Application note 63

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APPLICATION NOTE NO. 63

Calculating Calibration Coefficients for the Turner SCUFA (I, II, or III) Fluorometer/OBS
October 2002

The Turner SCUFA measures fluorescence and optional turbidity. The fluorescence channel is configured to detect either chlorophyll a (SCUFA II) or Rhodamine WT tracer dye (SCUFA III).

Note: SCUFA I measures only chlorophyll a, and is no longer available through Sea-Bird. Discussion of chlorophyll a calibration coefficients for SCUFA II is applicable to users who have a SCUFA I.

As listed in the SCUFA manual, all units are calibrated with primary standards at the Turner factory, with the 0 to 5 volt range equal to:

0 - 80 μg/l of chlorophyll a (SCUFA II)
0 - 200 ppb of Rhodamine (SCUFA III)
0 - 200 NTU (turbidity, SCUFA II or SCUFA III)

These can be used to calculate the factory default scale factor:

Scale Factor = [value at 5 V - value at 0 V] / 5 V

The user can customize the 0-5V SCUFA range to correspond to the expected data range with Turner's SCUFAsoft software, thus improving data resolution. Change the range, as needed, before you set up the configuration (.con) file in the Sea-Bird software.

Example: You expect a range of 0 – 10 μg/l for chlorophyll a fluorescence. If you do not change the default range, the maximum voltage will be:
0.62 V (= 10 μg/l / [80 μg/l / 5 V] )
This limits the resolution and multiplies the noise level of the instrument. Changing the range using SCUFAsoft to 0 - 10 μg/l provides the best results.

Note on Chlorophyll a Calibration

While the nominal Scale Factors based on factory default settings, and 0 Offset, can be used to obtain approximate values, field calibration for chlorophyll a 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, 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. It is only through the use of these calibrations that a meaningful and accurate measure of chlorophyll a can be obtained. 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.

See Turner’s SCUFA manual, Sections 2 through 4, for calibration details.

Setting Up Configuration (.con) File in SBE Data Processing or SEASAVE

Use the Configure menu to create / modify the .con file. See the software Help files for details.

(if optional turbidity included in SCUFA) Select the Turner SCUFA OBS. The software prompts for Scale Factor and offset and calculates turbidity as:

turbidity (NTU) = (Scale Factor * Voltage) + Offset

where

Scale Factor (NTU/volt) = (turbidity value at 5 volts - turbidity value at 0 volts) / 5 volts
(Turner calls the Scale Factor the calibration coefficient)

Offset (NTU) = OBS value at 0 volts

Select the Turner SCUFA fluorometer. The software prompts for Scale Factor, offset, fluorescence units, mx, my, and b (prompts for mx, my, and b only if SCUFA OBS was already entered in .con file) and calculates fluorescence as:

Chlorophyll a (SCUFA II) -- Equations shown are for units of µg/l; other units available

chlorophyll a (µg/l) = (Scale Factor * Voltage) + Offset

corrected chlorophyll a (µg/l) = (mx * chlorophyll a) + (my * NTU) + b

where

Scale Factor (µg/l-volt) = (chlorophyll a value at 5 volts - chlorophyll a value at 0 volts) / 5 volts
(Turner calls the Scale Factor the calibration coefficient)

Offset (µg/l) = chlorophyll a value at 0 volts

mx, my, and b = correction factors for correcting chlorophyll a data for turbidity effects

Rhodamine (SCUFA III) -- Equations shown are for units of ppb; other units available

Rhodamine (ppb) = (Scale Factor * Voltage) + Offset

corrected Rhodamine (ppb) = (mx * Rhodamine) + (my * NTU) + b

where

Scale Factor (ppb/volt) = (Rhodamine value at 5 volts - Rhodamine value at 0 volts) / 5 volts
(Turner calls the Scale Factor the calibration coefficient)

Offset (ppb) = Rhodamine value at 0 volts

mx, my, and b = correction factors for correcting Rhodamine data for turbidity effects

Example of Chlorophyll a Concentration Calculation:

If fluorometer Scale Factor = 14.5 µg/l-volts, and

Measured voltage from fluorometer = 4.65 volts

Calculated concentration (µg/l) = (Voltage * Scale Factor) + Offset
= (4.65 * 14.5) + 0 = 67.4 µg/l

Last modified: 06 Apr 2007

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