Tau Correction
Note the derivative term [tau(T,P) * δV/δt]; this correction term’s function is to improve the response of the
measured signal in regions of large oxygen gradients. However, this term also amplifies residual noise in the signal
(especially in deep water), and in some situations this negative consequence overshadows the gains in signal
responsiveness. In SEASAVE V7 and SBE Data Processing version 7.18b or later, this Tau correction can be
disabled if desired, deleting the entire derivative term from the equation for calculated oxygen.
Hysteresis Correction
Under extreme pressure, changes can occur in gas permeable Teflon membranes that affect their permeability
characteristics. Some of these changes (plasticization and amorphous/crystalinity ratios) have long time constants
and depend on the sensor’s time-pressure history. These slow processes result in hysteresis in long, deep casts. The
hysteresis correction algorithm (using H1, H2, and H3 values entered for the SBE 43 in the .con file) operates
through the entire data profile and corrects the oxygen voltage values for changes in membrane permeability as
pressure varies. At each measurement, the correction to the membrane permeability is calculated based on the
current pressure and how long the sensor spent at previous pressures.
Hysteresis responses of membranes on individual SBE 43 sensors are very similar, and in most cases the default
hysteresis parameters provide the accuracy specification of within 2% of true value. For users requiring higher
accuracy (±1 µmol/kg), the parameters can be fine-tuned, if a complete profile (descent and ascent) made preferably
to greater than 3000 meters is available. H1, the effect’s amplitude, has a default of -0.033, but can range from
-0.02 to -0.05 between sensors. H2, the effect’s non-linear component, has a default of 5000, and is a second-order
parameter that does not require tuning between sensors. H3, the effect’s time constant, has a default of 1450
seconds, but can range from 1200 to 2000. Hysteresis can be eliminated by alternately adjusting H1 and H3 in the
.con file during analysis of the complete profile. Once established, these parameters should be stable, and can be
used without adjustment on other casts with the same SBE 43.
Software
The following Sea-Bird software allows you to select the SBE 43 oxygen sensor (labeled Oxygen, SBE) and use the
Sea-Bird equation documented in this application note when setting up the configuration (.con) file for the CTD:
• SBE Data Processing version 7.18b or later
Enable / disable the Tau correction on the Miscellaneous tab in Data Conversion, if you are outputting
calculated oxygen at this step. Enable / disable the hysteresis correction on the Miscellaneous tab in Data
Conversion, if you are outputting oxygen voltage and/or calculated oxygen at this step.
You can also enable / disable the Tau correction on the Miscellaneous tab in Derive.
• SEASAVE V7 version 7.18b or later
Enable / disable the Tau correction and/or the hysteresis correction on the Miscellaneous tab in Configure
Inputs. Note that these corrections are applied to data displayed in the software and to calculated values output
by the software; however, raw oxygen voltage output by SEASAVE V7 to the CTD .hex file is not corrected.
• SEASAVE-Win32 version 5.39c or later
(The Sea-Bird equation is called the Murphy-Larson equation in this software, but performs the same basic
calculation as in SEASAVE V7 and SBE Data Processing. However, the hysteresis correction is not available
in this software. Additionally, to disable the Tau correction, the user must set tau20=0 in the .con file; this
deletes the term [tau(T,P) * δV/δt] from the calibration equation).
The latest version of the software is available for download from our website (www.seabird.com).
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