SBE 7 Microstructure Conductivity Sensor
The SBE 7 Microstructure Conductivity Sensor is a reliable, lightweight instrument intended for use in marine profiling applications where its high speed and spatial resolving power offer the ability to characterize small-scale ocean conductivity features. Where salinity variations are minimal, the conductivity signal offers unexcelled resolution of temperature structure. Used in conjunction with the SBE 8 Microstructure Temperature Sensor, the SBE 7 can provide comparably sensitive resolution of salinity and density fields.
Although the SBE 7 is intended primarily for determination of conductivity gradients, it is configured to respond also to absolute conductivity:
- The gradient sensitivity of the sensor is a function of its average zero-frequency response and may be evaluated by integrating that response.
- The functionality of the instrument is verified by its proper static response.
- It is accurate enough as an absolute sensor (typically within 0.005 S/m over periods of several hours) to provide useful data regarding absolute conductivity.
- Sensing element is a remote-cabled, probe-mounted, 2-terminal, platinized electrode cell of dual needle configuration; sensing element’s completely open construction provides nearly total freedom from fouling.
- Sensor probe is a small, stainless steel assembly that may be mounted up to 3 m from the sensor electronics housing. This arrangement minimizes effect of the instrument housing and its wake on the data.
- Cell electrodes are fabricated of solid platinum and are platinized to reduce effect of polarization impedance on sensor response.
- Aluminum housing; depths to 6800 m.
- Five-year limited warranty.
- Cabled remote probe (up to 3 m length) or probe mounted to sensor housing.
- AG or wet-pluggable MCBH connector.
The cell is driven by a sinusoidal voltage of approximately 0.24 Voltsrms at a frequency of 15 kHz. The resulting current is synchronously detected to provide a signal proportional to conductivity (transformer coupling is used to preclude the flow of galvanically induced currents into the electrodes). This signal is subsequently pre-emphasized so that the sensor’s output increases as a function of frequency. The effect of pre-emphasis is to magnify the sensor output for rapidly changing conductivity, therefore overcoming restrictions on system resolving power that would otherwise be imparted by the use of conventional (e.g., 16-bit) digitizers. The SBE 7’s pre-emphasis response magnifies a 100 Hz conductivity signal by a factor of 1000, facilitating acquisition of signals 1000 times smaller than could be characterized by conventional CTD sensors.
The spatial response characteristics of the SBE 7 are such that about 90% of the electrical current associated with the dual needle cell response flows within a volume of water less than 3 cm in diameter and mostly forward of the needle tips. General characteristics of similar sensors are described in a paper by Meagher, et al 1, which also discusses field experiences with instruments of this type.
1 Meagher, T.B.; Pederson, A.M.; and M.C. Gregg, “A Low-noise conductivity microstructure instrument”, IEEE-MIS Proceedings of Oceans ‘82.
|High-Speed Micro-Scale Resolution||-3 db at 100 cycles/m (sensor)
-3 db at 1000 Hz (electronics)
|Extremely Low Noise||2 x 10-7 (S/m) / (Hz 1/2)
Pre-emphasized response to overcome system thresholds
|Conductivity Sensing Element||2-terminal, dual-needle, platinized-electrode cell.
Cabled (up to 3m) remote stainless steel probe assembly.
|Input Power||± 15 V ± 3% at 25 mA|
|Voltage, +6 to -6 V for 0 to 6.5 S/m nominal at 0 frequency.
Output increases in proportion to frequency, beginning at 0.1 Hz and ending at 200 Hz.
|Housing & Depth Rating||Hard-anodized 7075-T6 aluminum, zinc anode protected,
4.8 cm diameter x 18 cm long,
6800 m depth rating
|Connector||6-pin Impulse AG 306 (1/2-20 thread) or MCBH-6MAL|
|Weight||0.8 kg in air, 0.4 kg in water|
What is the function of the zinc anode on some instruments?
A zinc anode attracts corrosion and prevents aluminum from corroding until all the zinc is eaten up. Sea-Bird uses zinc anodes on an instrument if it has an aluminum housing and/or end cap. Instruments with titanium or plastic housings and end caps (for example, SBE 37 MicroCAT) do not require an anode.
Check the anode(s) periodically to verify that it is securely fastened and has not been eaten away.
|7||MICROSTRUCTURE CONDUCTIVITY SENSOR - 6800 m depth, with probe. Includes Microfit software & complete documentation.|
|SBE 7 Probe Configuration Selections — MUST SELECT ONE|
|7-1[X]||Cabled remote probe (80013.2x), [X = length] up to 3 m||Remote probe (shown here) minimizes effect of instrument housing and its wake on data.|
|7-2||Probe (80013.4) mounted to sensor housing|
|SBE 7 Wet-Pluggable Connector Option|
|7-3||Wet-pluggable (MCBH) connector instead of XSG connector||
Wet-pluggable connectors may be mated in wet conditions. Their pins do not need to be dried before mating. By design, water on connector pins is forced out as connector is mated. However, they must not be mated or un-mated while submerged. Wet-pluggable connectors have a non-conducting guide pin to assist pin alignment & require less force to mate, making them easier to mate reliably under dark or cold conditions, compared to XSG/AG connectors. Like XSG/AG connectors, wet-pluggables need proper lubrication & require care during use to avoid trapping water in sockets.