User`s manual

Manual revision 016 Section 4: Deploying and Operating FastCAT SBE 49
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Equalizing Pressures when Mounted on a Vehicle
An object moving through water has a large pressure gradient around the
vehicle and/or any major protuberances. As in the discussion of equalizing
Bernoulli pressures with respect to the exhaust fitting, if the intake and exhaust
see a different pressure, there will be acceleration of the water in the
plumbing; acceleration in the plumbing overrides the constant flow provided
by the pump, resulting in temperature and conductivity data that can be
difficult to align because of changing flow rates.
Therefore, it is important to mount the FastCAT to the vehicle (AUV, ROV,
etc.) so that the plane that passes through the axis of the intake and exhaust is
parallel to the major surface that the FastCAT mounts to. Two examples of
appropriate mounting schemes are shown at left.
Profiling Speed when Profiling from a Ship
A profiling speed of approximately 1 meter/second usually provides good
quality data. However, the amount of ship motion, and the dynamic effect it
has on data quality, must be considered as operating conditions change. In
rough seas or other conditions (small boats) where the ship’s dynamic motion
is large, increase the profiling speed to as much as 2 to 3 meters/second to
reduce dynamic errors (spiking) caused by the rapidly changing ascent/descent
rate of the FastCAT (yo-yo effect).
In an unpumped CTD, slow profiling speeds can cause reduced flushing of the
conductivity cell, and salinity spiking can be severe in areas of strong
temperature gradients. Since the FastCAT’s pump creates and maintains a
constant and optimum flow, the FastCAT can be raised slowly to give
greater vertical resolution in the data, especially on lakes or protected bays,
or in other calm conditions. Adjust the ascent rate according to the amount of
ship motion (i.e., sea state). On a very calm lake, 10 cm/second is feasible if
used with a constant winch speed.
Spiking is sometimes seen in the derived values for salinity, density, or sound
velocity. Spiking results largely from a response time mismatch of the
conductivity and temperature sensors, especially when the profiling rate is non-
uniform. The amount of spiking depends on the temperature gradient, and is
much worse when coupled surface motion causes the instrument to stop - or
even reverse - its descent. In the event of heavy ship motion, it may be worth
letting the instrument free-fall. When very heavy seas cause severe ship motion
and result in periodic reversals of the instrument descent, the data set can be
greatly improved by removing scans taken when the pressure is not increasing.
Positioning Relative to Other Instruments and Package
Position the FastCAT so that other instruments and hardware do not thermally
contaminate the water that flows past the sensors. Position the FastCAT
forward of or at the front of the package.
Temperature Equilibration
Where water temperature is very different from the temperature at which the
FastCAT has been stored, better results are obtained if the FastCAT is allowed
to equilibrate to the water temperature at the surface (soak) for several minutes
before beginning a profile. The reason is not that the electronics are temperature
sensitive - they are not - but that the thermal influence of the instrument housing
on the water entering the cell will be reduced. If the difference between water
and storage temperature is extreme, allow more soak time.
Notes:
• The FastCAT performs real-time
alignment of conductivity and
temperature data if
ProcessRealTime=Y and
OutputFormat=1 or 3.
• See the SBE Data Processing Help
files and/or manual for information
on data processing modules that
can align conductivity and
temperature data, and correct data
for the influences of ship motion
and minimize salinity spiking.
Vehicle
Plane through intake
and exhaust parallel
with vehicle surface or
vehicle surface
tangent
Vehicle
Vehicle fin