Campbell Scientific OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors - Manual (Page 39)

OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
13. Terminology
110 Rule: 100 ppm of 100-μm suspended sand will scatter light with the same
intensity as 10 ppm of 10-μm suspended silt, other factors, such as size, shape
and color, remaining constant.
Backscatter/forward scatter: The interaction of light with suspended
particles, water molecules, and variations in refractive index that alters the
direction of light transport through a sample without changing the wavelength.
The angle between the direction of propagation of a source light beam and the
direction of a scattered beam is the scattering angle. Forward-scattering refers
to scattering angles less than 90º and backscattering refers scattering angle
equal to or greater than 90º.
Calibration Slope: NTU or SSC value per mV or mA of OBS output. It is
nearly constant in the linear region of the OBS response function but is a
function of sediment concentration elsewhere.
Daylight-Rejection Filter: An optical bandpass filter that transmits near
infrared light (760 to 1200 nm) and blocks visible light (390 to 760 nm).
Drift: A change in OBS output over time or with ambient temperature that is
unrelated to the sample turbidity or SSC.
Interference: Properties of the sample, the environment, or measurement
procedure that produce unintended and unknown shifts in turbidity or SSC.
Sample Volume: The water volume where the OBS infrared beam and
suspended particles interact and the scattered light is detected. OBS sample
volumes range from 25 to 12 x 10 4 mm 3 , i.e. BBs to tennis balls, depending on
turbidity.
SDVB: Styrene divinylbenzene beads, a calibration solution also known as
AMCO Clear ® .
Sensitivity: The ratio of output level (mV, mA, or NTU value) to suspended-
sediment concentration; for example, mV per mg L –1 , mA per mg L –1 , or NTU
per mg L –1 .
Suspended-Sediment Concentration (SSC): Mass or volume of sediment in a
sample divided by the volume (or weight) of the sample expressed as mg L –1
(or ppm).
Synchronous Detection: A signal-processing technique for rejecting d.c. shifts
wherein the gain is switched from –1 to 1 synchronously with the OBS clock.
This technique limits the bandwidth and noise of a circuit.
Temperature Coefficient: The change in OBS output per unit of ambient
temperature change expressed as ppm. For example, an OBS that indicates 100
NTU in a standard at 25 o C and 101 NTU in the same standard at 5 o C would
have a temperature coefficient of –500 ppm.
Turbidity (conceptual definition): A numerical expression, in relative units,
of the optical properties that cause water to appear hazy or cloudy when light to
be scattered and absorbed by suspended matter. Turbidity is caused by
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