Sea-Bird Electronics SBE 45 MicroTSG, User Manual

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3
Example
Three salinity bottles are taken during a CTD profile; assume for this discussion that shipboard analysis of the
bottle salinities is perfect. The
uncorrected CTD data (from Seasave V7) and bottle salinities are:
Approximate
Depth (m)
CTD Raw
Pressure (dbar)
CTD Raw
Temperature ( ° C) *
CTD Raw
Conductivity
(S/m)
CTD Raw
Salinity
Bottle
Salinity
200 202.7 18.3880 4.63421 34.9705 34.9770
1000 1008.8 3.9831 3.25349 34.4634 34.4710
4000 4064.1 1.4524 3.16777 34.6778 34.6850
* Temperatures shown are ITS-90 . However, the salinity equation is in terms of IPTS-68; you must convert
ITS-90 to IPTS-68 (IPTS-68 = 1.00024 * ITS-90) before calculating salinity. SEASOFT does this automatically.
The uncorrected salinity differences (CTD raw salinity - bottle salinity) are approximately -0.007 psu. To
determine conductivity drift, first correct the CTD temperature and pressure data. Suppose that the error in
temperature is +0.0015 °C uniformly at all temperatures, and the error in pressure is +0.5 dbar uniformly at all
pressures (drift offsets are obtained by projecting the drift history of both sensors from pre-cruise calibrations).
Enter these offsets in the configuration (.con or .xmlcon) file to calculate the corrected CTD temperature and
pressure, and calculate the CTD salinity using the corrected CTD temperature and pressure. This correction method
assumes that the pressure coefficient for the conductivity cell is correct. The CTD data with
corrected temperature
(ITS-90) and pressure are:
Corrected CTD
Pressure (dbar) Corrected CTD
Temperature ( ° C)
CTD Raw
Conductivity (S/m)
CTD Salinity
[T,P Corrected]
Bottle
Salinity
202.2 18.3865 4.63421 34.9719 34.9770
1008.3 3.9816 3.25349 34.4653 34.4710
4063.6 1.4509 3.16777 34.6795 34.6850
The salinity difference (CTD salinity – bottle salinity) of approximately -0.005 psu is now properly categorized as
conductivity error, equivalent to about -0.0005 S/m at 4.0 S/m.
Compute bottle conductivity (conductivity calculated from bottle salinity and CTD temperature and pressure) using
SeacalcW (in SBE Data Processing); enter bottle salinity for
salinity , corrected CTD temperature for ITS-90
temperature , and corrected CTD pressure for pressure :
CTD Raw Conductivity (S/m) Bottle Conductivity (S/m) [CTD - Bottle] Conductivity (S/m)
4.63421 4.63481 -0.00060
3.25349 3.25398 -0.00049
3.16777 3.16822 -0.00045
By plotting conductivity error versus conductivity, it is evident that the drift is primarily a slope change.
If α is the CTD conductivity computed with pre-cruise coefficients and β is the true bottle conductivity, then:
slope = (slope is typically > 1.0)
Using the above data, the slope correction coefficient for conductivity at this station is:
Slope = [(4.63421 * 4.63481) + (3.25349 * 3.25398) + (3.16777 * 3.16822)] /
[(4.63421 * 4.63421) + (3.25349 * 3.25349) + (3.16777 * 3.16777)] = +1.000138
Following Sea-Bird’s recommendation of assuming no offset error in conductivity, set offset to 0.0 .
Σ
( α
i
)( β
i
)
n
Σ
( α
i
)( α
i
)
i=1
n
i=1
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