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Frequently Asked Questions (FAQs) --
Recommended Practices

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FAQs have been organized into the following categories:

bullet Recommended practices (this page)
bullet General instrument questions
bullet General oceanographic questions
bullet Service
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bullet Data analysis and processing
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Our Glossary page is another good source of information.


What are the major steps involved in taking a cast with a Profiling CTD?

Following is a brief outline of the major steps involved in taking a CTD cast, based on generally accepted practices. However, each ship, crew, and resident technicians have their own operating procedures. Each scientific group has their own goals. Therefore, observe local ship and scientific procedures, particularly in areas of safety. Before the cruise a discussion of the planned work is advisable between the ship’s crew, resident technicians, and scientific party. At this time discuss and clarify any specific ship’s procedures.

Note: The following procedure was written for an SBE 9plus CTD operating with an SBE 11plus Deck Unit. Modify the procedure as necessary for your CTD.

10 to 15 minutes before Station:

  1. Review the next cast’s plan, including proposed maximum cast depth, bottom depth, and number of bottles to close and depths. If the cast will be close to the bottom, familiarize yourself with the bottom topography.
  2. Verify that all water samples have been obtained from the bottles from the previous cast. If so, drain the bottles and cock them. Hand manipulate each Carousel latch as you cock the bottle to ensure it is free to release and is not stuck in some way.
  3. Remove the soaker tubes from the conductivity cells.
  4. Remove any other sensor covers.
  5. With permission from the deck crew, power up the CTD. Check the Deck Unit front panel display to verify communication. Perform a quick frequency check of the main sensors.
  6. Start Seasave. Set up a fixed display. Select Do not archive data for this cast. Start acquisition and view the data to verify the system is operational.
  7. Clean optical sensor windows, and perform any required air calibration.
  8. Stop acquisition. Do Not turn the CTD Deck Unit off. Select begin archiving data immediately. Set up the plot scales and status line.

5 minutes before Station:

  1. Start the ship's depth sounder and obtain a good depth reading. Be careful reading the depth sounder; if it is improperly configured the trace will wrap around the plot and be incorrect. The bottom depth should be close to the expected charted depth.
  2. Fill out any parts of the cast log that can be done at this time.

On Station, On Deck:

  1. Verify the position and the bottom depth.
  2. The computer operator should begin filling out the software header.
  3. After receiving word from the bridge that they are on station and ready to begin, untie the CTD and move it into position. If this requires hydraulics, ensure you have the appropriate people in place and permission.
  4. Position the CTD under the block. Have the winchman remove any slack from the wire.
  5. Notify the computer room that the CTD is ready for launch. The computer room should start acquiring data.
  6. Obtain a barometric pressure reading and note it on the cast sheet.
  7. When the bridge, computer room, and winchman are ready (and you have permission to proceed), put the CTD in the water.
  8. Have the winchman lower the CTD to 10 meters (his readout), hold for 1 minute, and then bring it back to the surface. One operator should remain on deck to help the winchman see when to stop the CTD. The CTD should be far enough below the surface so that the package does not break the surface in the swells.

CTD Soaking at the Surface:

  1. Finish filling out the cast log. Re-check the bottom depth.
  2. Fill out the computer software log.
  3. Hold the CTD at the surface for at least 3 minutes.
  4. Check the status line to verify that the CTD values are correct. The pressure should be the soaking depth of the CTD. Comparing the CTD temperature and salinity to the ship's thermosalinograph is helpful. Log the information (CTD and thermosalinograph) on the cast sheet.

Starting the Cast:

  1. Call the winchman and have him start the cast down. Typical lowering speed is 1 m/sec, modified for conditions as needed.
  2. Watch the computer output and verify that the system is working.

During the Cast:

  1. Closely monitor the CTD output for malfunctions. Sudden noise in a channel is often a sign of a leaking cable. A periodically flashing error light on the Deck Unit is a sign of a bad spot in the slip rings. The modulo error count (usually on the status line) provides an indication of telemetry integrity; on a properly functioning system, there will be no modulo errors.
  2. Note any odd behavior or problems on the cast sheet. Keeping good notes and records is of critical importance. While you may remember what happened an hour from now, in the months that follow, these notes will be a vital link to the cruise as you process the data.
  3. Monitor the bottom depth. This is especially critical if the cast will be close to the bottom, or you are working in an area with varying topography such as in a canyon. Running the CTD into the bottom can cause serious (and expensive) damage.

