APPLICATION NOTE NO. 62
Calculating Calibration Coefficients for WET Labs ECO-AFL and ECO-FL Fluorometer, ECO-NTU Turbidity Meter, and ECO-FL-NTU Fluorometer/Turbidity Meter
Revised February 2011

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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 following WET Labs fluorometers and turbidity meters:

• ECO-AFL fluorometer -- older model, not in current production by WET Labs
• ECO-FL fluorometer -- FL, FLD, FL(RT), FL(RT)D, FLS, FLB, or FLSB
• ECO-NTU turbidity meter -- NTU(RT), NTU(RT)D, or NTUS
• ECO-FL-NTU -- combines a fluorometer and turbidity meter, and requires two channels in your CTD

The fluorometer has a response that is linear over the measurement range provided.

• ECO-AFL (no longer current production) -- Measurement range is approximately 0.02 - 100.0 µg/l or 0.04 - 200 µg/l.
• ECO-FL -- Measurement range can be adjusted with the analog scaling value. To change the range, connect the ECO-FL directly to the computer and use WET Labs' ECOView Host software to enter a new analog scaling value.

Analog Scaling Value / Range (enter in ECOView Host)	1	2	4 (factory default)
Chlorophyll a Fluorometer: Nominal Range (µg/l)	0 - 30	0 - 50	0 - 125
Rhodamine Fluorometer: Nominal Range (ppb)	0 - 55	0 - 110	0 - 230
Phycocyanin Fluorometer: Nominal Range (ppb)	0 - 100	0 - 200	0 - 400
Phycoerythrin Fluorometer: Nominal Range (ppb)	0 - 55	0 - 110	0 - 230
CDOM Fluorometer: Nominal Range (ppb)	0 - 125	0 - 250	0 - 500

• ECO-FL-NTU -- Measurement ranges for this combination fluorometer and turbidity meter is factory-configured, and cannot be adjusted in the field. For the fluorometer channel, the range is approximately 0 to 30, 50, 75, or 125 µg/l (see Turbidity Meter Calibration Coefficients below for the turbidity meter channel).

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 one of the following as a voltage sensor when editing the .con or .xmlcon file:

• Fluorometer - WET Labs CDOM for the CDOM fluorometer
• Fluorometer - WET Labs ECO-AFL/FL for all other fluorometers

The software prompts for Vblank and Scale Factor and calculates concentration as:

concentration (µg/l or ppb, as applicable) = (Vsample - Vblank) * Scale Factor     (see Note)

where:

• Vsample (volts) = in-situ output of the fluorometer
• Vblank (volts) = measured output for a seawater blank (pure, de-ionized water) with black tape over detector     (see Note)
• Scale factor (µg/l-volts or ppb/volts, as applicable) = multiplier
• The fluorometer comes with a calibration sheet that lists values for Vblank and Scale Factor (see Note). If you changed the analog scaling value (ECO-FL series only), change the Scale Factor to correspond.

Note: Calibration sheets from WET Labs for newer fluorometers may list Dark Counts instead of Vblank. Use the Dark Counts value in place of Vblank when setting up the configuration (.con or .xmlcon) file. The Dark Counts is supplied in terms of both voltage and of counts; enter the voltage values in the Sea-Bird software.

Example Chlorophyll a Concentration Calculation in Sea-Bird Software:

Vblank = 0.05 volts and Scale Factor = 12.35 µg/l-volts (from calibration sheet)

Measured voltage from fluorometer = Vsample = 4.65 volts

Calculated concentration (µg/l) = (Vsample - Vblank) * Scale Factor
                                             = (4.65 - 0.05) * 12.35 = 56.8 µg/l

While the factory-supplied Scale Factor can be used to obtain approximate values, field calibration is highly recommended. The relationship between fluorescence and chlorophyll a is highly variable, and is not easy to determine in the laboratory. Species distribution, ambient light level, and health of the stock are just some of the factors that affect the relationship. To accurately measure chlorophyll a concentration with a fluorometer, perform calibrations on seawater samples with concentrations of plankton populations that are similar to what is expected in-situ. Determine chlorophyll-a concentrations independently, and use those concentrations, as well as readings from the fluorometer, to determine the correct Scale Factor. The Scale Factor is correct as long as the condition of the plankton population does not change; the condition does change with season and geographic location.

Example Calculation of Scale Factor from field calibration:

Seawater sample analysis shows chlorophyll a concentration is 50 µg/l when fluorometer reads 3.2 volts; measured signal for seawater blank is 0.05 volts.

concentration (µg/l) = (Vsample - Vblank) * Scale Factor
50 = (3.2 - 0.05) * Scale Factor

Solving: Scale Factor = (50) / (3.2 - 0.05) = 15.87 µg/l

Enter new Scale Factor in configuration (.con or .xmlcon) file.

 

The turbidity meter has a response that is linear over the measurement range provided.

• ECO-NTU -- Measurement range can be adjusted with the analog scaling value. To change the range, connect the ECO-NTU directly to the computer and use WET Labs' ECOView Host software to enter a new analog scaling value.

Sea-Bird PN 24345, 24346, and 24348: Nominal Range (NTU)	0 - 30	0 - 60	0 - 125
Sea-Bird PN 24367: Nominal Range (NTU)	0 - 250	0 - 500	0 - 1000
Analog Scaling Value / Range (enter in ECOView Host)	1	2	4 (factory default)

• ECO-FL-NTU -- Measurement ranges for this combination fluorometer and turbidity meter is factory-configured, and cannot be adjusted in the field. For the turbidity meter channel, the range is approximately 0 to 10, 25, 100, 200, or 1000 NTU (see Fluorometer Calibration Coefficients above for the fluorometer channel).

Note: If you require m ^-1 sr ^-1 output, WET Labs can provide a secondary calibration for these instruments in m ^-1 sr ^-1 at an additional cost. Alternatively, you can purchase the ECO-BB, which comes calibrated to m ^-1 sr ^-1. See Application Note 87 for details on how to use m ^-1 sr ^-1 output with Sea-Bird CTDs and software.

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 the OBS/Nephelometer/Turbidity – Turbidity, WET Labs, ECO-NTU as a voltage sensor 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 turbidity meter 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 of counts; use the voltage values in the Sea-Bird software.

Note: The configuration file can only be saved as an .xmlcon file (not a .con file) if the 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 turbidity and NTU is highly 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 NTU with a turbidity meter, perform calibrations on seawater samples with distributions of particles that are similar to what is expected in-situ. Determine NTU independently, and use those values, as well as readings from the turbidity meter, 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: 15 Feb 2011

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