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Section 8. Factors Affecting OBS Response

in this region. OBS sensors are also more sensitive to mineral particles than

either bubbles or particulate organic matter by factors of four to six. In most

environments, interference from these materials can therefore be ignored. One

notable exception is where biological productivity is high and sediment

production from rivers and resuspension is low. In such an environment, OBS

signals can come predominately from plankton. Prop wash from ships and

small, clear mountain streams where aeration produces high bubble

concentrations are another probable source of erroneous turbidity readings.

0 20 40 60 80 100 120 140 160 180

Scattering Angle (

)

0.001

0.010

0.100

1.000

10.00

Scattering Intensity

Organic Material

Minerals

OBS

Backscatter (90 - 180

)

Bubbles

FIGURE 8.6-1. Scattering Intensity vs. Angle

8.7 Biological and Chemical Fouling

Sensor cleaning is essential during extended deployments. In salt water,

barnacle growth on an OBS sensor can obscure the IR emitter and/or detectors

and produce an apparent decline in turbidity. Algal growth in marine and fresh

waters has caused spurious scatter and apparent increases of OBS output. The

reverse has also been noted in fresh water where the signal increases after

cleaning the sensor window.

Prolonged operation in freshwater with high tannin levels can cause a varnish-

like coatings to develop on an OBS sensor that obscure the IR emitter and

caused an apparent decline in turbidity. Cleaning algal and tannin

accumulation off OBS sensors is required more often during the summer

because warm water and bright sunlight increase biological and chemical

activity. See Antifoulant Coatings for alternatives to cleaning.

8-5

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Campbell OBS-3A Especificaciones Pagina 51