Approaching the Bottom:

  1. Take extra care if the cast will take the CTD close to the bottom. Monitor the bottom depth, pinger, and altimeter, if available. As you get within 30 meters of the bottom, slow down the cast to 0.5 m/sec. If you wish to get closer than 10 m above the bottom, slow down to 0.2 m/sec. Keep in mind that ship roll will cause the CTD depth to oscillate by several meters.
    - If the CTD does touch bottom, it will be apparent from the sudden, low salinity spike. A transmissometer, if installed, will also show a sudden low spike.
  2. Adjust these numbers and procedures as conditions dictate to avoid crashing the CTD into the bottom.
  3. When the CTD reaches the maximum cast depth, call the winchman and stop the descent.
  4. Log a position on the cast sheet. If a bottle will be closed at the bottom, allow the CTD to soak for at least 1 minute (preferably several minutes) and then close the bottle. Verify that the software records the bottle closure confirmation.
  5. Start the CTD upcast. Stop the CTD ascent at any other bottle closure depths. For each bottle, soak for at least 1 minute (preferably several minutes) and then close the bottle.

End of the Cast:

  1. As the CTD approaches the surface, have someone help spot for the winchman. Stop the CTD below the surface. Close a bottle if desired.
  2. When ready, recover the CTD. Avoid banging the system against the ship.

CTD Back on Board:

  1. Stop data acquisition and power off the CTD.
  2. Move the CTD it into its holding area and secure it.
  3. See Application Note 2D: Instructions for Care and Cleaning of Conductivity Cells for details on rinsing, cleaning, and storing the conductivity cell. Fill the conductivity cell with clean DI (or 1% Triton-X) and secure the filler device to the CTD frame. Freezing water in a conductivity cell will break the cell.
  4. See Application Note 64: SBE 43 Dissolved Oxygen Sensor - Background Information, Deployment Recommendations, and Cleaning and Storage for details on rinsing, cleaning, and storing SBE 43 (membrane-type) dissolved oxygen sensors; see the SBE 63 manual for details on rinsing, cleaning, and storing SBE 63 optical dissolved oxygen sensors.
  5. Rinse any optical sensors.
  6. Rinse the water sampler latches with clean water.
  7. Draw water samples from the bottles.

After the Cast:

  1. Re-plot the data and look at any channels that were not displayed in real time.
  2. Perform diagnostics and take a first pass through processing.
    - Verify that the data is good (at least on a first-order basis) at this point, when you can still re-do the cast. Many casts are lost because they are not analyzed until months later, when the problems are discovered.
  3. Final processing may need to wait until bottle salts and post-cruise lab calibrations are available.


Should I collect water samples (close bottles) on the downcast or the upcast?

Most of our CTD manuals refer to using downcast CTD data to characterize the profile. For typical configurations, downcast CTD data is preferable, because the CTD is oriented so that the intake is seeing new water before the rest of the package causes any mixing or has an effect on water temperature.

However, if you take water samples on the downcast, the pressure on an already closed bottle increases as you continue through the downcast; if there is a small leak, outside water is forced into the bottle, contaminating the sample with deeper water. Conversely, if you take water samples on the upcast, the pressure decreases on an already closed bottle as you bring the package up; any leaking results in water exiting the bottle, leaving the integrity of the sample intact. Therefore, standard practice is to monitor real-time downcast data to determine where to take water samples (locations with well-mixed water and/or with peaks in the parameters of interest), and then take water samples on upcast.


Can I deploy my profiling CTD for monitoring an oil spill?

Sea-Bird CTDs can be deployed in oil; the oil will not cause long-term damage to the CTD. If the oil coats the inside of the conductivity cell and coats the dissolved oxygen sensor membrane, it can possibly affect the sensor’s calibration (and thus affect the measurement and the data). Simple measures can reduce the impact, as follows:

  1. To minimize the ingestion of oil into the conductivity cell and onto the DO sensor membrane:

SBE 19, 19plus, 19plus V2, 25, or 25plus CTD:

Set up the CTD so that the pump does not turn on until the CTD is in the water and below the layer of surface oil, minimizing ingestion of oil (however, some oil will still enter the system). Pump turn-on is controlled by two user-programmable parameters: the minimum conductivity frequency and the pump delay.

