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 (see Appendix II: Example
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 (see Appendix II: Example Temperature
Calibration Sheet)
In the .con file, use the pre-cruise calibration coefficients and use the calculated offset for the value of offset.
Note: In our SEASOFT-Win32 suite of programs, edit the CTD configuration (.con) 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).
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.
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.
181
To Next Page
Loading...