APPLICATION NOTE NO. 87
Calculating Calibration Coefficients for the WET Labs ECO-BB Scattering/Turbidity Meter
Revised August 2010

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(Note: This application note is for use with Seasave and SBE Data Processing version 7.20g and later. If you are using older software, click here for an earlier version of the application note, which is compatible with earlier software versions.)

This Application Note applies to the use of the WET Labs ECO-BB series of scattering/turbidity meters with Sea-Bird CTDs. The ECO-BB has a response that is linear over the measurement range provided.

WET Labs has changed the type of output available from the ECO-BB:

• For products currently sold by WET Labs:

• ECO-BB with 0 - 5 m ^-1 sr ^-1 range: WET Labs provides a calibration sheet for calculation of output in m ^-1 sr ^-1 .

• For products previously sold by WET Labs:

• ECO-BB with 0 - 25 NTU range: WET Labs provided two calibration sheets, for calculation of output in NTU or m ^-1 sr ^-1, as desired by the user.
• ECO-BB with 0 - 100 NTU range: WET Labs provided a calibration sheet for output in NTU only.

Note: If you require NTU output in a currently manufactured product, WET Labs ECO-NTU or ECO-FLNTU are calibrated for calculation of output in NTU. See Application Note 62.

Each calibration sheet provides a Dark Counts and Scale Factor consistent with the output units. Definition of the sensor in Sea-Bird software follows for m ^-1 sr ^-1 and NTU (NTU output available only for older ECO-BB meters).

Note: The ECO-BB comes with a calibration sheet that lists values for Dark Counts (instead of Dark Voltage) and Scale Factor. Each of these values is supplied in terms of both voltage and counts; use the voltage values in the Sea-Bird software.

1. Use the Configure Inputs menu in Seasave V7 (real-time data acquisition software), or the Configure menu in SBE Data Processing (post-processing software), to create / modify the .con or .xmlcon file (see software Help files).
2. Select OBS/Nephelometers/Turbidity – Turbidity Meter, WET Labs ECO-BB when editing the configuration file (the WET Labs ECO-BB turbidity meter was added to the list of voltage sensors in software version 7.20g; earlier versions do not list it).
   The software prompts for Scale Factor and Dark Voltage, and calculates:

b(qc) = (Voltage – Dark Voltage) * Scale Factor                     (b(qc) in units of m -1 sr -1)

where:

• Voltage (volts) = in-situ output of the ECO-BB
• Dark voltage (volts) = measured output for a seawater blank (pure, de-ionized water) with black tape over detector
• Scale factor (b(qc)/volts) = multiplier 

Note:

The configuration file can only be saved as an .xmlcon file (not a .con file) if the WET Labs ECO-BB turbidity meter is selected as one of the sensors.

To get NTU units, you will select the ECO-NTU instead of the ECO-BB in the Sea-Bird software.

1. Use the Configure Inputs menu in Seasave V7 (real-time data acquisition software), or the Configure menu in SBE Data Processing (post-processing software), to create / modify the .con or .xmlcon file (see software Help files).
2. Select OBS/Nephelometers/Turbidity – Turbidity Meter, WET Labs, ECO-NTU when editing the configuration file (the ECO-NTU turbidity meter was added to the list of voltage sensors in software version 7.20g; earlier software versions do not list it). The software prompts for Dark Voltage and Scale Factor, and calculates concentration as:

turbidity (NTU)  = (V – Dark Voltage) * Scale Factor

where:

• V (volts) = in-situ output of the turbidity meter
• Dark Voltage (volts) = measured output for a seawater blank (pure, de-ionized water) with black tape over detector
• Scale factor (NTU/volts) = multiplier

Note:

The configuration file can only be saved as an .xmlcon file (not a .con file) if the WET Labs ECO-NTU turbidity meter is selected as one of the sensors.

While the factory-supplied Scale Factor can be used to obtain approximate values, field calibration is highly recommended. The relationship between ECO-BB output and β(Θc) or turbidity is somewhat variable, and is not easy to determine in the laboratory. Particle shape and size are some of the factors that affect the relationship. To accurately measure with the ECO-BB, perform calibrations on seawater samples with distributions of particles that are similar to what is expected in-situ. Determine β(Θc) or turbidity independently, and use those values, as well as readings from the ECO-BB, to determine the correct Scale Factor. The Scale Factor is correct as long as the distribution of particle sizes and shapes does not change; the condition does change with season and geographic location.

Last modified: 08 Sep 2010

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