Set the minimum conductivity frequency for pump turn-on above the instrument’s zero conductivity raw frequency (shown on the conductivity sensor Calibration Sheet), to prevent the pump from turning on when the CTD is in air. Note that this is the same as our typical recommendation for setting the minimum conductivity frequency.
     For salt water and estuarine applications - typical value = zero conductivity raw frequency + 500 Hz
     For fresh/nearly fresh water - typical value = zero conductivity raw frequency + 5 Hz
If the minimum conductivity frequency is too close to the zero conductivity raw frequency, the pump may turn on when the CTD is in air as result of small drifts in the electronics. Another option is to rely only on the pump turn-on delay time to control the pump; if so, set a minimum conductivity frequency lower than the zero conductivity raw frequency.

Set the pump turn-on delay time to allow enough time for you to lower the CTD below the surface oil layer after the CTD is in the water (the CTD starts counting the pump delay time after the minimum conductivity frequency is exceeded). You may need to set the pump delay time to be longer than our typical 30-60 second recommendation.

The current minimum conductivity frequency and pump delay can be checked by sending the status command to the CTD (DS or GetCD, as applicable). Commands for modifying these parameters are:

SBE 9plus CTD:

Minimum conductivity frequency and pump delay are not user-programmable for the 9plus

If you are using your 9plus with the 11plus Deck Unit, the Deck Unit provides power to the 9plus. Without power, the pump will not turn on. At the start of the deployment, to ensure that you have cleared the surface oil layer before the pump turns on, do not turn on the Deck Unit until the 9plus is below the surface oil layer. Similarly, on the upcast, turn off the Deck Unit before the 9plus reaches the surface oil layer.

If your 9plus is equipped with the optional manual pump control, you can enable manual pump control via the Pump Control tab in Seasave V7’s Configure Inputs dialog box. Once enabled, you can turn the pump on and off from Seasave V7’s Real-Time Control menu. Do not turn the pump on until the CTD is below the surface oil layer. On the upcast, turn the pump off before the CTD reaches the surface oil layer.

  1. To reduce the effect of the ingestion of oil into the conductivity cell and onto the DO sensor membrane or optical window:

After each recovery, rigorously follow the cleaning and storage procedures in the following application notes ‑

Quick Reference Sheets for Oil Spill Deployment:


What are the major steps involved in deploying a moored instrument?

Application Note 83: Deployment of Moored Instruments contains a checklist, which is intended as a guideline to assist you in developing a checklist specific to your operation and instrument setup.


What are the recommended practices for mating and unmating connectors?

It is important to prepare and mate connectors correctly, both in terms of the costs to repair them and to preserve data quality. Leaking connectors cause noisy data and even potential system shutdowns.

Application Note 57: Connector Care and Cable Installation describes the proper care and installation of connectors for Sea-Bird instruments. The Application Note covers connector cleaning and cable or dummy plug installation, locking sleeve installation, and cold weather tips.


How can I tell if my connectors have leaked, and what do I do about corrosion on connector pins?

If there has been leakage, it will show up as green-colored corrosion product. Performing the following steps can usually reverse the effect of the leak:

  1. Thoroughly clean the connector with water, followed by alcohol.
  2. Give the connector surfaces a light coating of silicon grease.

Re-mate the connectors properly ‑ see Application Note 57: Connector Care and Cable Installation and nine-minute video covering O-ring, connector, and cable maintenance.


What are the recommended practices for replacing a bulkhead connector?


What are the recommended practices for inspecting and cleaning o-rings and mating surfaces?

