APPLICATION NOTE NO. 39
Application note in pdf format
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.
Select Fluorometer, Chelsea Aqua 3 as a voltage sensor when setting up the configuration (.con) file in Sea-Bird software. The software calculates:
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 |
NOTE: The CTD configuration (.con) file is edited using the Configure menu (in SEASAVE or SBE Data Processing in our SEASOFT-Win32 suite of programs) or the Configure Inputs menu in SEASAVE V7.
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) 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.
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