User`s manual

Manual revision 016 Section 4: Deploying and Operating FastCAT SBE 49
24
Real-Time Data Corrections
Typical post-processing of CTD data includes the following:
• Aligning data − Data misalignment is caused by physical misalignment
of sensors in depth, inherent time delay of sensor responses, and water
transit time in a plumbing line. Aligning conductivity, temperature, and
pressure (C, T, and P) ensures that derived parameters are calculated with
measurements from the same parcel of water, eliminating spiking caused
by misaligned values in areas with sharp gradients.
• Filtering data − Filtering matches the T and C sensor time constants.
• Correcting for conductivity cell thermal mass − The glass cell stores
heat, causing water in the cell to be a different temperature than the
temperature sensor measured a moment earlier. Removing thermal mass
effects from measured conductivity provides improved results in areas
with steep temperature gradients.
Performing these calculations in post-processing allows you to make
adjustments to the measured data before calculating derived parameters.
SBE Data Processing, part of our Seasoft suite, can perform this processing on
recorded data, if a complete C, T, and P time series is recorded. However, the
FastCAT, which samples and transmits data at 16 Hz, is frequently integrated
with an AUV or ROV. The AUV/ROV may record data on command, with
uneven time intervals between each recorded measurement. The lack of a true
time series from the FastCAT for these applications prevents aligning, filtering,
and correcting for cell thermal mass in post-processing.
Therefore, the FastCAT can be programmed to process data in real-time if
sampling autonomously. This provides C, T, and P output that is already
aligned, filtered, and corrected for cell thermal mass, and also provides
calculated and output salinity and sound velocity (if applicable) based on these
corrected values.
If real-time corrections are enabled (ProcessRealTime=Y) and the output
format is converted hex or decimal (OutputFormat=1 or 3), the FastCAT
processes the data as follows:
1. Advance temperature by user-programmed amount (TAdvance=).
2. Filter temperature and conductivity with cosine filter and five-scan window.
3. Apply cell thermal mass correction to conductivity, with user-
programmed values for alpha (Alpha=) and tau (Tau=).
4. Calculate averaged data (if NAvg= is greater than 1) based on corrected
values of C, T, and P.
5. Calculate salinity (if OutputSal=Y) and sound velocity (if OutputSV=Y)
based on corrected, averaged values of C, T, and P.
Example: Autonomous Sampling Setup for Real-Time Data Processing (user input in bold)
See either of the autonomous sampling examples above for general setup. Send the following commands to enable and
establish parameters for real-time data processing: output converted decimal data, enable real-time data corrections,
advance temperature 0.0625 seconds, and use alpha and tau of 0.03 and 7 respectively, for cell thermal mass correction.
S>OUTPUTFORMAT=3
S>PROCESSREALTIME=Y
S>TADVANCE=0.0625
S>ALPHA=0.03
S>TAU=7
Notes:
• The FastCAT does not perform real-
time corrections if the output format
is raw hex or raw decimal
(OutputFormat=0 or 2), even if
ProcessRealTime=Y.
• See Appendix V: Real-Time Data
Correction Algorithms for details.
Note:
Cell thermal mass corrections
should not be applied to
freshwater data. It can give bad
results, due to the way the
derivative dc/dT is calculated in
regions where conductivity
changes are very small. Currently,
the SBE 49 firmware does not permit
you to apply just aligning and filtering,
without also applying cell thermal
mass corrections.