Conductivity Sensor
SBE 4

Print version     Configuration options & accessories -  4C or 4M

DESCRIPTION

SBE 4 series conductivity sensors are modular, self-contained instruments that measure conductivity from 0 to 7 S/m (Siemens/meter), thus covering the full range of lake and oceanic applications. The sensors (Version 2; S/N 2000 and higher) have electrically isolated power circuits and optically coupled outputs to eliminate any possibility of noise and corrosion caused by ground loops. Interfacing is also simplified by the square-wave variable frequency output signal (nominally 2.5 to 7.5 kHz, corresponding to 0 to 7 S/m). The sensors offer improved temperature compensation, smaller fit residuals, and faster turn-on stabilization times. Supply voltage range is 6 to 24 volts.

The SBE 4C is a primary sensor for the SBE 9plus CTD Underwater Unit and SBE 25 and 25plus Sealogger CTD. Available in 6800 meter (22,300 ft) aluminum or 10500 meter (34,400 ft) titanium housing, the SBE 4C has a quick-disconnect fitting to simplify plumbing to the CTD pump. The SBE 4M is intended for long-term moored deployments.  Available with a 3400 meter (11,100 ft) or 6800 meter (22,300 ft) aluminum housing or 10500 meter (34,400 ft) titanium housing, the SBE 4M is supplied without the quick-disconnect fitting.

The sensing element is a cylindrical, flow-through, borosilicate glass cell with three internal platinum electrodes. The electrode arrangement offers distinct advantages over inductive or open external field cells. Because the outer electrodes are connected together, electric fields are confined inside the cell, making the measured resistance (and instrument calibration) independent of calibration bath size or proximity to protective cages or other objects. The cell resistance controls the output frequency of a Wien Bridge oscillator circuit. A unique Sea-Bird design feature introduces a fixed conductivity offset, permitting the instrument to measure conductivity down to 0 for fresh water work.

APPLICATION

Because of the SBE 4's low noise characteristics, hybrid frequency measuring techniques (used in Sea-Bird's CTD instruments) may be used to obtain rapid sampling with very high temporal and spatial resolution. The SBE 4 is ideally suited for obtaining horizontal data with towed systems or vertical data with lowered systems. Because of its small size, it is especially useful for moorings, portable CTD systems, or through-the-ice work.

SPECIFICATIONS ¹

Measurement Range: 0.0 - 7.0 Siemens/meter (S/m)

Initial Accuracy: ± 0.0003 S/m

Stability ²: 0.0003 S/m/month

Resolution ³: 0.00004 at 24 Hz

Time Response ⁴: 0.060 seconds (pumped)

Settling Time: 0.7 seconds to within 0.0001 S/m

Supply Voltage: 6 - 24 VDC

Supply Current: 18 mA at 6V; 12 mA at 10 - 24 V

Signal Output: 1 V square wave capacitively coupled

^¹ Typical specifications, referenced to NIST-traceable calibration.
^² Not applicable in areas of high biofouling activity or highly contaminated waters, or if procedures in Application Note 2D are not followed.
³ Achieved with SBE 9 CTD. In custom applications, resolution will depend on the frequency measuring technique used.
⁴ Time to reach 63% of final value following a step change in conductivity.

CALIBRATION

Sea-Bird calibrates the sensors over the range of 2.6 to 6 S/m in a computer-controlled bath, using natural seawater; a water sample at each point is compared to IAPSO seawater using a Guildline Autosal. A least-squares fitting technique (also including a zero conductivity point in air) yields calibration coefficients for use in the following equation:

Conductivity [S/m] = ( g + hf ² + if ³ + jf ⁴ ) / 10 (1 + dt + ep)

where f is instrument frequency [kHz], t is temperature [°C], p is pressure [decibars], and d is thermal coefficient of expansion (3.25 x 10 ^-06) and e is bulk compressibility (-9.57 x 10 ^-08) of the borosilicate cell. The resulting coefficients g, h, i, and j are listed on the calibration certificate. Residuals are typically less than 0.0002 S/m.

Sample Calibration Data

Calibration data for sensor serial number = 2020
Calibration date: 30 May 97
Practical Salinity Scale 1978: C(35,15,0) = 4.2914 [Siemens/meter]

g = -1.05697877e+01     h =  1.42707291e+00       i = -4.32023820e-03        j =  4.53455585e-04

ADDITIONAL INFORMATION / LINKS:

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

• Print version of this specification sheet / brochure
• Application Notes:
  - 2D: Instructions for Care and Cleaning of Conductivity Cells
  - 6: Determination of Sound Velocity from CTD Data
  - 10: Compressibility Compensation of Sea-Bird Conductivity Sensors
  - 70: Installing Anti-Foulant Device Mount Kit on SBE 4, 16, 19, and 21 Conductivity Cells
  - 71: Desiccant Use and Regeneration (drying)

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

• Configuration options & accessories:
  - Details for SBE 4M & 4C
  - Listing for all products
• Cables
• Mount kits
• Spare parts

Links to Other Instruments of Interest

• SBE 9plus CTD -- SBE 4C is conductivity sensor used on this CTD
• SBE 25plus Sealogger CTD -- SBE 4C is conductivity sensor used on this CTD
• SBE 26plus Wave and Tide Gauge -- SBE 4M is optional conductivity sensor for this instrument
• SBE 53 BPR Bottom Pressure Recorder -- SBE 4M is optional conductivity sensor for this instrument

Specifications are subject to change without notice.

Sea-Bird Home     Phone: (+1) 425-643-9866     E-mail: seabird@seabird.com