AN39: Calculating Calibration Coefficients for the Chelsea Aquatracka Fluorometer

Document Number: 
39
Publication Date: 
Wednesday, February 24, 2010
AttachmentSize
appnote39Feb10.pdf158.24 KB

Discussion

The Chelsea Aquatracka fluorometer measures Chlorophyll-a concentrations in the approximate range of 0.01 µg/l to 100 µg/l with a logarithmic output of nominally 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 Aqua 3 as a voltage sensor when editing the configuration file; the software prompts for Vb, V1, Vacetone, scale factor, slope and offset. Seasoft calculates chlorophyll concentration as:

concentration [µg/l] = slope * (10 exp (V/sf) - 10 exp (VB) ) / (10 exp (V1) - 10 exp (Vacetone) ) + offset

where

V fluorometer output voltage in-situ
slope nominally 1.0; user may adjust to fit field data 1
sf scale factor (1.0 except as follows; 2.0 for versions of the SBE 9 having input amplifiers with a gain of 2)
VB electrical zero (from Chelsea calibration sheet) 2
V1 fluorometer output voltage at 1 µg/l (from Chelsea calibration sheet)
Vacetone fluorometer output voltage at zero chlorophyll (from Chelsea calibration sheet) 3
offset nominally 0.0; user may adjust to fit field data 1

Notes:

1 Users may wish to adjust the fluorometer readings to conform to measured concentrations. For example, from two in-situ samples and with the slope and offset set to 1.0 and 0.0 respectively, a user might observe that Seasoft indicated a concentration of 0.390 when the concentration determined from a water sample was 0.450, and 0.028 when the concentration determined from a water sample was 0.020. A linear regression to these data would give slope = 1.188 and offset = -0.013. The introduction of these fit coefficients in the configuration (.con or .xmlcon) file would then correct the indicated concentrations. In general, VB, V1, and Vacetone as listed on the original Chelsea factory calibration sheet can be used throughout the life of the instrument, with subsequent calibrations used to adjust slope and offset alone.

2 Chelsea defines VB as ideally being equal to the clean water response of the fluorometer. They suggest substituting the electrical zero (also referred to as VBdefault) when the clean water reading is unknown and hard to obtain — as is usually the case.

3 Vacetone is determined with acetone in a special calibration cell (available from Chelsea Instruments), NOT WITH THE FLUOROMETER IMMERSED IN ACETONE. Exposing the fluorometer lenses to acetone will PERMANENTLY DAMAGE THE FLUOROMETER LENSES.

 

Application Note Revision History

Date Description
  Initial release.
February 2001 Indicate that applies to Aquatracka fluorometer.
September 2001 Add references to modifying .con file using Configure menu in Seasave or SBE Data Processing in Windows software.
June 2006 Indicate that need to select Fluorometer, Chelsea Aqua 3 in software.
May 2007 Incorporate Seasave V7.
February 2010 - Change Seasoft-Win32 to Seasoft V2.
- Add information on .xmlcon file.
- Update address.

 

Applies To