California Cooperative Oceanic Fisheries Investigations

CTD General Practices: System Description, Deployment, Data Aquisition, & Maintenance

SUMMARY: Since 1993, the CalCOFI program has deployed a Seabird 911 CTD mounted on a 24-bottle rosette during seasonal, quarterly cruises off California. The CTD-rosette is lowered into the ocean to 515m, depth-permitting, on 75 hydrographic stations using the ship's conductive-wire winch. Data from the sensors are transmitted up the conductive wire and displayed real-time on a data aquistion computer. Discrete seawater samples are collected in 10L bottles at specific depths determined by the chlorophyll maximum and mixed layer depth. These samples are analyzed at sea and used to assess the CTD sensor data quality plus measure additional properties. Processed CTD sensor data are compared to the seawater sample data and corrected when necessary. Preliminary data are available on CalCOFI's website, calcofi.org, while the cruise is at sea when internet is available. Preliminary processed data files are online shortly after the cruise returns. Final, publication-quality bottle & CTD data are available once the bottle data have been fully processed & scrutinized.

1. Basic CTD Components

The Seabird 911/911plus CTD configuration has evolved since 1993.  Components are added or upgraded as new sensor technology becomes available.

The current (since Nov 2009) CTD & sensors configuration: 

• SBE9plus CTD with SBE11 v2 Deck Unit (CTD 911plus); rated to 6800m 
• dual SBE3plus fast response temperature sensors (T); rated to 6800m 
• dual SBE4C conductivity sensors (C); rated to 6800m 
• dual SBE43 oxygen sensors (O2); rated to 7000m 
• dual SBE5T pumps; rated to 10,500m 
• Seapoint Chlorophyll Fluorometer; passive flow (not pumped), mounted on rosette, not shuttered; rated to 6000m 
• Wetlabs C-Star Transmissometer; 25cm 660nm, passive flow; rated to 6000m
• Satlantic ISUS Nitrate sensor; since 0411; v1 ISUS powered by an external 12v battery, passive flow; rated to 1000m 
• Seabird SBE-18 pH sensor; since 0911; rated to 1200m 
• Datasonics/Teledyne-Benthos PSA-916 Altimeter; mounted unobstructed & low; rated to 6000m 
• Biospherical Remote Photoradiometer (PAR) QSP-2300; rated to 2000m; alternate model QSP-200L; rated to 1000m 
• Biospherical Surface Photoradiometer (PAR) QSR-240; attached to deck unit
• Remote Depth Readout SBE14; attached to deck unit; allows winch operator to see CTD depth

2. Preparation & Deployment

Weather-permitting, the CTD and bottles are prepared for deployment 20 minutes prior to station arrival. 
CTD-rosette preparations on CalCOFI cruises include: 

• Prep the electronics: removal of fresh-water rinse tubes attached to the pumps; removal of the PAR protective cap;removal of the pH sensor cap; RBS rinse (using a squirt bottle) the transmissometer lenses to eliminate surface film.
• Prep the rosette bottles: 24-10 liter bottles are propped open by stretching the spring-loaded end caps back and securing their nylon lanyards to the proper carousel position. Bottle breathers & sample-drawing valves are checked for closure. Tag lines are attached to the upper rosette ring and once secured, the deck/eye-bolt lines are untied.
• Electronics warmup: eight minutes prior to station arrival, the CTD deck unit is turned on, powering up the CTD electronics. After filling out the header form (see example), data aquisition is started. The ISUS nitrate sensors power cable is attached to the battery to allowseveral minutes warmup prior to deployment.
• ~Two minutes before deployment, the CTD's pressure reading on-deck is logged on the console ops form. This value is monitored at the beginning and end of the cast for shifts in the pressure baseline. It's median on-deck reading should be ~0. If the on-deck pressure become greater than +-0.3db, a corrective pressure sensor offset should be applied and documented in the CTD cast notes. It is important to wait several minutes after turning on the CTD deck unit before assessing the deck pressure.
  

