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Current Manual - SBE 9plus,
11plus V2, &
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Configuration options & accessories - SBE
9plus, 11plus V2,
& 17plus
V2
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 intercompared 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.
ADDITIONAL INFORMATION / LINKS:
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.