Wet Labs ECO VSF, User Manual

Page: 22 / 32

16 ECO VSF User’s Guide (VSF) Revision AN 14 Sept. 2011
Temperature correction—Output from an LED reference detector is provided, which gives an
indication of relative LED intensity during operation.
Obtaining angular values in inverse meters—The primary angular values for each angle of
backscattering are directly calculated by ECOView or alternatively can be applied upon raw
data downloaded from the instruments. Determination is made by subtracting the clean water
offset from the measured value and multiplying the result by the scaling factors provided in
the calibration sheet.
The scaling coefficients for these values are determined by our instrument calibration process
as described in Section 6.2.
5.2 Determining other Angle-Specific Coefficients
Other angular scattering coefficients can be determined through interpolation between the
measured angles. At this time WET Labs offers no preference in the type of curve fit to use
for deriving these coefficients.
5.2.1 Determining Backscattering Coefficient
The most accurate method of determining the backscattering coefficient, b
b
, from the VSF
measured at the three angles of the ECO VSF is as follows:
1.
Multiply the corrected  values by 2  sin  to convert to a polar steradian area.
2. Fit a 3rd order polynomial to the three measured points and a fourth value of  radians
= 0 (sin (  radians) = 0).
3. Integrate under the curve fit from  /2 to  radians.
4. Add 1.3 percent to the backscattering estimate.
Testing this approach with all the Petzold (1972) VSFs results in a maximum error of
about 1 percent.
(Petzold, T.J., 1972 . Volume scattering functions for selected natural waters. Scripps Institution of
Oceanography, Visibility Laboratory, San Diego, CA, SIO Ref. 71–78.)