APPLICATION NOTE NO. 81
Entering Calibration Coefficients for Campbell Scientific/D&A OBS-3+ Optical Backscatter Sensor
Revised October 2010
Note: See Application Note 16 for the OBS-3 (not +).
The OBS-3+ measures turbidity by the optical backscatter method. A light source illuminates a water sample, and a photodetector converts the radiated light to photocurrent; the amount of photocurrent depends mainly on the area of the illuminated particles, but also on particle size, shape, and reflectivity. The signal is processed, generating an output voltage related to NTUs.
The OBS-3+ is available in 4 measurement ranges/models. Within each model, range is determined by connection to the appropriate bulkhead connector pin -- pin 5 for the 1X range or pin 4 for the 4X range. A jumper cable may be purchased from Sea-Bird to allow the range to be changed by inserting the jumper cable in line with the original cable purchased. Note that by connecting to both pins 4 and 5 (using 2 A/D channels on your CTD), you could use the OBS-3+ to simultaneously monitor both the 1X and 4X ranges, which may be useful for applications where you expect to encounter both low and high turbidity.
|1X Range (pin 5)||4x Range (pin 4)|
|250 / 1000||0 - 250||0 - 1000|
|500 / 2000||0 - 500||0 - 2000|
|1000 / 4000||0 - 1000||0 - 4000|
|2000 / 4000 *||0 - 2000||0 - 4000|
*Capable of 0 – 2000 and 0 – 8000 NTU. However, 0 – 8000 NTU is only calibrated to 0 – 4000 NTU.
Select the OBS, D & A OBS 3+ OBS/Nephelometer when setting up the configuration (.con or .xmlcon) file in Sea-Bird software. The software prompts for A0, A1, and A2, and calculates turbidity as:
NTU = (A2 * V 2) + (A1 * V) + A0
V = voltage from sensor (milliVolts)
A0, A1, and A2 = calibration coefficients from D & A calibration sheet
Example: From the Campbell Scientific/D&A Calibration Certificate for a
1000/4000 NTU sensor, at 1x ‑
NTU = (8.07146 x 10 -6 * V2) + (0.191247 * V) - 0.160 = (A2 * V2) + (A1 * V) + A0
A0 = -0.160
A1 = 0.191247
A2 = 8.07146 x 10 -6
|October 2005||Initial release.|
|October 2005||- Add information that OBS-3+ now
supported by Seasave / SBE Data Processing (version 5.37), do not
need to use User Polynomial to define.
- Add information about NTU value being dependent on the meter used, with different readings from different meters.
|May 2007||Incorporate Seasave V7; eliminate discussion of SEASOFT-DOS.|
|February 2010||- Add note referencing application
note 16 for the older OBS-3 (not +).
- Add Campbell Scientific to title, and change reference to consult D&A to Campbell Scientific instead.
- Change Seasoft-Win32 to Seasoft V2.
- Add information on .xmlcon configuration file.
- Update address.
|October 2010||Rearranged equation to NTU = (A2 * V
2) + (A1 * V) + A0 instead of
NTU = A0 + (A1 * V) + (A2 * V 2), to match Campbell Scientific calibration sheet and eliminate customer confusion.
Last modified: 26 Dec 2012
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