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SBE 911plus

Print version     Current Manual - SBE  9plus, 11plus V2, & 17plus V2 (older manuals)    Configuration options & accessories - SBE 9plus, 11plus V2, & 17plus V2

SBE 9plus CTD with optional dual T & C sensors
SBE 9plus Underwater Unit shown with optional dual T & C sensors

Sea-Bird's 911plus CTD is the primary oceanographic research tool chosen by the world's leading institutions.





SBE 11plus CTD Deck Unit
SBE 11plus V2 Deck Unit



The 911plus CTD system includes:

For real-time data collection, an electro-mechanical sea cable (single- or multi-conductor), a slip-ring equipped winch, and computer for data display and logging are typically supplied by the user. An optional SBE 17plus V2 Searam Recorder and Auto Fire Module provides in-situ recording and self-contained CTD operation, and can be user-programmed to trigger bottle closure on a Carousel Water Sampler, eliminating the need for the Deck Unit, conductive sea cable, and slip-ring equipped winch.

SBE 9plus Underwater Unit

The standard SBE 9plus underwater unit has an aluminum housing rated to 6800 meters (22,300 ft), and is supplied with one conductivity and one temperature sensor (fitted with a TC Duct and constant-flow pump), and an internally mounted, temperature-compensated Paroscientific Digiquartz pressure sensor for 6800 meter (10,000 psia) full scale range. Input channels and bulkhead connectors are provided for an optional second (redundant) pair of temperature and conductivity sensors. Other standard features include an 8-channel, 12-bit A/D converter with differential inputs and low pass filters, and high-power capability for support of commonly used auxiliary sensors (e.g., SBE 43 dissolved oxygen, SBE 18 or SBE 27 pH, transmissometer, fluorometer, ambient light, altimeter), a modem channel for real-time water sampler control, and a port for connection of an optional bottom contact switch. Options include:

SBE 11plus V2 Deck Unit

SBE 11plus V2 Deck Unit includes RS-232 and IEEE-488 computer interfaces, a modem channel for real-time water sampler control (including water sampler control push buttons and status lights), NMEA 0183 interface for adding GPS position to CTD data, 12-bit A/D input channel for surface PAR sensor, switch-selectable 115/230 VAC operation, audio tape interface (data backup), LED readout for raw data, and audible bottom contact (or altimeter) alarm. The 11plus V2 also provides a remote pressure output (useful as an input signal for towed vehicle control) and a programmable serial ASCII data output containing up to seven variables in computed engineering units. Calibration coefficients are stored in EEPROM, and a separate microcontroller converts raw CTD data to temperature, depth, salinity, etc. The 11plus V2 is shipped in a free-standing cabinet  with a hardware kit for mounting in a standard 19-inch electronics rack.

SBE 17plus V2 Searam Memory and Auto Fire Module (see the Searam brochure for details)

The Searam provides battery power for the SBE 9plus CTD and SBE 32 Carousel Water Sampler, memory for CTD data recording, and autonomous water sampler control (Auto-Fire). Each time the Searam's magnetic on/off switch is activated, date, time, and sequential cast number is recorded. The 16 Mbyte memory provides approximately 15 hours recording of C, T, and P at 24 Hz (6 hours with all auxiliary channels used), and the Searam can average samples for longer recording endurance. It can be programmed before deployment to decode pressure data from the 9plus and fire bottles at user-programmed depths, allowing autonomous water sampling and CTD recording without a conductive sea cable. Data upload requires about 12 minutes per Mbyte at 38,400 baud. The Searam includes a Nickel-Metal hydride (NiMH) rechargeable battery park and a charger.

When the SBE 9plus is equipped with the Searam, the system is referred to as the 917plus.

SOFTWARE - Seasoft© V2

Supplied with each SBE 911plus, Seasoft calculates a suite of seawater parameters, including salinity, density, buoyancy, sound velocity, etc., and fully supports auxiliary sensors for oxygen, light transmission, PAR, fluorescence, and many other variables. Seasoft provides real-time plots or number readouts while saving raw data to a disk file from which an ASCII or binary intermediate file in engineering units may subsequently be made. Post-processing utilities provide bin averaging, wild point editing, filtering, time-aligning, and color video graphing / hard copy plotting of profiles, waterfall overlays, and density-contoured TS plots. When operating the 911plus with a water sampler, complete bottle housekeeping files based on firing confirmations are recorded. Seasoft is upgraded frequently; new versions are supplied free to 911plus users and posted on our website for easy access.



