pg
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25
2.9
Turbidity
Sensor Overview
Turbidity is the indirect measurement of the suspended solid concentration in water and is typically
determined by shining a light beam into the sample solution and then measuring the light that is scattered
off of the particles which are present. The suspended solid concentration is an important water quality factor
and is a fundamental measure of environmental change. The source of the suspended solids varies in nature
(examples include silt, clay, sand, algae, organic matter) but all particles will impact the light transmittance
and result in a turbidity signal.
The EXO Turbidity sensor employs a near-infrared light source and detects scattering at 90 degrees of the
incident light beam. According to ASTM D7315 method,
this type of turbidity sensor has been characterized as a
nephelometric near-IR turbidimeter, non-ratiometric
#
.
This method calls for this sensor type to report values in
formazin nephelometric units (FNU). FNU is the default
calibration unit for the EXO sensor but users are able to
change calibration units to nephelometric turbidity units
(NTU), raw sensor signal (RAW), or total suspended solids
(TSS) assuming the user enters the appropriate correlation
data.
The RAW value is a value unaffected by user calibrations
and provides a range from 0-100, representing the per cent
of full scale that the sensor detects in a sample.
1.11
#
ASTM D7315-07a “Test Method for Determination of
Turbidity Above 1 Turbidity Unit (TU) in Static Mode.”
Specifications
Default Units
FNU
Temperature
Operating
Storage
-5 to +50°C
-20 to +80°C
Range
0 to 4000 FNU
Accuracy
0-999 FNU: 0.3 FNU or
±2% of reading, whichever is
greater; 1000-4000 FNU: ±5%
of reading
Response
T63<2 sec
(see pg 12)
Resolution
0-999 NTU: 0.01 FNU
1000-4000 FNU: 0.1 FNU
Sensor Type
Optical, 90° scatter
Optics:
Excitation
.
860±15 nm
(continued)
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