mud will therefore indicate the

correct concentration only part of the time. There are no simple remedies for

this problem. The obvious thing to do is to take a lot of water samples and

analyze them in the laboratory. This is not always practical during storms

when the errors are likely to be largest. Do not rely solely on OBS sensors to

monitor suspended sediments when particle size or composition are expected to

change with time at a monitoring site.

0 50 100 150 200 250

Grain Diameter (microns)

0.2

0.4

0.6

0.8

1.0

Gain (V/g/l)

Gain = 8.3(D)

-0.6

FIGURE 8.2-1. Effects of Particle Size

8.3 High Sediment Concentrations

At high sediment concentrations, particularly in suspensions of high clay and

silt, the infrared radiation from the emitter can be so strongly attenuated along

the path connecting the emitter, the particle, and the detector, that backscatter

decreases with increasing sediment concentration. For mud, this occurs at

concentrations greater than about 5,000 mg/l. FIGURE 8.3-1 shows a

calibration in which sediment concentrations exceed 6 g/l cause the output

signal to decrease. It is recommended not to exceed the specified turbidity or

suspended sediment ranges unless calibrations extend over range “A” on

FIGURE 8.3-1.

8-2

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Campbell OBS-3A Especificaciones Pagina 48