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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

)(β

)

Σ (α

)(α

)

i=1

1 2 ... 78 79 80 81 82 83 84 85 86 87 88 ... 103 104

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