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CTD Operation
SBE 911plus

Brochure/specifications in pdf format     Manual - SBE 9plus, 11plus V2, & 17plus V2     Configuration - SBE 9plus, 11plus V2, & 17plus V2

SENSOR OPERATION

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.

SYSTEM OPERATION

The SBE 11plus V2 Deck Unit provides power to the seacable, 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 seacable. 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.

Seacable 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 seacable. The SBE 9plus receives this voltage (minus the seacable 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 seacable (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 seacable 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 seacables 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.

ADDITIONAL INFORMATION / LINKS:

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

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

Links to Other Instruments of Interest

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Last modified: 06-Apr-2007
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