  Measurement Range Initial Accuracy Typical Stability Resolution at 24 Hz Time Response 1 Master Clock Error Contribution2
Conductivity 0 - 7 S/m
(0 - 70 mmho/cm)
0.0003 S/m
(0.003 mmho/cm)
0.0003 S/m
(0.003 mmho/cm) per month
0.00004 S/m
(0.0004 mmho/cm)
0.065 second 0.00005 S/m
Temperature -5 to +35 °C 0.001 °C 0.0002 °C per month 0.0002 °C 0.065 second 0.00016 °C
Pressure 0 to full scale -- 1400/2000/4200/6800/10,500 m
(2000/3000/6000/10,000/15,000 psia)
0.015% of full scale 0.02% of full scale per year 0.001% of full scale 0.015 second 0.3 dbar with 6800 m (10,000 psia)
pressure sensor
A/D Inputs 0 to +5 volts 0.005 volts 0.001 volts per month 0.0012 volts 5.5 Hz 2-pole
Butterworth Low
 Pass Filter

1Single pole approximation including sensor and acquisition system contributions.
2 Based on five-year worst-case error budget including ambient temperature influence of 1 ppm total over -20 to +70 °C plus 1 ppm first year drift plus four additional year's drift at 0.3 ppm per year.


  Weight in air, kg (lbs) Weight in water, kg (lbs) Dimensions, mm (in.)
SBE 9plus with Aluminum housing 25 (55) 16 (35) 952 x 330 x 305 
(37.5 x 13 x 12)
SBE 9plus with Titanium housing 29 (65) 20 (45)
SBE 11plus 10 (23) Not applicable 132 x 432 x 432 
(5.2 x 17 x 17)
SBE 17plus with Aluminum housing 8 (18) 3.7 (8) 711 x 99
(28 x 3.9)
SBE 17plus with Titanium housing 12 (26) 7.3 (16)



The temperature sensor (SBE 3plus) is a compact module containing a pressure-protected, high-speed thermistor and Wien-bridge-oscillator interface electronics. The thermistor is the variable element in the Wien bridge, while a precision Vishay resistor and two ultra-stable capacitors form the fixed components.

The conductivity sensor (SBE 4C) is similar in operation and configuration to the temperature sensor, except that the Wien-bridge variable element is the cell resistance (cell resistance is the reciprocal of cell conductivity).

The Digiquartz® pressure sensor also provides a variable frequency output. Embedded in the pressure sensor is a semiconductor temperature sensor used to compensate the small ambient temperature sensitivity of the Digiquartz.

The calibration information for each sensor (C, T, and P) is contained in a series of numeric coefficients used in equations relating frequency to the measured parameter.


The SBE 11plus V2 Deck Unit provides power to the sea cable, decodes the data arriving from the underwater unit, and interfaces to a computer via RS-232 or IEEE-488. Push buttons and status lights for SBE 32 Carousel Water Sampler operation are provided, and there are connections for back-up data recording and playback using an audio tape recorder. The SBE 9plus underwater unit comprises modular temperature and conductivity sensors, a small external pump, and a main housing supplied with surface power from the sea cable. Electronics in the main housing provide three primary functions: regulation of the several voltage levels required by the internal circuits, external sensors, and pump; acquisition (digitization) of sensor signals; and data telemetry.

Sea cable Power Supply

Unlike competing CTD systems in which the deck unit supplies a fixed current, the SBE 11plus V2 presents a constant voltage to the sea cable. The SBE 9plus receives this voltage (minus the sea cable I-R drop), regulates it to a constant value, and presents it to a high-efficiency DC/DC converter that generates the system supply voltages (+14.3/-13.5, +8, and +5). Two advantages derive from this method: less power is lost in the sea cable (and more is delivered to the underwater unit); and the underwater unit is not required to dissipate unneeded power (freeing the user of the need to monitor and adjust the surface sea cable supply).