  1. Remove any water from the o-rings and mating surfaces with a lint-free cloth or tissue.
  2. Visually inspect the o-rings and mating surfaces for dirt, nicks, cuts, scratches, lint, hair, and any signs of corrosion; these could cause the seal to fail. Clean the surfaces, and clean or replace the o-rings as necessary.
  3. Apply a light, even coat of 100% silicon o-ring lubricant (Parker Super O Lube) to the o-rings and mating surfaces. For an end cap o-ring, a ball of lubricant the size of a pea is about all that is needed. Too much lubricant can cause the seal to fail as much, if not more, than no grease. Do not use petroleum-based lubricant (car grease, Vaseline, etc.), as it will cause premature failure of the rubber.
  4. After lubricating the o-ring, immediately reassemble the end cap or connector, verifying that no hairs or lint have collected on the lubricated o-ring.

Nine-minute video covering O-ring, connector, and cable maintenance.
Short, silent video of application of lubricant to o-ring.
Short, silent video of application of lubricant to o-ring mating surface (note the use of a plastic dental syringe -- no sharp points to scratch the housing -- to apply the lubricant).


How often should I replace o-rings?

End Cap O-Rings: We recommend scheduled replacement of end cap o-rings approximately every 3 years, to prevent leaks caused by normal o-ring wear.

Connector O-Rings: Replacing connector o-rings requires de-soldering and re-soldering the connector wires, which makes it a more difficult task. Therefore, we recommend replacement of connector o-rings when needed, not on a routine, scheduled basis.


How should I handle my CTD to avoid cracking the conductivity cell?

Shipping: Sea-Bird carefully packs the CTD in foam for shipping. If you are shipping the CTD or conductivity sensor, carefully pack the instrument using the original crate and packing materials, or suitable substitutes.

Use: Cracks at the C-Duct end of the conductivity cell are most often caused by:

Note: If a Tygon tube attached to the conductivity cell has dried out, yellowed, or become difficult to remove, slice (with a razor knife or blade) and peel the tube off of the conductivity cell rather than twisting or pulling the tube off.


What is an Anti-Foulant Device? How often should I replace it? Does it require special handling?

The Anti-Foulant Device is an expendable device that is installed on each end of the conductivity cell, so that any water that enters the cell is treated. Anti-Foulant Devices are typically used with moored instruments (SBE 16, 16plus, 16plus-IM, 16plus V2, 16plus-IM V2, 37-SM, 37-SMP, 37-SMP-IDO, 37-SMP-ODO, 37-SI, 37-SIP, 37-SIP-IDO, 37-SIP-ODO, 37-IM, 37-IMP, 37-IMP-IDO, 37-IMP-ODO), thermosalinographs (SBE 21 and 45), moored profilers (SBE 52-MP), and drifters (SBE 41/41CP Argo float CTDs), and optionally with SBE 19plus, 19plus V2, and 49 profilers.

Useful deployment life varies, depending on several factors. We recommend that customers consider more frequent anti-foulant replacement when high biological activity and strong current flow (greater dilution of the anti-foulant concentration) are present. Moored instruments in high growth and strong dilution environments have been known to obtain a few months of quality data, while drifters that operate in non-photic, less turbid deep ocean environments may achieve years of quality data. Experience may be the strongest determining factor in specific deployment environments. Sea-Bird recommends that you keep track of how long the devices have been deployed, to allow you to purchase and replace the devices when needed.

Shelf Life and Storage: The best way to store Anti-Foulant Devices is in an air-tight, opaque container. The rate of release of anti-foulant is based on saturation of the environment. The anti-foulant will release until the environment is fully saturated (100% saturated) and then it will no longer release any anti-foulant. So if you keep Anti-Foulant Devices sealed well in an air-tight container, theoretically they will stay good for extended periods of time. Exposure to direct sunlight can also affect the release of anti-foulant; we recommend storage in an opaque container.



What are the recommended practices for deploying in frazil or pancake ice, or deploying at low temperatures?


Large numbers of Sea-Bird conductivity instruments have been used in Arctic and Antarctic programs.

Special accommodation to keep temperature, conductivity, oxygen, and optical sensors at or above 0 C is advised. Often, the CTD is brought inside protective doors between casts to achieve this.

Conductivity Cell

There are several considerations to weigh when contemplating deployments in frazil or pancake ice and at low temperatures in general:

The above considerations apply to all known conductivity sensor types, whether electrode or inductive types. 

