Glider Payload CTD with Pump

Glider Payload CTD (GPCTD)

Measures conductivity, temperature, and pressure, and optionally, dissolved oxygen. Modular, low-power profiling instrument for autonomous gliders.

The GPCTD is a modular, low-power profiling instrument for autonomous gliders with the high accuracy necessary for research, inter-comparison with moored observatory sensors, updating circulation models, and leveraging data collection opportunities from operational vehicle missions. The externally powered, continuously pumped CTD consumes only 175 mW recording at 1 Hz (190 mW for real-time data). One Alkaline D cell could operate the CTD continuously for 114 hours (9.5 days at 50% duty cycle, profiling continuously at 1 Hz on every glider upcast); one Lithium DD cell could provide 48 days continuous profiling on every upcast. Data are output in engineering units.

The GPCTD can optionally be equipped with an SBE 43F Dissolved Oxygen sensor, but does not support other auxiliary sensors.

FEATURES

  • Conductivity, Temperature, Pressure, and (optional) Dissolved Oxygen (modular SBE 43F DO sensor).
  • Pressure-proof module allows for exchange of CTD (and DO sensor) without opening glider pressure hull.
    • Assembly visible on glider exterior consists of intake sail (with integral T-C duct and anti-foulant device), internal field conductivity cell, and exhaust sail with pump connections. Intake sail allows measurements to be made outside vehicle’s boundary flow (where old water is thermally contaminated by vehicle). Pump pulls water into intake sail, past temperature sensor, through anti-foulant device and conductivity cell, and out exhaust sail (preventing exhaust re-circulation and Bernoulli pressure differences from changing flow rate). Outside of conductivity cell is free-flushed. minimizing salinity errors. Connecting neck, electronics, pump, and DO sensor are in a flooded space inside hull, placed so that tubing lengths are minimized (between conductivity cell and pump intake, and from pump outlet to sail exhaust fitting), sharp bends are avoided, and pump and tubing are oriented to avoid trapping air that will interfere with pump priming.
  • RS-232 interface, memory, real-time output, no batteries (for use on vehicles that can supply power).
  • Four sampling modes: Continuous (1 Hz), Fast Interval (5-14 sec intervals), Slow Interval (15-3600 sec intervals; CTD only), and Polled.
    • Continuous sampling time series suitable for corrections (e.g. response filtering, alignment, thermal mass correction) for dynamic errors in data.
    • File headers (maximum 1000) contain beginning and ending sample numbers, sampling mode and interval, and starting date/time.
  • Unique flow path, pumping regimen, and expendable anti-foulant device, for maximum bio-fouling protection.
  • Pump-controlled, T-C ducted flow to minimize salinity spiking.
  • Depths to 350 or 1500 m.
  • Field-proven design based on Argo float CTD, with more than 10,000 Argo float CTDs deployed.
  • Seasoft© V2 Windows software package (setup, data upload, data processing).
  • Five-year limited warranty.

COMPONENTS

  • Unique internal-field conductivity cell permits use of expendable anti-foulant device, for long-term bio-fouling protection.
  • Aged and pressure-protected thermistor has a long history of exceptional accuracy and stability.
  • Pressure sensor with temperature compensation is available in four strain-gauge ranges (to 2000 m).
  • (optional) Oxygen sensor is field-proven, individually calibrated SBE 43F Dissolved Oxygen sensor.
  • For Continuous and Fast Interval sampling, pump runs continuously, providing bio-fouling protection and correlation of CTD (and optional DO) measurements.

OPTIONS

  • Strain-gauge pressure sensor in one of 4 ranges (to 2000 m).
  • SBE 43F interface in GPCTD, and modular SBE 43F Dissolved Oxygen Sensor in 600 or 7000 m housing.
  • Plastic (350 m) or titanium (1500 m) housing.

Measurement Range

Conductivity 0 to 9 S/m (calibrated 0 to 6 S/m)
Temperature -5 to +42 °C (calibrated +1 to +32 °C)
Pressure 0 to 100 / 350 / 1000 / 2000 m (calibrated to full scale)

Initial Accuracy

Conductivity In calibration range: ± 0.0003 S/m; Outside calibration range 1: ± 0.0010 S/m
Temperature In calibration range: ± 0.002 °C; Outside calibration range 1: ± 0.004 °C
Pressure In calibration range: ± 0.1% of full scale range;

1 Due to fit extrapolation errors.

Typical Stability

Conductivity 0.0003 S/m per month
Temperature 0.0002 °C per month
Pressure 0.05% of full scale range per year

Resolution

Conductivity 0.00001 S/m
Temperature 0.001 °C
Pressure 0.002% of full scale range