Signal Acquisition and Data Telemetry

Connectors on the SBE 9plus bottom end cap supply power to (and receive variable frequencies from) the modular conductivity and temperature sensors. The C and T variable frequencies plus the internal Digiquartz frequency are routed to separate counters that are allotted exactly 1/24 second to derive 24-bit binary values representative of each sensor frequency. Sea-Bird's hybrid counter technique combines integer and period counting to produce digital results that are simultaneous (time coincident) integrals of C, T, and P. The 9plus provides four bulkhead connectors for optional auxiliary sensor inputs. Each connector provides +14.3 volts power and permits access to two differentially amplified and low pass filtered digitizer channels with 0 to 5 volt range and 12-bit resolution. Binary data from the entire suite of C, T, P, and auxiliary sensors are transmitted serially 24 times per second using a 34560 Hz carrier differential-phase-shift-keyed (DPSK) technique. This telemetry system is suitable for all single and multi-conductor sea cables having a conductor resistance of 350 ohms or less.

Subcarrier Modem and SBE 32 Carousel Water Sampler Control

A 300 baud full duplex FSK subcarrier modem (2025/2225 Hz downlink; 1070/1270 Hz uplink) provides a separate communications channel  for control of the Carousel. Bottles can be fired with push buttons on the deck unit's front panel, through SEASOFT©, or via a separate computer connected directly to the modem port on the deck unit's back panel. There is no interruption of CTD power or data during the bottle firing process. An optional interface card in the SBE 9plus permits control of older multi-bottle sampler types, and the modem channel is also available as a general purpose RS-232 interface for custom user applications.


Following consultation with the US National Institute of Standards and Technology, Sea-Bird's metrology lab was configured to achieve temperature precision of 50 µK and accuracy of 0.0005° C. To obtain this performance, premium primary references, including four Jarrett water triple-point cells (with maintenance bath) and an Isotech gallium melt cell, are operated in conjunction with two YSI 8163 standards-grade platinum resistance thermometers and an ASL Model F18 Automatic Temperature Bridge. IAPSO standard seawater and a Guildline 8400B AutoSal provide the highest obtainable salinity accuracy of 0.002 PSU.

SBE 911plus temperature and conductivity sensors are calibrated in low-gradient, computer-controlled baths capable of transferring Sea-Bird metrology lab accuracies within 0.0005° C and 0.001 PSU.

Calibration data from Sea-Bird's computer-controlled baths are collated to produce certificates showing the latest results. Overplots of previous calibrations allow the user to judge the stability of the sensor over time.

SBE 911plus Digiquartz pressure calibrations are performed by Paroscientific, Inc. using a DH Model 5206 Primary Pressure Standard (oil-operated dead weight tester) certified to an accuracy of 0.01% of reading (0.7 dbar at 6800 meters depth). Cross-comparison against independent standards has established the consistent accuracy of the Paroscientific calibrations.

Accuracy and stability of the SBE 911plus quartz master clock is judged against a Spectracom Model 8163 reference oscillator phase-locked to the U.S. National Institute of Standards and Technology's WWVB 60 kHz broadcast signal.


The SBE 911plus CTD produces profiles of ocean temperature, salinity, and density at the highest possible absolute accuracy, because its performance under both static and dynamic conditions has been optimized. Static accuracy (as demonstrated in an equilibrated calibration bath) ensures that the deep-ocean readings will be correct and allows meaningful comparison of results obtained by different researchers at different times and places. Dynamic accuracy is necessary to present water column features in clear detail, and is critical for maintaining absolute accuracy under oceanic (non-equilibrated) conditions. This is because salinity, density, and other oceanographic variables are calculated from separate measurements of pressure, temperature, and conductivity. If the calculated values are to be correct, the separate measurements must be made at the same time and on the same sample of water.

