APPLICATION NOTE NO. 63
Calculating Calibration Coefficients for the Turner
SCUFA (I, II, or III) Fluorometer/OBS
Revised February 2010
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).
Notes:
1. SCUFA I measures only chlorophyll a. Discussion of chlorophyll a calibration coefficients
for SCUFA II is applicable to users who have a SCUFA I.
2. Turner no longer sells the SCUFA.
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:
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 or .xmlcon) 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.
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.
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
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: 02 Jun 2010
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