Miscellaneous

Sampling Speed 1 Hz (1 sample/sec)
External Power Requirements 8 to 20 VDC nominal.
CTD only: 175 mW recording at 1 Hz; 190 mW transmitting real-time data.
CTD & DO: 265 mW recording at 1 Hz; 280 mW transmitting real-time data.
Memory 8 Mbytes; 699,000 samples CTD (194 hours at 1 Hz), or 559,000 samples CTD & DO (155 hours at 1 Hz)
Data Format Real-time data and uploaded data in decimal or Hex: S/m, °C, decibars, DO frequency.
Housing, Depth Rating, & Weight CTD & pump: Plastic, 350 m, in air 1.0 kg, in water 0.2 kg; Titanium, 1500 m, in air 1.2 kg, in water 0.4 kg
SBE 43F DO sensor: Plastic, 600 m, in air 0.3 kg, in water 0.1 kg; Titanium 7000 m, in air 0.4 kg, in water, 0.2 kg

 


Forward End View at left; Side View at right
Note: Oxygen connector optional if SBE 43F not ordered.


With SBE 43F Oxygen Sensor (plumbing approximate)

The list below includes (as applicable) the current product brochure, manual, and quick guide; software manual(s); and application notes.

For older Glider Payload CTD product manuals, organized by instrument firmware version, click here.

Title Type Publication Date PDF File
Glider Payload CTD Brochure Product Brochure Wednesday, August 5, 2015 GliderPayloadCTDBrochureAug15.pdf
Glider Payload CTD Manual Product Manual Monday, June 27, 2016 GliderPayloadCTD_006.pdf
SBE Data Processing Manual Software Manual Thursday, May 26, 2016 SBEDataProcessing_7.26.0.pdf
Improving CTD Data from Gliders by Optimizing Sample Rate and Flow Past Sensors White Paper Monday, August 1, 2011 ONT-Aug11-CJanzenArticle.pdf
AN02D: Instructions for Care and Cleaning of Conductivity Cells Application Notes Monday, June 13, 2016 appnote2DJun16.pdf
AN06: Determination of Sound Velocity from CTD Data Application Notes Tuesday, February 2, 2010 appnote06Aug04.pdf
AN10: Compressibility Compensation of Sea-Bird Conductivity Sensors Application Notes Tuesday, May 7, 2013 appnote10May13.pdf
AN27D: Minimizing Strain Gauge Pressure Sensor Errors Application Notes Wednesday, May 18, 2016 appnote27DMay16.pdf
AN31: Computing Temperature and Conductivity Slope and Offset Correction Coefficients from Laboratory Calibrations and Salinity Bottle Samples Application Notes Monday, June 13, 2016 appnote31Jun16.pdf
AN38: TC Duct Fundamentals Application Notes Tuesday, July 10, 2012 appnote38Jul12.pdf
AN42: ITS-90 Temperature Scale Application Notes Wednesday, May 18, 2016 appnote42May16.pdf
AN57: Connector Care and Cable Installation Application Notes Tuesday, May 13, 2014 appnote57Jan14.pdf
AN68: Using USB Ports to Communicate with Sea-Bird Instruments Application Notes Friday, October 19, 2012 appnote68Oct12.pdf
AN69: Conversion of Pressure to Depth Application Notes Monday, July 1, 2002 appnote69.pdf
AN73: Using Instruments with Pressure Sensors at Elevations Above Sea Level Application Notes Thursday, May 19, 2016 appnote73May16.pdf
SeatermV2© is a terminal program launcher for setup and data upload of Sea-Bird instruments developed or redesigned in 2006 and later. The common feature of this generation of instruments is the ability to output status responses in XML. SeatermV2 is part of our Seasoft V2 software suite.
Version 2.6.1 released June 1, 2016
SeatermV2.6.1-b12.exe for Windows XP/Vista/7


SBE Data Processing© consists of modular, menu-driven routines for converting, editing, processing, and plotting of oceanographic data acquired with Sea-Bird profiling CTDs, thermosalinographs, and the SBE 16 and 37 families of moored CTDs. SBE Data Processing is part of our Seasoft V2 software suite.
Version 7.26.2 released September 6, 2016


How can I tell if the conductivity cell on my CTD is broken?

Conductivity cells are made of glass, which is breakable.

  • If a cell is cracked, it typically causes a salinity shift or erratic data.
  • However, if the crack occurs at the end of the cell, the sensor will continue to function normally until water penetrates the epoxy jacket. Post-cruise calibration results will reveal whether or not water has penetrated the epoxy jacket.

