SBE 41 Argo CTD for Autonomous Profiling Floats

SBE 41/41CP CTD Module for Autonomous Profiling Floats (Argo)

Autonomous profiling CTD for Argo and other programs, supplying more than 90% of the annual Argo program floats.


The SBE 41 CTD module was originally developed in 1997 for integration with a sub-surface oceanographic float called ALACE (Autonomous Lagrangian Circulation Explorer) (Davis, et. al., 1992). ALACE floats were neutrally buoyant at depth, where they were carried by currents until periodically increasing their displacement and slowly floating to the surface. At the surface, the ARGOS satellite system could determine its current position. The long time-series of successive position data on hundreds of individual floats gave insights into deep ocean circulation.

Early in the evolution of ALACE floats, researchers added temperature and conductivity sensors to the float to obtain a temperature/salinity profile during the float's ascent to the surface. The ALACE float soon became known as PALACE (Profiling ALACE), and PALACE floats transmitted temperature and salinity data in addition to providing drift track data. However, the quality of the salinity data on early PALACE floats deteriorated quickly due to fouling. The Sea-Bird SBE 41 CTD module was developed in response to the scientific need for highly accurate salinity measurements that remained stable for the three to five year operational life of the PALACE float. The results have been truly remarkable. Today, SBE 41s are producing salinity data accurate to within 0.005 [PSU] for more than 3 years. The value of near-real-time oceanographic data transmission from multiple floats is evidenced by the ability of oceanographers to monitor and predict the influence of El Niño weather patterns on the surface of the North Atlantic Ocean (Riser, 1998).

The success of PALACE floats and the SBE 41 led to the Argo Program, which began in 1999 ( The program goal was to establish and maintain a working population of 3000 floats dispersed throughout the world, transmitting near-real time temperature and salinity from the upper 2000 meters of the ocean. That goal was reached by the end of 2007; there are more than 3700 deployed floats as of January 2015, as shown on the float distribution map below.

Today, Sea-Bird manufactures approximately 1000 SBE 41/41CP CTDs per year, supplying more than 90% of the annual Argo program requirement as well as a growing market in non-Argo float applications.

Features of the SBE 41/41CP Design

  • The SBE 41/41CP uses the proven MicroCAT Temperature, Conductivity, and Pressure sensors. The CTD is shipped fully calibrated, and has demonstrated excellent long-term stability, eliminating the need for post-deployment tampering of the calibration to force agreement with the local TS.
  • The SBE 41/41CP has carefully engineered anti-foul protection, with anti-foulant devices, a U-shaped flow path, and a pump.
    - On the float’s ascent, as the float approaches 10 – 5 decibars beneath the ocean’s surface, the pump turns off. The U-shaped flow path prevents sea surface oils and contaminants from being ingested while proceeding through the ocean surface skin and sitting at the surface during data transmittal.
    - Between profiles the pump is off. The U-shaped flow path prevents water flow through the system caused by waves or currents; minute amounts of anti-foulant concentrate inside the conductivity cell to minimize bio-fouling.
  • The SBE 41/41CP has proven TC-Ducted flow over the temperature sensor and into the conductivity sensor. Salinity spiking is minimized because the TC-Duct and pump precisely coordinate the T and C responses.
  • The SBE 41/41CP premium strain-gauge pressure sensor has thermistor correction for ambient temperature effects.
  • The SBE 41 pump delivers 40 ml/sec flow for 2.5 seconds per measurement of T, C, & P, and exhausts through a zero-thrust port.
  • The SBE 41CP pump delivers 10 ml/sec flow continuously during the profile.

The CTD sensors and electronics (shown below) are mounted to an Argo top end cap (satellite whip antenna not shown).

Photo A: Sea-Bird CTD module with guard installed


Photo B: Module with guard removed to show conductivity cell


Photo C: Module showing mounted PCB

Photo D: Opposite side of Photo C, showing pressure sensor


SBE 41 or 41CP?

Both the SBE 41 and 41CP measure conductivity, temperature, and pressure, and can be equipped to also measure dissolved oxygen (see Recent Developments below). The sensors on the SBE 41 and 41CP are identical; the difference lies in the circuitry and sampling protocols:

  • The SBE 41 spot samples on command. It has no internal memory, and transmits the data to the float controller as each measurement is made. The float controller stores the data, and transmits it when the float reaches the surface. This system is typically used with low bandwidth satellite telemetry such as Argos, which can accommodate only a limited stream of data.
  • The SBE 41CP is designed to perform a Continuous Profile, sampling at 1 Hz as the float ascends to the surface, and storing the data in internal memory. The float controller requests and transmits the data when the float reaches the surface. The SBE 41CP can spot sample on command in addition to / in combination with continuous sampling. For example, a float can be programmed to spot sample in deep water, where parameters of interest are fairly stable, and then to perform a continuous profile as the float ascends through shallower depths, providing the desired level of detail while minimizing the data to be transmitted.

For both the SBE 41 and 41CP, the system can be programmed to transmit all the data, or just the parameters of interest (C, T, P; T and P; P only; or C, T, P, and D.O.).

Additional Sensors

  • SBE 63 Optical Dissolved Oxygen Sensor integrated with Argo CTDs:
    The SBE 63, in production since 2011, can be integrated with SBE 41/41CPs on several float types, including a Sea-Bird Navis Float. Currently, the SBE 63 requires an RS-232 interface in the float controller; a fully integrated version (eliminating the need for the RS-232 interface in the controller) is expected to be available at a future date.
  • Biogeochemical Sensors:
    Biogeochemical sensors, such as the WET Labs ECO Triplet, can be integrated with the SBE 41CP on several float types, including a Sea-Bird Navis Float. The ECO Triplet is three sensors in one, providing any combination of fluorescence (chlorophyll, CDOM, phycoerythrin, phycocyanin, rhodamine, or uranine) and scattering (blue , green, or red) measurements.
  • Surface Temperature and Salinity Sensor (STS) integrated with Argo CTDs:
    The trade-off for achieving sensor stability over the lifespan of the Argo floats was the forfeiture of very near surface temperature and salinity information to prevent ingestion of sea surface oils and contaminants into the conductivity cell. With increasing interest in measuring surface salinity in the world’s oceans, Sea-Bird developed an integrated system that continues to gather high accuracy salinity and temperature data using the pumped SBE 41CP CTD between 2000 – 5 decibars, while co-deploying a free-flow sensor called STS in the upper 20 db to capture the surface temperature and salinity conditions. STS is a free-flushing sensor, and is expected to ingest sea surface oils and contaminants that may alter the sensor calibration over time. To correct any drift in STS, both the SBE 41CP CTD and the STS take measurements near the float park depth and again in the upper ocean, just before the SBE 41CP pump is turned off, and then STS continues sampling through the ocean surface. STS data can then be corrected by recalibrating it based on the comparison measurements to the clean data from the SBE 41CP (more details).

SBE 41/41CP Manufacturing and Calibration

Argo CTD Calibration Baths (4 of 6 shown)

Argo CTDs in various stages

Additional Information and References


ITS-90 ± 0.002 0.0002
per year
Conductivity IAPSO Standard Seawater ± 0.002
(equivalent salinity)
0.001per year
(equivalent salinity)
Pressure Deadweight tester & pressure reference ± 2 dbar 0.8 dbar
per year


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

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.

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.

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.


  • 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.

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.


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).
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.