Specifications
5
Correcting for Temperature Drift Based on Pre- and Post-Cruise Laboratory Calibrations
Suppose a temperature sensor is calibrated (pre-cruise), then immediately used at-sea, and then returned for post-
cruise calibration. The pre-and post-cruise calibration data can be used to generate an offset correction for data
obtained between the pre- and post-cruise calibrations.
Calibration coefficients are calculated with the post-cruise calibration. Using the pre-cruise bath data and the post-cruise
calibration coefficients, a mean residual over the calibration temperature range is calculated.
residual = instrument temperature bath temperature
Sea-Bird calculates and prints the value for the residual on the temperature calibration sheet.
To correct temperature data taken between pre- and post-cruise calibrations:
Offset = b * (residual / n)
where
b = number of days between pre-cruise calibration and the cast to be corrected
n = number of days between pre- and post-cruise calibrations
residual = residual from calibration sheet as described above
In the .con file, use the pre-cruise calibration coefficients and use the negative of the offset for the value of offset
(offset is added, so to remove negative drift a positive number is entered in the .con file).
Note: The CTD configuration (.con) file is edited using the Configure menu (in SEASAVE or SBE Data Processing in
our SEASOFT-Win32 suite of programs) or the Configure Inputs menu in SEASAVE V7.
Example
Instrument was calibrated (pre-cruise), used at sea for 4 months, and returned for post-cruise calibration.
Using pre-cruise bath data and post-cruise coefficients, the calibration sheet shows a mean residual of
-0.2 millidegrees C (-0.0002 °C).
For preliminary work at sea, use the pre-cruise calibration coefficients and slope = 1.0, offset = 0.0.
After the cruise, correct temperature data obtained during the cruise for drift using properly scaled values of
correction coefficients:
For data from the end of the first month (30 days) at sea:
Offset = b * (residual / n) = 30 * (0.0002 / 120) = - 0.00005;
Convert data using pre-cruise coefficients and +0.00005 as the offset in the .con file.
(Notice the change in sign for the offset entry in the .con file)
For data from the end of the second month (60 days) at sea:
Offset = b * (residual / n) = 60 * (0.0002 / 120) = - 0.0001;
Convert data using pre-cruise coefficients and +0.0001 as the offset in the .con file.
For data from the end of the third month (90 days) at sea:
Offset = b * (residual / n) = 90 * (0.0002 / 120) = - 0.00015;
Convert data using pre-cruise coefficients and +0.00015 as the offset in the .con file.
For data from the end of the 4-month cruise:
Offset = - 0.0002;
Convert data using pre-cruise coefficients and +0.0002 as the offset in the .con file, or using post-cruise
coefficients and 0 as the offset in the .con file.