Inspect the cell thoroughly and make sure that it isn’t cracked or abused in any way.

  • (SBE 9plus, 25, or 25plus) If the readings are good at the surface but erratic at depth, it is likely that the problem is in the cable or the connector, not the conductivity cell. Check the connections, making sure that you burp the connectors when you plug them in (see Application Note 57: Connector Care and Cable Installation). Check the cable itself (swap with a spare cable, if available).
  • If the readings are incorrect at the surface but good after a few meters, it is likely that the problem is flow-related. Verify that the pump is working properly. Check the air bleed valve (the white plastic piece in the Y-fitting, which is installed on vertically deployed CTDs) to see if it is clogged; clean out the small hole with a piece of fine wire supplied with your CTD.
  • If the readings are incorrect for the entire cast, there may be an incorrect calibration coefficient or the cell may be cracked.
  • Check the conductivity calibration coefficients in the configuration (.con or .xmlcon) file.
  • Do a frequency check on the conductivity cell. Disconnect the plumbing on the cell. Rinse the cell with distilled or de-ionized water and blow it dry (use your mouth and not compressed air, as there tends to be oil in the air lines on ships). With the cell completely dry, check the frequency reading. It should read within a few tenths of a Hz of the 0 reading on your Calibration Sheet. If it does not, something is wrong with the cell and it needs to be repaired.

What are the recommended practices for connectors - mating and unmating, cleaning corrosion, and replacing?

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.

Checking for Leakage and Cleaning Corrosion on Connectors:

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 9-minute video covering O-ring, connector, and cable maintenance.

Replacing Connectors:

  • The main concern when replacing a bulkhead connector is that the o-rings on the connector and end cap must be prepared and installed correctly; if they are not, the instrument will flood. See the question below for general procedure on handling o-rings.
  • Use a thread-locking compound on the connector threads to prevent the new connector from loosening, which could also lead to flooding.
  • If the cell guard must be removed to open the instrument, take extra care not to break the glass conductivity cell.

What is an Anti-Foulant Device? Does it affect the conductivity cell calibration? 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-IM, 37-IMP, 37-IMP-IDO, 37-IMP-ODO), thermosalinographs (SBE 21 and 45), glider CTDs (Glider Payload CTD), moored profilers (SBE 52-MP), and drifters (SBE 41/41CP Argo float CTDs), and optionally with SBE 19plus, 19plus V2, and 49 profilers.

Anti-Foulant Devices have no effect on the calibration, because they do not affect the geometry of the conductivity cell in any way. The Anti-Foulant Devices are mounted at either end of the conductivity cell. For an in-depth explanation of how Sea-Bird makes the conductivity measurement, see Conductivity Sensors for Moored and Autonomous Operation.

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.

Note that the anti-foulant device does not actually dissolve, so there is no way to visually determine if the anti-foulant device is still effective.

The cost of the anti-foulant devices is small compared to the deployment costs, so we recommend that you replace the devices before each deployment. This will provide the maximum bio-fouling protection, resulting in long-term data quality. 

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.

Handling:

  • For details, refer to the Material Safety Data Sheet, enclosed with the shipment and available on our MSDS page.
  • Anti-Foulant Devices are not classified by the U.S. DOT or the IATA as hazardous material.

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

General

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

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.

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

  • Ensure that the instrument is at or above water temperature before it is deployed. If the cell gets colder than 0 to -2 ºC while on deck, when it enters the water a layer of ice forms inside the cell as the cell warms to ocean temperature. If ice forms inside the conductivity cell, 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.
  • For accurate measurements, keep ice out of the sensing region of the conductivity cell. The conductivity measurement involves determining the electrical resistance of the water inside the sensor. Ice is essentially a non-conductor. To the extent that ice displaces the water, the conductivity will register (very) misleadingly low. Some type of screening is necessary to keep ice out of the cell. This is relatively easy to arrange for the Sea-Bird conductivity cell, which is an electrode-type cell, because its sensing region is totally inside a long tube; plastic mesh could be positioned at each end and would have zero effect on accuracy and stability.

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

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.

  • Solution of 1% Triton in sterile seawater (use 0.5-micron filtered seawater or boiled seawater),   or
  • Brine solution (distilled seawater or homemade salt solution that is higher than 35 psu in salinity).

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 and SBE 63 Dissolved Oxygen sensor, avoid prolonged exposure to freezing temperature, including during shipment. Do not store the with water (fresh or seawater), Triton solution, alcohol, or glycol in the plenum. The best precaution is to keep the sensor indoors or in some shelter out of the cold weather.

Can I use a pressure sensor above its rated pressure?

