APPLICATION NOTE NO. 81

Entering Calibration Coefficients for D & A Instruments OBS-3+ Optical Backscatter Sensor
Revised October 2005

Application note in pdf format

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.

*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) file in Sea-Bird software (SEASAVE or SBE Data Processing). The software prompts for A0, A1, and A2, and calculates turbidity as:

NTU = A0 + (A1 * V) + (A2 * V ²)

where:
V = voltage from sensor (milliVolts)
A0, A1, and A2 = calibration coefficients from D & A calibration sheet

Example: From D & A’s Calibration Certificate for a 1000/4000 NTU sensor, at 1x ‑
NTU = -0.160 + (0.191247 * V) + (8.07146 x 10 ^-6 * V²) = A0 + (A1 * V) + (A2 * V²)
A0 = -0.160
A1 = 0.191247
A2 = 8.07146 x 10 ^-6

Notes:

• The CTD configuration (.con) file is edited using the Configure menu in SEASAVE or SBE Data Processing in our SEASOFT-Win32 programs.
• NTUs are relative and arbitrary units. All the NTU value means is that a particular turbidity meter responds to a particular water sample as if it contained a certain amount of the turbidity standard (by interpolation) used for its calibration. The NTU value has no relation to the physical and optical properties of the suspended particulate matter in the water sample that caused the response, unless one was establish by an independent calibration with the field material. In other words, the NTU value for a water sample will depend on the meter used, because there are not standard designs like for transmissometers, and also on the standard used for the meter’s calibration. For example, you might measure 15 NTU in a well-mixed lake with a Sea Point sensor, 19 NTU with an OBS sensor, and 11 NTU with a YSI sensor. Consult D & A (http://www.d-a-instruments.com) for discussions of turbidity.
• The OBS-3+ is adjusted at the factory for a nominal range and sensitivity for a given gain setting. Sensor sensitivity is dependent upon the characteristics of the particles that are being measured in the water. As a result, the sensor should be calibrated before and after use in different water types, using samples of the type of water being studied. Consult D & A (http://www.d-a-instruments.com) for the correct calibration procedures.

Last updated: 06 Apr 2007

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