Some additional recommendations in deploying a Sea-Bird conductivity sensor when there is any chance of freezing:

If deploying at low temperatures but no surface frazil or pancake ice is present, rinse the conductivity cell in one of the following salty solutions (salty water depresses the freezing point) to prevent freezing during deployment. But this does not mean you can store the cell in one of these solutions outside . . . it will freeze.

Note that there is still a risk of forming ice inside the conductivity cell if deploying through frazil or pancake ice on the surface, if the freezing point of the salt water is the same as the water temperature. Therefore, we recommend that you deploy the conductivity cell in a dry state for these deployments.

Commercially available alcohol or glycol antifreezes contain trace amounts of oils that will coat the conductivity cell and the electrodes, causing a calibration shift, and consequently result in errors in the data. Do not use alcohol or glycol in the conductivity cell.

Temperature Sensor

In general, neither the accuracy of the temperature measurement nor the survival of the temperature sensor will be affected by ice.

Oxygen Sensor

For the SBE 43 Dissolved Oxygen sensor, avoid prolonged exposure to freezing temperature, including during shipment. Do not store the SBE 43 with water (fresh or seawater), Triton solution, alcohol, or glycol in the plenum. The best precaution is to keep the SBE 43 indoors or in some shelter out of the cold weather.


Does it matter if I deploy my moored instrument, which includes a conductivity sensor, in a horizontal or vertical position?

Yes, vertical is usually preferable. In the presence of consistent currents and suspended sediment, we have seen instances where a horizontal conductivity cell is scoured by the abrasive effect of the flow. When scouring is particularly intense, the electrodes can be stripped of their electroplated platinum-black coating, driving the calibration toward fresher readings. Sedimentation (silting) in the cell also drives the readings fresh of correct.

Mounting the instrument vertically avoids abrasive flow and sediment build-up while allowing wave motions and Bernoulli pressures to flush the cell.

Note that some moored sensors (SBE 37-SIP, 37-SIP-IDO, 37-SIP-ODO, 37-SMP, 37-SMP-IDO, 37-SMP-ODO, 37-IMP, 37-IMP-IDO, 37-IMP-ODO) have a required orientation because of their u-shaped plumbing configuration. Refer to the instrument manual for details.


How many/what kind of spares should I have on ship for my instrument?

Very few Sea-Bird instruments completely fail due to component malfunction or manufacturing defects. However, we see a reasonably large number that require repairs of some sort. Most of these are simply due to the user breaking the equipment through rough handling, accidents, or lack of maintenance. It always best to plan for the worst case.

Parts most likely to be damaged are cables, connectors, and sensors (specifically the conductivity cell). Cables and connectors are easily replaced and spares should always be carried. After a sensor is replaced, the instrument must be re-calibrated, so it is really not practical to carry spare cells or temperature probes. If you start carrying many spare boards and sensors you are better off (both in cost and efficiency) having whole spare instruments on board.

Carrying at least 1 complete set of spares, with 3 sets of cables, connectors and dummy plugs, is recommended. How fast you can get spares from shore to the ship should dictate how many spare systems you need to have on board.

Note: See the question below for spares recommendations specific to the SBE 9plus.


How many/what kind of spares should I have on ship for my SBE 9plus?

The most complete backup system would be another SBE 9plus, to allow for very rapid system swaps. This is important if your stations are close together and there is limited time between CTD casts. However, it is the most expensive option.

The next step down would be an SBE 9plus without sensors. In this case, a system failure would require swapping sensors and pumps to the new unit. This is not difficult, but it is somewhat time consuming. If you have several hours between casts it should not be a problem.

The next option would be to carry spare boards and try and troubleshoot the problem and replace boards. If you have a technician that can do this it is not a bad option. However, it requires some clean and dry lab space to open the CTD and work. You will also have to properly re-seal the CTD. Based upon experience, the SBE 9plus does not fail very often. The most common failure is the main DC-to-DC converter. Other than that, there are very few system failures. However, there are several components that can be damaged through mistakes or misuse. The most catastrophic, other that losing the whole CTD, is to plug the sea cable into the bottom contact connector on the bottom end cap; if this happens, several circuit boards will be destroyed (Note: In 2007 Sea-Bird began using a female bulkhead connector on the 9plus for the bottom contact switch, to differentiate from the sea cable connector and prevent this error. If desired, older CTDs can be retrofitted with the female connector.).