Digiquartz pressure sensors are used in the SBE 9plus, 53, and 54. The SBE 16plus V2, 16plus-IM V2, 19plus V2, and 26plus can be equipped with either a Druck pressure sensor or a Digiquartz pressure sensor. All other instruments that include pressure use a Druck pressure sensor.

  • The overpressure rating for a Digiquartz (as stated by Paroscientific) is 1.2 * full scale. The sensor will provide data values above 100% of rated full scale; however, Sea-Bird does not calibrate beyond the rated full scale.
  • The overpressure rating for a Druck (as stated by Druck) is 1.5 * full scale. The sensor will provide data values above 100% of rated full scale; however, Sea-Bird does not calibrate beyond the rated full scale.

Note: If you use the instrument above the rated range, you do so at your own risk; the product will not be covered under warranty.

How accurate is salinity measured by my CTD? What factors impact accuracy?

One of the reasons that this is not a simple question is that there are several factors to take into consideration regarding the error margin for practical salinity measurements. Salinity itself is a derived measurement from temperature, conductivity, and pressure, so any errors in these sensors can propagate to salinity. For example, Oour initial accuracy specification for the SBE 3plus temperature sensor and SBE 4 conductivity sensor on an SBE 9plus CTD is approximately equivalent to an initial salinity accuracy of 0.003 PSU (note that conductivity units of mS/cm are roughly equivalent in terms of magnitude to PSU).

However, another issue to consider is that this accuracy is defined for a clean, well-mixed calibration bath. In the ocean, some of the biggest factors that impact salinity accuracy are 1) sensor drift from biofouling or surface oils for conductivity in particular and 2) dynamic errors that can occur on moving platforms, particularly when conditions are rapidly changing, which will be true for all sensors that measure salinity. Sea-Bird provides recommendations, design features such as a pumped flow path, and data processing routines to align and improve data for the salinity calculation to account for thermal transients and hysteresis, and to match sensor response times.  Depending on the environment and the steepness of the gradient, and after careful data processing, this may continue to have an impact on salinity on the order of 0.002 PSU or more, for example. For more details, see Application Note 82.

Lastly, note that salinity in PSU is calculated according to the Practical Salinity Scale (PSS-78), which is defined as valid for salinity ranges from 2 – 42 PSU.

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:

  • Hitting the bottom, which can cause the T-C Duct to flex, resulting in cracking at the end of the cell.
  • Removing the soaker tube from the T-C duct in a rough manner, which also causes the T-C Duct to flex. Pulling the soaker tube off at an angle can be especially damaging over time to the cell. Pull the soaker tube off straight down and gently.
  • Improper disassembly of the T-C ducted temperature and conductivity sensors (SBE 25, 25plus, and 9plus) when removing them for shipment to Sea-Bird for calibration. See Shipping SBE 9plus, 25, and 25plus Temperature and Conductivity Sensors for the correct procedure.

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 are the typical data processing steps recommended for each instrument?

Section 3: Typical Data Processing Sequences in the SBE Data Processing manual provides typical data processing sequences for our profiling CTDs, moored CTDs, and thermosalinographs. Typical values for aligning, filtering, etc. are provided in the sections detailing each module of the software. This information is also documented in the software's Help file. To download the software and/or manual, go to SBE Data Processing.

Which Sea-Bird profiling CTD is best for my application?

Sea-Bird makes four main profiling CTD instruments, as well as several profiling CTD instruments for specialized applications.

In order of decreasing cost, the four main profiling CTD instruments are the SBE 911plus CTD, SBE 25plus Sealogger CTD, SBE 19plus SeaCAT Profiler CTD, and SBE 49 FastCAT CTD Sensor:

