APPLICATION NOTE NO. 61
Calculating Calibration Coefficients for the Chelsea Minitracka Fluorometer
Revised February 2010

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The Chelsea Minitracka fluorometer supplied with your Sea-Bird CTD measures Chlorophyll-a concentrations in the range of 0.03 to 100 μg/l with a linear output of 0 to +4 volts.

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 Fluorometer, Chelsea Minitracka as a voltage sensor when editing the configuration file; the software prompts for V_acetone , V_acetone-100 , and Offset. SEASOFT calculates Chlorophyll-a concentration as:

concentration (μg/l) = (100 * [V - V_acetone] / [V_acetone-100 - V_acetone ]) + Offset

where:

V (volts) = in-situ output of the Chelsea Minitracka

V_acetone (volts) = measured output for pure acetone with 0 μg/l chlorophyll-a; also called V_{acetone-blank}

V_acetone-100 (volts) = measured output with 100 μg/l chlorophyll-a dissolved in 1 liter of acetone

Offset (μg/l) = term used to adjust to fit field data; nominally 0.0

The fluorometer comes with a calibration sheet that lists values for V_acetone and V_acetone-100 . (see Note at bottom of page)

Example of Concentration Calculation:

V_acetone-100 = 3.102 volts and V_acetone = 0.076 volts (from calibration sheet)

Offset = 0

Measured voltage from fluorometer = V = 2.53 volts

Calculated concentration (μg/l) = (100 * [V - V_acetone] / [V_acetone-100 - V_acetone ]) + Offset
= (100 * [2.53 - 0.076] / [3.102 - 0.076]) + 0 = 81.10 μg/l

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 calibration coefficients. It is only through the use of these calibrations that a meaningful and accurate measure of chlorophyll-a can be obtained. The calibration coefficients are only correct as long as the condition of the plankton population does not change; the condition does change with season and geographic location.

Shown below is an example of how to adjust V_acetone-100 in Sea-Bird’s software to reflect calibration data.

Example of Calculation of V_acetone-100 from field calibration for Sea-Bird’s data processing software:

Seawater sample analysis indicates chlorophyll-a concentration is 90 μg/l when fluorometer reads 3.2 volts.

Calculated concentration (μg/l) = (100 * [V - V_acetone] / [V_acetone-100 - V_acetone ]) + Offset
90 = (100 * [3.2 - 0.076] / [V_acetone-100 - 0.076]) + 0

Solving for V_acetone-100 :

V_acetone-100 = (100 / 90) (3.2 - 0.076) + 0.076 = 3.547 volts (input in the configuration [.con or .xmlcon] file)

Factors such as fouling, scratches, or lamp degradation reduce the sensitivity of the sensor. Enter an Offset to adjust the calibration to fit the field data.

NOTE: For chlorophyll in acetone calibrations, always use the calibration cell, as direct contact with acetone will permanently damage the fluorometer.

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