If the budget allows it, we recommend getting a complete backup SBE 9plus, including sensors. If there is any problem, return the malfunctioning instrument for repair and continue sampling with the spare instrument. A complete backup also provides you with spare sensors, so you can rotate 1 set through calibration and continue to operate.


How will my CTD be affected by adjacent objects?

Sea-Bird’s CTDs are not directly affected by adjacent objects, unlike some CTDs that shift their calibration due to proximity effects. However, the CTD can only measure the water it sees. There are 2 concerns to keep in mind when mounting the CTD:


What are the safety concerns/procedures if the instrument floods? Can the instrument explode?

While a CTD leak can result in a dangerous situation, it is not common. Pressure housings may flood under pressure due to dirty or damaged o-rings, or other failed seals, causing highly compressed air to be trapped inside. For example, a housing that floods at 5000 meters depth holds an internal pressure of more than 7000 psia. If this happens, a potentially life-threatening situation can occur when the instrument is brought to the surface. The CTD will not explode. If it does flood and develop pressure inside, the end cap can be shot out of the housing if a technician tries to open the unit without releasing the pressure first.

Possible causes of flooding include:

It is important to visually inspect the instrument for damage before each survey. A cracked bulkhead connector is usually easy to spot.

If the instrument is unresponsive to commands or shows other signs of flooding or damage, see the Recovery section in your instrument manual for details specific to your instrument. For most instruments, follow these precautions:

  1. Every time you open the instrument, loosen each end cap screw a few turns. If the end cap follows the screws out, there is pressure in the housing.
  2. If pressure in the housing is indicated:
    A. Point the instrument in a safe direction away from people.
    B. Loosen 1 of the bulkhead connectors very slowly, at least 1 turn, to release the pressure safely (bulkhead connectors are the black connectors on the end cap, where the cables attach to the instrument). This opens an O-ring seal under the connector. Look for signs of internal pressure (hissing or water leak). If internal pressure is detected, let it bleed off slowly past the connector o-ring. Then, you can safely remove the end cap.

In general, instruments do not flood. However, be aware of the potential for flooding so that if a problem arises you will be able to safely deal with it.


Is it necessary to put my instrument in water to test it? Will I destroy the conductivity cell if I test it in air?

It is not necessary to put the instrument in water to test it. It will not hurt the conductivity cell to be in air.

If there is a pump on the instrument, it should not be run for extended periods in air.


How should I store my conductivity sensor if there is danger of freezing?

When freezing is possible, we recommend that the conductivity sensor be stored dry. Remove larger droplets of water by blowing through the cell. Do not use compressed air, which typically contains oil vapor. Attach a length of Tygon tubing to each end of the conductivity cell to close the cell ends. See Application Note 2D: Instructions for Care and Cleaning of Conductivity Cells for details.

When CTDs are exposed to deck temperatures consistently below freezing an additional concern needs to be addressed. On deployment, parts of the CTD that are colder than the freezing point of seawater will form a thin layer of ice. If ice forms inside the conductivity cell, then measurements will be low of correct until the ice layer melts and disappears. Thin layers of ice will not hurt the conductivity cell, but repeated ice formation on the electrodes will degrade the conductivity calibration (at levels of 0.001 to 0.020 psu), and thicker layers of ice can lead to glass fracture and permanent damage of the cell. Special accommodation to keep temperature, conductivity, oxygen, and optical sensors at or above 0 °C is advised. Often at high latitude the CTD is brought inside protective doors between casts to achieve this.


What are the recommended practices for cleaning and lubricating winch cables?

This topic is covered in detail on the UNOLS (University-National Oceanographic Laboratory System) website; see


What are the recommended practices for splicing cables?

Sea-Bird typically recommends using the Dam/Blok and EverGrip products from PMI Industries. DamBlok makes the electrical splice and EverGrip provides the strain relief on the cable. See an example of how these products can be used.

For a quick electrical splice in the field using commonly available materials, the UNOLS (University-National Oceanographic Laboratory System) website provides a procedure using hot glue and heat shrink: GLUE.pdf. Numerous cycles of deployment to great depths could compromise the seal, but it may be useful for a quick fix.

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