  • The SBE 911plus is the world’s most accurate CTD. Used by most leading oceanographic institutions, the SBE 911plus is recognized for superior performance, reliability, and ease-of-use. Features include: modular conductivity and temperature sensors, Digiquartz pressure sensor, TC-Ducted Flow and pump-controlled time response, 24 Hz sampling, 8 A/D channels and power for auxiliary sensors, modem channel for real-time water sampler control without data interruption, and optional 9600 baud serial data uplink. The SBE 911plus system consists of: SBE 9plus Underwater Unit and SBE 11plus Deck Unit. The SBE 9plus can be used in self-contained mode when integrated with the optional SBE 17plus V2 Searam. The Searam provides battery power, internal 24 Hz data logging, and an auto-fire interface to an SBE 32 Carousel Water Sampler to trigger bottle closures at pre-programmed depths.
  • The SBE 25plus Sealogger is the choice for research work from smaller vessel not equipped for real-time operation, or use by multi-discipline scientific groups requiring configuration flexibility and good accuracy and resolution on a smaller budget. The SBE 25plus is a battery-powered, internally-recording CTD featuring the same modular C & T sensors used on the SBE 9plus CTD, an integral strain gauge pressure sensor, 16 Hz sampling, 2 GB of memory, TC-Ducted Flow and pump-controlled time response, and 8 A/D channels plus 2 RS-232 channels and power for auxiliary sensors. Real-time data can be transmitted via RS-232 simultaneous with data recording. The SBE 25plus integrates easily with an SBE 32 Carousel Water Sampler or SBE 55 ECO Water Sampler for real-time or autonomous operation.
  • The SBE 19plus V2 SeaCAT Profiler is known throughout the world for good performance, reliability, and ease-of-use. An economical, battery-powered, internally-recording mini-CTD, the SBE 19plus V2 is a good choice for basic hydrography, fisheries research, environmental monitoring, and sound velocity profiling. Features include 4 Hz sampling, 6 differential A/D channels plus 1 RS-232 channel and power for auxiliary sensors, 64 MB of memory, and pump-controlled conductivity time response. Real-time data can be transmitted via RS-232 simultaneous with data recording, The SBE 19plus V2 integrates easily with an SBE 32 Carousel Water Sampler or SBE 55 ECO Water Sampler for real-time or autonomous operation.
  • The SBE 49 FastCAT is an integrated CTD sensor intended for towed vehicle, ROV, AUV, or other autonomous profiling applications. Real-time data ‑ in raw format or in engineering units ‑ is logged or telemetered by the vehicle to which it is mounted. The SBE 49’s pump-controlled, TC-ducted flow minimizes salinity spiking, and its 16 Hz sampling provides very high spatial resolution of oceanographic structures and gradients. The SBE 49 has no memory or internal batteries. The SBE 49 integrates easily with an SBE 32 Carousel Water Sampler or SBE 55 ECO Water Sampler for real-time operation.

The specialized profiling CTD instruments are the SBE 52-MP Moored Profiler, Glider Payload CTD, and SBE 41/41CP Argo CTD module:

  • The SBE 52-MP Moored Profiler is a conductivity, temperature, pressure sensor, designed for moored profiling applications in which the instrument makes vertical profile measurements from a device that travels vertically beneath a buoy, or from a buoyant sub-surface sensor package that is winched up and down from a bottom-mounted platform. The 52-MP's pump-controlled, TC-ducted flow minimizes salinity spiking. The 52-MP can optionally be configured with an SBE 43F dissolved oxygen sensor.
  • The Glider Payload CTD measures conductivity, temperature, and pressure, and optionally, dissolved oxygen (with the modular SBE 43F DO sensor). It is a modular, low-power profiling instrument for autonomous gliders with the high accuracy necessary for research, inter-comparison with moored observatory sensors, updating circulation models, and leveraging data collection opportunities from operational vehicle missions. The pressure-proof module allows glider users to exchange CTDs (and DO sensors) in the field without opening the glider pressure hull.
  • Argo floats are neutrally buoyant at depth, where they are carried by currents until periodically increasing their displacement and slowing rising to the surface. The SBE 41/41CP CTD Module obtains the latest CTD profile each time the Argo float surfaces. At the surface, the float transmits in-situ measurements and drift track data to the ARGOS satellite system. The SBE 41/41CP can be integrated with Sea-Bird's Navis float or floats from other manufacturers. The SBE 41N CTD is integrated with Sea-Bird's Navis Float with Integrated Biogeochemical Sensors and Navis BGCi + pH Float with Integrated Biogeochemical Sensors.

See Product Selection Guide for a table summarizing the features of our profiling CTDs.

How often do I need to have my instrument and/or auxiliary sensors recalibrated? Can I recalibrate them myself?

General recommendations:

  • Profiling CTD — recalibrate once/year, but possibly less often if used only occasionally. We recommend that you return the CTD to Sea-Bird for recalibration. (In principle, it is possible for calibration to be performed elsewhere, if the calibration facility has the appropriate equipment andtraining. However, the necessary equipment is quite expensive to buy and maintain.) In between laboratory calibrations, take field salinity samples to document conductivity cell drift.
  • Thermosalinograph — recalibrate at least once/year, but possibly more often depending on the degree of bio-fouling in the water.
  • DO sensor —
    — SBE 43 — recalibrate once/year, but possibly less often if used only occasionally and stored correctly (see Application Note 64), and also depending on the amount of fouling and your ability to do some simple validations (see Application Note 64-2)
    — SBE 63 — recalibrate once/year, but possibly less often if used only occasionally and stored correctly and also depending on the amount of fouling and your ability to do some simple validations (see SBE 63 manual)
  • pH sensor —
    — SBE 18 pH sensor or SBE 27 pH/ORP sensor — recalibrate at the start of every cruise, and then at least once/month, depending on use and storage
    — Satlantic SeaFET pH sensor — recalibrate at least once/year. See FAQ tab on Satlantic's SeaFET page for details (How often does the SeaFET need to be calibrated?).
  • Transmissometer — usually do not require recalibration for several years. Recalibration at the manufacturer’s factory is the most practical method.

