APPLICATION NOTE NO. 48
Revised May 2007
Entering Calibration Coefficients for the Seapoint Turbidity Meter
The Seapoint Turbidity Meter measures turbidity by detecting scattered light from suspended particles in
the water using dual 880 nm light sources and dual silicon photodiode detectors with visible light blocking
filters. The design of the optical sensors confines the sensing volume to within 5 centimeters of the
sensor windows.
Sensitivity of the Seapoint Turbidity Meter is determined by two control lines that allow the user to change the
range and resolution as required for a particular application. Jumper cables may be purchased from
Sea-Bird to allow the sensor range to be changed by inserting the jumper cable in line with the original
purchased cable.
Gain Sensitivity (mV/FTU) Range (FTU)
100X 200 25
20X 40 125
5X 10 500
1X 2 < 750
In our SEASOFT-Win32 suite of programs, edit the CTD configuration (.con) file using the Configure Inputs
menu in SEASAVE V7 (real-time data acquisition software) or the Configure menu in SBE Data Processing
(data processing software). Select OBS/Nephelometers – OBS, Seapoint Turbidity as a voltage sensor when
editing the .con file; the software prompts for the gain and scale factor. SEASOFT calculates turbidity as:
OUTPUT (FTU) = 500 * Scale Factor * Voltage
Gain
where
• Gain is as described above.
• Scale Factor can be used to adjust the calibration. When greater accuracy is desired, calibrate the
sensor prior to deployment using a sample from the measurement site. When the sensor is calibrated
using a suspended particle sample, enter the Scale Factor to correct any discrepancies in the equation.
Factors such as fouling or scratches reduce the sensitivity of the sensor. Consult the Seapoint
operating manual for maintenance and calibration
procedures.
Response to turbidity levels greater than 750 FTU is non-linear. The linear values calculated by our software
will be incorrect for water samples with turbidity levels greater than 750 FTU. An approximate response can
be calculated using the second-order polynomial equation:
Voltage = 2.2 x 10
-3
(FTU) - 3.0 x 10
-7
(FTU
2
)
Sea-Bird Electronics, Inc.
1808 136th Place NE
Bellevue, WA 98005
US
Phone: (425) 643-9866
Fax: (425) 643-9954
E-mail: seabird@seabird.com
Web: www.seabird.com
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