APPLICATION NOTE NO. 16
Entering Calibration Coefficients for D&A Instrument's OBS-3 Optical Backscatter Sensor
Revised October 2012
Note: See Application Note 81 for the OBS-3+ sensor.
The gain of the OBSTM sensor was set at the factory at a value of 400 FTU (Formazin Turbidity Units) per volt, and is designed to produce signal voltages in the range of 0 - 5 volts as required by your Sea-Bird CTD. The factory-preset gain should be sufficient for most, if not all, oceanographic conditions. If the preset gain is not appropriate for your application, it is possible to make a hardware change of the gain by adjusting resistance values on the circuit board within the pressure housing. The OBS Instruction Manual contains instructions for this procedure.
The OBS sensor sensitivity is dependent upon the characteristics of the particles in the water that are being measured. As a result, the sensor should be calibrated before and after use in different water types, using particles from the water that is being measured. Consult the OBS Instruction Manual for the correct calibration procedures. The resulting sensitivity, in mg/l or FTUs per volt, is entered in the CTD configuration (.con or .xmlcon) file.
Configuration File Entry
In our Seasoft V2 suite of programs, edit the CTD configuration (.con or .xmlcon) 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, Backscatterance (D & A) as a voltage sensor when editing the configuration file; the software prompts for gain and offset.
|September 2001||Previously referred to SEACON in discussions of .con files. Add references to modifying .con file using Configure menu in Seasave or SBE Data Processing in Windows software.|
|July 2001||Add information about offset term, and clarify units.|
|May 2007||Incorporate Seasave V7.|
|March 2008||Update to include V2 SeaCATs (16plus V2, 16plus-IM V2, 19plus V2).|
|February 2010||- Change Seasoft-Win32 to Seasoft V2.
- Add information on .xmlcon files.
- Update address.
|October 2012||Update to include SBE 25plus.|
Sea-Bird Home Phone: (+1) 425-643-9866 Fax: (+1) 425-643-9954 E-mail: email@example.com