Profiling CTDs:

We often have requests from customers to have some way to know if the CTD is out of calibration. The general character of sensor drift in Sea-Bird conductivity, temperature, and pressure measurements is well known and predictable. However, it is very difficult to know precisely how far a CTD calibration has drifted over time unless you have access to a very sophisticated calibration lab. In our experience, an annual calibration schedule will usually maintain the CTD accuracy to within 0.01 psu in Salinity.

Conductivity drifts as a change in slope as a result of accumulated fouling that coats the inside of the conductivity cell, reducing the area of the cell and causing an under-reporting of conductivity. Fouling consists of both biological growth and accumulated oils and inorganic material (sediment). Approximately 95% of fouling occurs as the cell passes through oil and other contaminants floating on the sea surface. Most conductivity fouling is episodic, as opposed to gradual and steady drift. Most fouling events are small and mostly transitory, but they have a cumulative affect over time. A severe fouling event, such as deployment through an oil spill, could have a dramatic but only partially recoverable effect, causing an immediate jump shift toward lower salinity. As fouling becomes more severe, the fit becomes increasingly non-linear and offsets and slopes no longer produce adequate correction, and return to Sea-Bird for factory calibration is required. Frequently checking conductivity drift is likely to be the most productive data assurance measure you can take. Comparing conductivity from profile to profile (as a routine check) will allow you to detect sudden changes that may indicate a fouling event and the need for cleaning and/or re-calibration.

Temperature generally drifts slowly, at a steady rate and predictably as a simple offset at the rate of about 1-2 millidegrees per year. This is approximately equal to 1-2 parts per million in Salinity error (very small).

Pressure sensor drift is also an offset, and annual comparisons to an accurate barometer to determine offset will generally keep the sensor within specification for several years, particularly as the sensors age over time.

Can I brush-clean and replatinize the conductivity cell myself? How often should this be done?

Brush-cleaning and replatinizing should be performed at Sea-Bird. We cannot extend warranty coverage if you perform this work yourself.

The brush-cleaning and replatinizing process requires specialized equipment and chemicals, and the disassembly of the sensor. If performed incorrectly, you can damage the cell. Additionally, the sensor must be re-calibrated when the work is complete.

Sea-Bird determines whether brush-cleaning and replatinizing is required based upon how far the calibration has drifted from the original calibration. Typically, a conductivity sensor on a profiling CTD requires brush-cleaning and replatinizing every 5 years.

I sent my conductivity sensor to Sea-Bird for calibration, and you also performed a Cleaning and Replatinizing (C &P). You sent the instrument back with 2 sets of calibration data. What does this mean?

The post-cruise calibration contains important information for drift calculations. The post-cruise calibration is performed on the cell as we received it from you, and is an indicator of how much the sensor has drifted in the field. Information from the post-cruise calibration can be used to adjust your data, based on the sensor’s drift over time. See Application Note 31: Computing Temperature and Conductivity Slope and Offset Correction Coefficients from Laboratory Calibrations and Salinity Bottle Samples.

If the sensor has drifted significantly (based on the data from the post-cruise calibration), Sea-Bird performs a C & P to restore the cell to a state similar to the original calibration. After the C & P, the sensor is calibrated again. This calibration serves as the starting point for future data, and for the sensor’s next drift calculation.

The C & P tends to return the cell to its original state. However, there are many subtle factors that may result in the post-C & P calibration not exactly matching the original calibration. Basically, the old platinizing is stripped off and new platinizing is plated on. Anything in this process that alters the cell slightly will result in a difference from the original calibration. We compare the calibration after C & P with the original calibration, not to make any drift analysis, but to make sure we did not drastically alter the cell, or that the cell was not damaged during the C & P process.