Time response and spatial mismatches not only create spiking, but also produce bias errors that are indistinguishable from static errors because they cannot be averaged out. For example, if the temperature sensor responds slowly, averaging its readings through a temperature gradient will produce a bias error of sign opposite to the gradient. Similarly, the spiking caused by a mismatch in time-response of the temperature and conductivity sensors will bias the results unless the correct time lag is applied in post-processing. Corrections are possible in practice only if the sensor time responses are constant, a condition that cannot be met by free-flushing (unpumped) conductivity sensors. The time responses of free-flushing sensors are inevitably affected by the influence of ship-coupled motion on profiling speed.

To obtain the highest possible absolute accuracy, the SBE 911plus CTD incorporates certain key features:

The temperature accuracy that can be achieved under controlled laboratory conditions with an SPRT (Standards-grade Platinum Resistance Thermometer) cannot be obtained in the ocean with the industrial-grade PRTs used in competing CTD instruments. The 911plus thermistor sensor's better ocean accuracy derives from its 10 times higher sensitivity and 100 times higher absolute resistance (at the ice-point, the thermistor resistance changes by about 1 ohm/mK while the resistance of a PRT changes by about 0.001 ohm/mK), its inherently fast response that eliminates the need for fast and slow sensor combinations (and the errors that arise when merging data from separate sensors), and because it is not measurably affected by shock and vibration.

Sea-Bird's conductivity cell designs reflect our recognition that the primary causes of conductivity errors are mineral and biological deposits on the sensor, proximity effects arising from external fields, and uncontrolled time-responses. Deposits occur with all conductivity sensor designs (they are more serious with sensors that are smaller than Sea-Bird's) and can be minimized by periodic detergent and bleach cleaning of the cell. The four-electrode and inductive-cell types used on competing CTDs have significant external fields that often completely preclude high-accuracy laboratory calibration and that lead to in-situ proximity errors induced by guards, mounting brackets, and other nearby sensors. Sea-Bird's totally internal field conductivity cell eliminates proximity errors, permits constant-flow pumping to control time response, and is connected to the temperature sensor by the TC Duct to ensure that the measurements of T and C are made on exactly the same water.

The highest possible pressure accuracy is obtained by using the Paroscientific Digiquartz® pressure sensor. The inexpensive pressure sensors used in other CTD systems have excessive hysteresis and thermal transient errors, requiring costly sensor-specific characterization and tedious post-processing. Sea-Bird's choice of a costly, but dramatically superior, pressure sensor eliminates most of these errors before they get into the data set. Careful shock mounting of the Digiquartz has resulted in negligible failure rates.

The SBE 911plus' modular sensors can be calibrated in well-insulated temperature/salinity baths that have smaller gradients and better accuracy than baths build to accommodate (and absorb the heat produced by) an integrated CTD. Unlike some competing sensor designs where trim pots are adjusted and drift history is lost each time a calibration is performed, the Sea-Bird calibrations are preserved as sets of numerical coefficients. As a result, all calibrations of Sea-Bird sensors can be compared and a complete drift history established (Sea-Bird maintains such histories - some of them spanning more than 20 years - on thousands of sensors). The information in these histories continues to play an important role in Sea-Bird's ongoing improvements to its sensor designs.

The SBE 911plus sensors can be calibrated separately without significant loss of overall CTD accuracy because the SBE 9plus digitizes the temperature, conductivity, and pressure sensor output signals by frequency counting, an inherently binary process whereby a count either registers or does not. Cable resistance, connector properties, and noise cannot degrade the overall system acquisition accuracy, which is limited only by the stability of a quartz master clock. Errors attributable to this clock are demonstrably negligible.

While competing designs occasionally offer elegant solutions to part of the CTD measurement problem, we have carefully balanced the engineering trade-offs to get better overall results. The SBE 911plus has the ability -- under conditions of rapidly changing temperature and immense pressure loading -- to obtain the independent measurements precisely coordinated in space and time that are the essence of CTD accuracy. Its design is a synthesis of ideas based upon a thorough understanding of the marine environment, the operational requirements of oceanographers, and the fundamental principles affecting CTD accuracy. System Engineering has made the 911plus CTD the World's Most Accurate CTD.


Documentation -- manual, photos, technical papers, application notes, etc.

Sales Information -- options, accessories, cables, mount kits, spares, etc.

Software -- components of Seasoft V2

Links to Other Instruments of Interest

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Specifications are subject to change without notice.

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