• The CTD-rosette is launched and held just below surface; enough wire is paid out so the bottle tops do not break surface when the ship rolls.
• The winch readout is zeroed and the CTD is sent to 10 meters for ~2 minutes to purge air from the system, allow the pumps to turn on (triggered by seawater contact; status is verified on the CTD computer screen). This gives the sensors a few minutes to stabilize and thermally equilibrate after sitting on-deck..
  
• Communicating with the winch operator using intercom or radio, the CTD operator requests the CTD return to just below surface. Holding at surface for one minute to log data and verify T, C, & O2 sensor correctness & agreement between the primary and secondary pairs.
• If everything looks good, the CTD is lowered to 515m, depth-permitting, at 30m/min for the first 100m then 60m/min to terminal depth.
• If the bottom depth is less than 515m, the CTD is lowered to 10m above the bottom, according to the altimeter reading, not wire readout. After the wire settles and if conditions permit, the CTD depth is adjusted to ~5m above the bottom.
  

3. Data Acquistion & Seawater Collection

Our CTD data acquistion system is a Microsoft Windows computer running Seasave v7, Seabird's data acquisiton program. Calibration coefficients for each sensor are entered during CTD setup and termination before the first cast. Data are logged at 24hz to insure maximum resolution & flexibility in post-cast data processing; 24Hz data allow re-calculation of derived values using different post-cast or post-cruise coefficients. The SBE11 deck unit v1 auto-applies a 0.073ms offset to the primary conductivity. The SBE11 deck unit v2, used since Jul 2009, auto-applies a 0.073ms offset to both primary & secondary conductivity sensors. 
During the cast, Seasave's main plot window displays real-time temperature, salinity, oxygen, and fluorometry versus depth. Seasave has a 4 parameters-per-plot limit so additional plots are used to display other sensor profiles. A fixed-data window lists real-time data in numeric form so T, C, & S values may be transcribed to the CTD console operations log prior to bottle closure. 

1. When the CTD arrives to the target depth, time, wire out, depth, T, C, S, & alt (if near bottom) are written on the console ops form. This usually takes at least 20secs, the minimum flushing time before closing a bottle. 
2. In Seasave, the 'create marker' command is initiated followed by the 'fire bottle' command. When the bottle closure confirmation is received by the deck unit, the 'bottles fired' will increment by one. The CTD operator records the confirmation time on the bottle depth record, then checkmarks the bottle confirmation boxes.
3. When the first bottle has closed, the bottle-closure confirmation time, latitude, longitude, and bottom depth (from echo sounder), are recorded on the form's CTD-at-Depth sidebar. A 500m CTD cast takes ~50mins so the GPS position & time recorded during the first bottle trip becomes the station cast information for the bottle (IEH) data.
4. The CTD-rosette is raised to the next target bottle depth at 60m/min, conditons permitting. Console ops logging and bottle closure steps (1 & 2) are repeated until the CTD-rosette is back at surface and final bottle closed.
5. The CTD-rosette is recovered using taglines and once on deck, re-secured to the deck eyes with short lines.
6. The deck pressure is recorded on the console ops form and data aquisition is halted.
7. The ISUS power cable is disconnected & seawater samples drawn.

4. Water Sampling

Seawater samples are drawn from the 10L rosette bottles once the CTD-rosette has been secured. Oxygen samples are drawn first, followed by DICs, salts, nutrients, chlorophylls (from depths 200m or less), and LTER's suite of samples. Please refer to the specific water sampling or analytical method for more information.

 5. Quality Control

The CTD electronics and sensors are reliable and stable when properly serviced and maintained. CalCOFI has established some standard practices over time to keep the CTD functioning properly. 