Family . Housing Pressure Sensor/Range Pump Speed Oxygen Sensor Interface
GPCTD . 1 – 350 m (plastic) 2 – 100 m strain gauge 0 – Standard 0 – None
    2 – 1500 m (titanium) 3 – 350 m strain gauge   1 – SBE 43F
      5 – 1000 m strain gauge    
      6 – 2000 m strain gauge    

Example: GPCTD.2601 is a GPCTD with 1500 m housing, 2000 m strain gauge pressure sensor, standard pump speed, and interface for SBE 43F dissolved oxygen sensor (interface board, bulkhead connector, and dummy plug; does not include oxygen sensor). See table below for description of each selection:

PART # DESCRIPTION NOTES
GPCTD

Glider Payload CTD (GPCTD) for measuring conductivity, temperature, and pressure, and optionally dissolved oxygen (with DO interface and modular SBE 43F DO sensor). Includes modular pump, AF24173 Anti-Foulant Device, Seasoft software, and complete documentation (order interface cable separately).


 

GPCTD Housing (Depth) Selections MUST SELECT ONE
GPCTD.1x0x 350 meter plastic CTD housing with pump in plastic housing  
GPCTD.2x0x 1,500 meter titanium CTD housing with pump in titanium housing
GPCTD Strain Gauge Pressure Sensor Selections MUST SELECT ONE
GPCTD.x20x 100 m strain gauge pressure sensor Pressure sensor is installed in connector end cap, & is not field replaceable / swappable. While highest pressure rating gives you most flexibility in using GPCTD, it is at expense of accuracy & resolution. It is advantageous to use lowest range pressure sensor compatible with your intended maximum operating depth, because accuracy & resolution are proportional to pressure sensor's full scale range. For example, comparing 2000 & 7000 m sensors:
  • 100 m sensor:
    initial accuracy = 0.1 m (= 0.1% * 100 m),
    resolution = 0.002 m (= 0.002% * 100 m)
  • 2000 m sensor:
    initial accuracy = 2 m (= 0.1% * 2000 m),
    resolution = 0.04 m (= 0.002% * 2000 m)
GPCTD.x30x 350 m strain gauge pressure sensor
GPCTD.x50x 1000 m strain gauge pressure sensor
GPCTD.x60x 2000 m strain gauge pressure sensor
GPCTD Dissolved Oxygen Interface Selections MUST SELECT ONE
GPCTD.xx00 No Dissolved Oxygen sensor interface  
GPCTD.xx01 Include optional Dissolved Oxygen sensor interface board, bulkhead connector, and dummy plug for interfacing with SBE 43F Dissolved Oxygen Sensor

This option is required to provide electronics & bulkhead connector interface to DO sensor, but does not include DO sensor (select GPCTD-4a or 4b below for DO sensor). If you intend to purchase and integrate DO sensor later, Sea-Bird recommends that you select GPCTD.xxx1 now; cost for including it when you order GPCTD is small compared to retrofit cost for adding it later.

GPCTD Dissolved Oxygen Options
GPCTD-4a Integrate SBE 43F Dissolved Oxygen Sensor with 600 m plastic housing (interface cable 171558 included)

SBE 43F is a frequency output version of our SBE 43 Dissolved Oxygen Sensor, & has same performance specifications. Select depth rating for SBE 43F compatible with depth rating of your instrument housing.

Note: GPCTD.xxx1 electronics & bulkhead connector also required.

GPCTD-4b Integrate SBE 43F Dissolved Oxygen Sensor with 7000 m titanium housing (interface cable 171558 included)
GPCTD Spares & Accessories
5M.1302 GPCTD SBE 5 pump, plastic housing, IE55W connector One pump ships with GPCTD; these are spares. Order pump with plastic housing for GPCTD with plastic CTD housing; order pump with titanium housing for GPCTD with titanium CTD housing.
5M.2302 GPCTD SBE 5 pump, titanium housing, IE55W connector
801542 AF24173 Anti-Foulant Device pair (spare, bagged, labeled for shipping) Anti-foulant devices fit into anti-foulant device cup. One ships installed in GPCTD; these are spares.
Useful life varies, depending on several factors. We recommend that customers consider more frequent replacement when high biological activity & strong current flow (greater dilution of anti-foulant concentration) are present. Moored instruments in high growth & 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 strongest determining factor in specific deployment environments.
801944 Data I/O cable, 4-pin IE55 to DB-9S with power leads, 2.5 m (DN 33511) Interface with a computer, for GPCTD setup and data upload.
171558 Cable, SBE 43F, IE55 3-pin to 3-pin, 0.5 m (DN 32561) Connect optional SBE 43F to standard IE-55 bulkhead connector on GPCTD. Cable is included with GPCTD if ordered with GPCTD-4a or -4b; this is spare.
172581 Pump cable, 2-pin IE55 to 2-pin IE55, 0.5 m (DN 33474) Connect pump to GPCTD pump bulkhead connector. Cable is included with GPCTD; this is spare.
30388 Tygon tubing, 1/2" ID X 3/4" OD, 2 m - main plumbing Main plumbing between GPCTD, (optional) SBE 43F dissolved oxygen sensor, and pump.
30579 Tygon tubing, 3/8" ID X 1/2" OD, 2 m - fits over ends of SBE 43 plenum Fits over ends of optional SBE 43 plenum; main plumbing fits over this.

