CTD PROTOCOLS AND SALINITY
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SUMMARY: A commercially-available instrument package is
lowered into the ocean on a conducting cable to obtain real-
time, high-resolution profiles of temperature, conductivity
and pressure from which salinity and depth are calculated.
Additional sensors also detect dissolved oxygen concentration
and phytoplankton fluorescence. Water samples are collected
in bottles attached to the rosette sampler which can be
activated by a surface control unit.
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1. Principle
High vertical resolution environmental data are collected with
a SeaBird CTD having external temperature (T), conductivity (C) and
dissolved oxygen (DO) sensors and an internal pressure sensor. There
is also a provision for adding fluorescence (F) and other sensors.
A General Oceanics 24-place pylon and an aluminum rosette containing
24 12-liter Niskin bottles is used to obtain water samples from
predetermined depths. The CTD and rosette are deployed on a 3-conductor
cable allowing for the real-time display of data and acquisition, and
for tripping the bottles in areas of interest in the water column.
The CTD system takes 24 samples per second of pressure (P), T, C, DO
and F. The raw data are stored both on the computer and, for redundancy,
on VHS-format video tapes.
2. CTD Data Collection
2.1. CTD data are taken with a SeaBird SBE-09 CTD having internal
pressure sensor, and external temperature, conductivity and
dissolved oxygen sensors.
2.2. The CTD is mounted in an aluminum Scripps-type rosette with 24
places for Niskin bottles. Water samples are taken on the upcast
for calibration of the conductivity and oxygen sensors.
2.3. CTD data are recorded during both down and up casts. When the
Niskin bottles are tripped, an event mark is made in the data
files to record the time and CTD data.
2.4. One deep cast to approximately 4700 m (total water depth = 4750 m)
is made on each cruise.
2.5. Ten to twelve consecutive casts to 1000 m are made over 36 hours
to span the local inertial period (~31 hour) and three semi-diurnal
tidal cycles. This sampling is performed so that tidal or near-
inertial variability can be estimated and removed, thereby
preventing an aliasing of the data records by these components.
During this 36 hour intensive sampling period, most of the water
samples for the GOFS program objectives are collected.
2.6. In 1989, SeaBird introduced a ducting arrangement which is designed
to minimize salinity spiking. This "T-C" duct has been used in
cruises since HOT-11.
3. CTD Sensors and Calibrations
3.1. Pressure: CTD pressure is measured with a 6,000 dbar
Paroscientific Digiquartz pressure transducer with an internal
temperature sensor. Pressure calibrations are conducted twice
yearly against a reference Paroscientific pressure standard
using a dead weight pressure tester to impose test pressures.
The pressure standard is recalibrated at the Northwest Regional
Calibration Center (NWRCC) every two years. Pressure is
corrected for thermal shock effects using a linear response
method (Chiswell, 1990):
P = aPm + h * T + b
where: *= temporal convolution operator
h= impulse response function
T= water temperature
Pm= measured pressure
The a and b calibration coefficients are determined from a
calibration at constant temperature and pressure. The impulse
response function, h, is computed by measuring pressure
perturbations when the CTD is plunged into a cold water bath.
3.2. Temperature sensor: CTD temperature sensors can be removed and
calibrated independently. The SeaBird model SBE-3-02/F temperature
sensors are calibrated annually at the NWRCC. A check on the
temperature sensors is maintained by intercomparing our pool of
sensors between each cruise. Two sensors (#741 and #866) were
used during 1989. These sensors were calibrated at Northwest
Regional Calibration Center prior to HOT-1, and again in November
1989. Between each cruise, they were compared against each other
and against two additional sensors acquired in August 1989. These
intercomparisons allowed us to map sensor drift that occurred
between the NWRCC calibrations. Each HOT program data report
will provide a Table of temperature sensor corrections for the
period of investigation.
3.3. Conductivity sensor: The SeaBird model SBE-4 conductivity cell
is calibrated by comparing CTD-recorded conductivities with
conductivities computed from the discrete water samples. At
least three samples are used per cast; salinity minimum,
salinity maximum and mixed layer. On three casts, together
comprising the "WOCE deep-cast," much higher vertical resolution
sampling of the salinity is done to get the best possible
salinity profile.
3.4. Dissolved oxygen (DO) sensor: A SeaBird model SBE-13 dissolved
oxygen sensor is used employing a polargraphic sensor,
manufactured by SensorMedics. The DO sensor consists of a
teflon membrane covering a layer of KCl gel. A constant
voltage applied across two electrodes results in a current
nearly proportional to the activity of oxygen diffusing across
the membrane. This current and the temperature of the cell
are measured, and DO is calculated using an algorithm based on
Owens and Millard (1985):
DO = (a1 OC + a2) OSAT (P,T,S) exp (a3 T + a4 OT + a5 p + a6 dOC/dt)
where: OC= sensor current
OT= sensor temperature
OSAT= saturation concentration of oxygen
P = pressure
T = temperature
S = salinity
The coefficients a1,....,a6 are determined from a nonlinear
least-squares fit against check samples taken from bottles
during the deep cast. Because the DO sensor shows considerable
hysteresis, the calibration is made using the down cast values
of OC, OT, P, T, S at the same density levels of the bottles.
3.5. Fluorescence sensor: Stimulated fluorescence is measured using
a Sea-Tech flash fluorometer with an excitation wavelength of
425 nm peak emission and a peak response at 685 nm (30 nm FWHM).
4. Salinity Determinations
Salinity samples are collected directly from the Niskin bottles
into 250 ml polyethylene bottles and stored at room temperature,
in the dark, for subsequent analysis at our shore-based laboratory.
The time between sample collection and analysis is generally less
than 1 week. Prior to analysis, the samples are equilibrated to
laboratory temperature and the salinity measured using an AGE
model 2100 Minisal salinometer which is calibrated against IAPSO
standard (Wormley) seawater. Typical precision of replicate
analyses from the same water sample is 0.0003 o/oo; for triplicate
sample bottles the precision is less than 0.001 o/oo. The effects
of sample storage in polyethylene bottles was systematically
evaluated during year 1 of the HOT program. The data, summarized
in the 1988-89 HOT Data Report, indicate that S o/oo changes were
negligible.
5. CTD Post-processing
Once the CTD data are collected and laboratory-determined pressure
and temperature calibrations are applied, they are subjected to
screening and quality control; they are checked for spikes or missing
data caused by electrical interference in the hydrowire. Spikes are
removed with a 9-point median filter, and missing data are replaced
with interpolated values. After this initial screening, the data
are averaged to 1/2-second values. Pressure and conductivity are
then corrected for thermal hysterisis effects. This processing in
the time-domain is required to allow for correction of lags between
the C- and T- sensor responses. After the data have been reduced to
1/2-second values, corrections derived from the calibration methods
described above are applied to the conductivity and dissolved oxygen.
Finally, the data are pressure-sorted to remove effects of shiproll
(i.e., only data taken when the CTD is moving downwards are kept),
and averaged into 2 decibar values.
6. References
Chiswell, S. M. Dynamic reaponse of SeaBird CTD pressure sensors
to temperature. In prep.
Owens, W. B. and R. C. Millard, Jr. 1985. A new algorithm for
CTD oxygen calibration. Journal of Physical Oceanography, 15,
621-631.
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