Example Calculation of Scale Factor from field calibration:
Seawater sample analysis shows chlorophyll-a is 50 μg/l when fluorometer reads 3.2 volts; measured signal for
seawater blank is 0.05 volts.
concentration (μg/l) = (Vsample - Vblank) * Scale Factor → 50 = (3.2 - 0.05) * Scale Factor
Solving: Scale Factor = (50) / (3.2 - 0.05) = 15.87 μg/l → Enter new Scale Factor in configuration [.con] file.
Turbidity Meter Calibration Coefficients
The ECO-FL-NTU’s turbidity meter has a response that is linear over the measurement range provided: 0 to 25 NTU.
The turbidity channel in the ECO-FL-NTU is not currently directly supported in Sea-Bird software. However, you can
set up the turbidity channel as a User Polynomial in the configuration (.con) file, which allows you to define an
equation to relate the sensor output voltage to calculated engineering units:
Value = a0 + (a1 * V) + (a2 * V
2
) + (a3 * V
3
)
Wet Labs defines turbidity as:
turbidity (NTU) = (Vsample - Vblank) * Scale Factor
where:
Vsample (volts) = in situ output of the turbidity meter
Vblank (volts) = measured output for a seawater blank (pure, de-ionized water)
Scale factor (NTU/volts) = multiplier
The ECO-AFL-NTU comes with a calibration sheet that lists values for Vblank and Scale Factor for the
turbidity meter.
Setting the Wet Labs equation equal to the user polynomial equation and calculating a0, a1, a2, and a3:
(Vsample – Vblank) * scale factor = a0 + (a1 * V) + (a2 * V
2
) + (a3 * V
3
)
Expanding the left side of the equation and using consistent notation (Vsample = V),
scale factor * V – scale factor * Vblank = a0 + (a1 * V) + (a2 * V
2
) + (a3 * V
3
)
Rearranging:
(– scale factor * Vblank) + (scale factor * V) = a0 + (a1 * V) + (a2 * V
2
) + (a3 * V
3
)
a0 = - scale factor * Vblank a1 = scale factor a2 = a3 = 0
In our SEASOFT-Win32 suite of programs, edit the CTD configuration (.con) 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 the User Polynomial for the turbidity channel when editing the .con file; the software prompts for a0,
a1, a2, and a3 as well as a sensor name (which will appear in the data file header).
While the factory-supplied Scale Factor can be used to obtain approximate values, field calibration is highly
recommended. The relationship between turbidity and NTU is highly variable, and is not easy to determine in the
laboratory. Particle shape and size are some of the factors that affect the relationship. To accurately measure NTU with
a turbidity meter, perform calibrations on seawater samples with distributions of particles that are similar to what is
expected in situ. Determine NTU independently, and use those values, as well as readings from the turbidity meter, to
determine the correct Scale Factor. The Scale Factor is correct as long as the distribution of particle sizes and
shapes does not change; the condition does change with season and geographic location.
248
To Next Page
Loading...