Many cables, mount kits, and spare parts can be ordered online.

Cables

  • 801944 To computer COM port with power leads, 2.5 m, DN 33511
  • 172581 To pump, 0.5 m, DN 33474
  • 171558 To SBE 43F DO sensor, 0.53 m, DN 32561

Spare Parts

  • 801542 AF24173 Anti-Foulant Device (pair, bagged, labeled for shipping)
  • 30388 Tygon tubing, 1/2" ID X 3/4" OD, 2 m — main plumbing
  • 30579 Tygon tubing, 3/8" ID X 1/2" OD, 2 m — fits over ends of SBE 43 plenum; main plumbing fits over this

 

Compare Profiling CTDs (Conductivity, Temperature, and Pressure)

SBE Sampling Rate Channels for Auxiliary Sensors Memory Power Real-Time Data Comments
Internal External
SBE 911plus CTD (9plus CTD & 11plus Deck Unit) 24 Hz

8 A/D

16 Mb with optional SBE 17plus V2
(with optional SBE 17plus V2)
World's most accurate, high resolution CTD, premium sensors, multi-parameter support, water sampler control.
SBE 25plus Sealogger CTD 16 Hz 8 A/D;
2 RS-232
2 Gb
May require SBE 36 CTD Deck Unit & PDIM
High-resolution logging CTD with multi-parameter support. Water sampler control with SBE 33 Carousel Deck Unit.
SBE 25 Sealogger CTD
8 Hz 7 A/D 8 Mb
May require SBE 36 CTD Deck Unit & PDIM
Replaced by SBE 25plus in 2012. Water sampler control with SBE 33 Carousel Deck Unit.
SBE 19plus V2 SeaCAT Profiler CTD 4 Hz 6 A/D;
1 RS-232
64 Mb
May require SBE 36 CTD Deck Unit & PDIM
Personal CTD, small, self-contained, adequate resolution. Water sampler control with SBE 33 Carousel Deck Unit.
SBE 19plus SeaCAT Profiler CTD
4 Hz 4 A/D; optional PAR 8 Mb
May require SBE 36 CTD Deck Unit & PDIM
Replaced by SBE 19plus V2 in 2008. Water sampler control with SBE 33 Carousel Deck Unit.
SBE 19 SeaCAT Profiler CTD
2 Hz 4 A/D 1 - 8 Mb
May require SBE 36 CTD Deck Unit & PDIM
Replaced by SBE 19plus in 2001. Water sampler control with SBE 33 Carousel Deck Unit.
SBE 49 FastCAT CTD Sensor 16 Hz      
May require SBE 36 CTD Deck Unit & PDIM
For towed vehicle, ROV, AUV, or other autonomous profiling applications. Water sampler control with SBE 33 Carousel Deck Unit.
SBE 52-MP Moored Profiler CTD & (optional) Dissolved Oxygen Sensor 1 Hz 1 frequency channel for dissolved oxygen sensor 28,000 samples   Intended for moored profiling applications on device that is winched up and down from a buoy or bottom-mounted platform.
SBE 41/41CP CTD Module for Autonomous Profiling Floats (Argo) OEM CTD for sub-surface oceanographic float that surfaces at regular intervals, transmits new drift position and in situ measurements to ARGOS satellite system. CTD obtains latest temperature and salinity profile for transmission on each ascent. Also available is a Navis Autonomous Profiling Float, Navis BGCi Autonomous Profiling Float with Integrated Biogeochemical Sensors, and Navis BGCi + pH Autonomous Profiling Float with Integrated Biogeochemical Sensors
Glider Payload CTD (GPCTD) and Slocum Glider Payload CTD OEM CTD for autonomous gliders. Generic Glider Payload CTD (GPCTD) is modular, low-power profiling instrument that measures C, T, P, and (optional) Dissolved Oxygen. Slocum Glider Payload CTD provides retrofit/replacement for CTDs on Slocum gliders. Designs share many features, but there are differences in packaging, sampling abilities, power consumption, and installation (see individual data sheets).
Notes:
1. See Application Note 82: Guide to Specifying a CTD.
2. products are no longer in production. Follow the links above to the product page to retrieve manuals and application notes for these older products.