Sea-Bird Electronics SBE 45 MicroTSG, User Manual, Page: 114 / 143

2
To correct conductivity data taken between pre- and post-cruise calibrations:
islope = 1.0 + (b / n) [(1 / postslope) - 1.0]
where
islope = interpolated slope; this is the value to enter in the configuration (.con or .xmlcon) file
b = number of days between pre-cruise calibration and the cast to be corrected
n = number of days between pre- and post-cruise calibrations
postslope = slope from calibration sheet as calculated above (see Appendix I: Example Conductivity
Calibration Sheet )
In the configuration (.con or .xmlcon) file, use the
pre-cruise calibration coefficients and use islope for the value of
slope.*
Note: 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).
For typical conductivity drift rates (equivalent to -0.003 PSU/month), islope does not need to be recalculated more
frequently than at weekly intervals.
* You can also calculate preslope. If
α is the conductivity computed from post-cruise bath data (temperature and
frequency) using pre-cruise calibration coefficients and β is the true conductivity in the post-cruise bath , then:
preslope = (preslope is typically > 1.0)
In this case, pre-cruise calibration coefficients would be used and:
islope = 1.0 + (b / n) (preslope - 1.0)
Correcting for Conductivity Drift Based on Salinity Bottles Taken At Sea
For this situation, the pre-cruise calibration coefficients are used to compute conductivity and CTD salinity. Salinity
samples are obtained using water sampler bottles during CTD profiles, and the difference between CTD salinity and
bottle salinity is used to determine the drift in conductivity.
In using this method to correct conductivity, it is important to realize that differences between CTD salinity and
hydrographic bottle salinity are due to errors in conductivity, temperature, and pressure measurements, as well as
errors in obtaining and analyzing bottle salinity values. For typical Sea-Bird sensors that are calibrated regularly,
70 - 90% of the CTD salinity error is due to conductivity calibration drift, 10 - 30% is due to temperature calibration
drift, and 0 - 10% is due to pressure calibration drift. All CTD temperature and pressure errors and bottle errors must
first be corrected before attributing the remaining salinity difference as due to CTD conductivity error and proceeding
with conductivity corrections.
Σ
( α
i
)( β
i
)
n
Σ
( α
i
)( α
i
)
i=1
n
i=1
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