User`s manual

Manual revision 016 Appendix V: Real-Time Data Correction Algorithms SBE 49
67
The best diagnostic of proper alignment is the elimination of salinity spikes
that coincide with very sharp temperature steps. Some experimentation with
different advances is required to find the best alignment. To determine the best
alignment for your FastCAT, plot 10 meters of uncorrected temperature and
salinity data at a depth that contains a very sharp temperature step. For the
downcast, when temperature and salinity decrease with increasing pressure:
• A negative salinity spike at the step means that temperature lags
conductivity (temperature sensor sees step after conductivity sensor does),
and that temperature should be advanced a positive number of seconds.
• Conversely, a positive salinity spike means that temperature leads
conductivity (temperature sensors sees step before conductivity sensor
does), and that temperature should be advanced a negative number of
seconds. Note that this behavior is not expected for a FastCAT, because of
the inherent characteristics of the sensor response times and plumbing.
Therefore, the FastCAT’s real-time data processing does not support a
negative temperature advance; see the SBE Data Processing manual to
perform this correction in post-processing if necessary.
Shown below is a comparison of FastCAT data with different temperature
advances; the best results (least salinity spiking) are seen with an advance of
0.0625 seconds.
Temperature advance 0 0.032 0.0625 0.094 seconds
Spiking minimized with
0.0625 second advance
Note: Data shown
has been filtered,
had conductivity cell
thermal mass
corrections applied,
and loops caused by
pressure reversals
removed.