APPLICATION NOTE NO. 48

Entering Calibration Coefficients for the Seapoint Turbidity Meter
Revised May 2007

Application note in pdf format

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.

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 (FTU²)

Last modified: 30-Jun-2009

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