• De-ionized or freshwater rinses: post-cast the plumbed-pumped sensors (2 pairs of T, C, O2, & pump) are flushed with de-ionized or Milli-Q water to minimize bio-fouling.
• The carousel is hosed with fresh water to reduce mis-trips from bio-fouling or inorganic particulate buildup. A vinyl rosette cover is used when the CTD-rosette needs protection from contaminants or debris.
• PAR and pH sensor (stored in buffer) are capped when on-deck.
• Deck tests are performed before the first cast to derive transmissometer coefficients based on in-air and blocked light path voltage readings. A chlorophyll standard is mounted on the fluorometer to test maximum response voltage. Deck tests are performed regularly throughout the cruise to monitor transmissometer and fluorometer stability and response.
• At-sea analyses of seawater samples allow bottle data to be compared to sensor data quickly, particularly salinities. When bottle salts are analyzed, the bottle salinity calculation is immediately compared to the CTD value and flagged if significantly different.  This allows early detection of analytical equipment or CTD sensor malfunction. Oxygen, chlorophyll, and nutrients data comparisons are less immediate but when data look suspect, this ability helps identify real vs faulty measurements.

6. Equipment/Supplies

1. 2 - Seabird SBE9plus CTDs with sensors; the primary package is inventoried in section 1; sensors without backups: ISUS nitrate sensor, pH sensor, deck unit remote depth readout 
2. 2 - deck units: primary SBE11v2; backup SBE11v1
3. 2 - Windows XP micro-computers with 2 serial ports; deck unit, & GPS interface cables.
4. 2 - SBE32 carousels; plus 3 spare trigger assemblies
5. Console operations forms plus clipboard
6. Timer, for 2 minute soak at surface
7. 2 - 24 place aluminum rosette frame
8. 2 - sets of 24 10L Niskin bottles; plus 4 spare bottles; multitude of spare parts
9. 2 - sets of 24 nylon lanyards for Niskin bottles
10. Termination toolkit and supplies - please refer to termination documentation for info on CalCOFI CTD wire termination techniques.
    • butane soldering wand, solder, butane
    • adhesive-lined shrink tubing: 1/8"
    • Scotch 130 electrical splicing tape
    • Scotch 33 electrical tape
    • Scotch-kote electrical coating
11. Cable grips (Chinese finger), thimble, and shackle to attached sea cable to the rosette
12. 3 - taglines with detachable hooks
13. 3 - 1m deck lines to secure the rosette on deck
14. 4L Milli-Q filled carbuoy with hose for flushing the plumbed sensors post-cast 
15. Hose, for freshwater rinse of carousel and other components post-cast
16. Stainless steel hose clamps: 100 - size 80 for mounting Niskin bottles to the rosette; misc others to mounted the CTD, ISUS, battery, and sensors to the frame.
17. Turner Designs fluorometer standard for SCUFA (fits Seapoint fluorometer) for deck calibration; black rubber card for transmissometer deck test
18. RBS or Micro in a squirt bottle for rinsing the transmissometer lenses before deployment. RBS or Micro are residue-free soaps in dilute Milli-Q solutions.
19. CTD cable servicing kit containing silicone grease; electrical contact cleaner; cotton swabs; Kim-wipes
20. 3 - Wetlabs 12v batteries and charging station or cable for ISUS nitrate sensor.

7. Maintenance

CalCOFI sends all CTD electronics to their respective manufacturer for service and maintenance. The temperature, conductivity, and oxygen sensors are serviced & re-calibrated after use on two consecutive cruises (~150 deployments). Routine Seabird carousel maintainance is performed by the CalCOFI-SIO Technical Group (CSTG). When repairs or five-year service are needed, the carousel is sent to Seabird. PAR sensors are serviced by Biospherical every three years. 
General protocol is any sensor is returned for repair if the sensor fails or data quality diminishes. The SBE9+ CTD ('fish') is routinely serviced every five years. The aluminum-frame rosette is repaired or modified at SIO's Research Support Shop whenever necessary.

8. References

1. Sea-Bird Electronics, Inc, 2009. SBE 9plus Underwater Unit Users Manual, Version 012
2. Sea-Bird Electronics, Inc, 2009. SBE 11plus V2 Deck Unit Users Manual, Version 012
3. Sea-Bird Electronics, Inc, 1998. SBE 32 Carousel Water Sampler Operating and Maintenance Manual