Campbell Scientific OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors - Manual (Page 16)

OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
Turbidity is caused by suspended and dissolved matter such as sediment,
plankton, bacteria, viruses, and organic and inorganic dyes. In general, as the
concentration of suspended matter in water increases, so will its turbidity, and
as the concentration of dissolved light-absorbing matter increases, turbidity
will decrease. Descriptions of the factors that affect turbidity are given in
Section 11, Factors that Affect Turbidity and Suspended-Sediment
Measurements.
Like all other optical turbidity monitors, the OBS response
depends on the size, composition, and shape of suspended
particles, and for this reason, the sensor must be
calibrated with suspended solids from the waters to be
monitored.
There is no ‘standard’ turbidimeter design or universal formula for converting
NTU values to physical units such as mg L –1 or ppm. NTU values have no
intrinsic physical, chemical, or biological significance. Empirical correlations
between turbidity and environmental conditions, established through field
calibration, can be useful in water-quality investigations.
5.3 Design Details
OBS sensors detect suspended matter in water and turbidity from the relative
intensity of light backscattered at angles ranging from 90 o to 165 o , in clean
water. A 3D schematic of the main components of the OBS-3+ is shown in
FIGURE 5-1. The OBS300 has the same components but they are arranged
differently.
The OBS light source is a Vertical-Cavity Surface-Emitting Laser diode
(VCSEL), which converts 5 mA of electrical current to 2000 µW of optical
power. The detector is a low-drift silicon photodiode with enhanced NIR
responsivity, the ratio of electrical current produced per unit of light power in
A W –1 . A light baffle prevents direct illumination of the detector by the light
source and in-phase coupling that would otherwise produce large signal biases.
A daylight-rejection filter blocks visible light in the solar spectrum and reduces
ambient-light interference. In addition to the filter, a synchronous detection
circuit is used to eliminate the bias caused by ambient light. The VCSEL is
driven by a temperature-compensated voltage-controlled current source
(VCCS).
The interface between the optics and the water sample is a window made of
cast optical epoxy.
Window transmittance must remain constant in order to
prevent calibration drift, so keeping the OBS window clean
is the most important maintenance item; see Section 10,
Maintenance.
CAUTION
CAUTION
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