AN61: Calculating Calibration Coefficients for the Chelsea Minitracka Fluorometer

Document Number: 
61
Publication Date: 
Tuesday, February 2, 2010
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appnote61Feb10.pdf152.04 KB

Discussion

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 Vacetone , Vacetone-100 , and Offset. Seasoft calculates Chlorophyll-a concentration as:

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

where:

  • V (volts) = in-situ output of the Chelsea Minitracka
  • Vacetone (volts) = measured output for pure acetone with 0 μg/l chlorophyll-a; also called Vacetone-blank
  • Vacetone-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 Vacetone and Vacetone-100 . (see Note at bottom of page)

Example of Concentration Calculation:

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

Offset = 0

Measured voltage from fluorometer = V = 2.53 volts

Calculated concentration (μg/l) = (100 * [V - Vacetone] / [Vacetone-100 - Vacetone ]) + 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 Vacetone-100 in Sea-Bird’s software to reflect calibration data.

Example of Calculation of Vacetone-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 - Vacetone] / [Vacetone-100 - Vacetone ]) + Offset
90 = (100 * [3.2 - 0.076] / [Vacetone-100 - 0.076]) + 0

Solving for Vacetone-100 :

Vacetone-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.

 

Application Note Revision History

Date Description
  Initial release.
February 2010 - Delete information about Seasoft-DOS.
- Add information on .xmlcon file.
- Update address.