Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i

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.


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 ‰; for triplicate sample bottles the precision is less than 0.001 ‰. 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 ‰ 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.