OBS501 Smart Turbidity Meter with ClearSensor
®
Technology
19
Descriptions of the factors that affect turbidity are given in Appendix
Factors that Affect Turbidity and Suspended-Sediment Measurements
(p. E-1)
.
Like all other optical turbidity monitors, the response depends on the size, the
composition, and the shape of suspended particles. For this reason, for
monitoring concentrations,
the sensor must be calibrated with suspended
sediments from the waters to be monitored
. There is no “standard”
turbidimeter design or universal formula for converting TU values to physical
units such as mg/l
or ppm. TU values have no intrinsic physical, chemical, or
biological significance. However, empirical correlations between turbidity and
environmental conditions, established through field calibration, can be useful
in water-quality investigations.
The USGS has an excellent chapter on turbidity measurements in their
“National Field Manual for the Collection of Water-Quality Data”:
http://water.usgs.gov/owq/FieldManual/Chapter6/Section6.7_v2.1.pdf
Historically, most turbidity sensor manufacturers and sensor users labeled the
units NTUs, for Nephelometric Turbidity Units. ASTM and the USGS have
come up with the following unit classifications that are applicable to the
OBS501:
Optical Backscatter
FBU
Formazin Backscatter Unit
Sidescatter
FNU
Formazin Nephelometric Unit
Ratio Back and Sidescatter
FNRU
Formazin Nephelometric Ratio Unit
The document “U.S. Geological Survey Implements New Turbidity Data-
Reporting Procedures” details the units:
http://water.usgs.gov/owq/turbidity/TurbidityInfoSheet.pdf
8.2 Vertical-Cavity Surface-Emitting Laser Diode
OBS501 sensors detect suspended matter in water and turbidity from the
relative intensity of light backscattered at angles ranging from 125°
to 170° and
at 90° for the sidescatter measurement. A 3D schematic of the main
components of the sensor is shown in FIGURE
. The OBS501 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 detectors are low-
drift silicon photodiodes with enhanced NIR responsivity. NIR responsivity is
the ratio of electrical current produced per unit of light power in air or water. 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).
