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DESCRIPTION
The SBE 31 is a variation of the SBE 11 CTD Deck Unit which contains the encoding electronics usually found in the SBE 9 CTD Underwater Unit. The SBE 31 is most useful as a general-purpose laboratory counter or (in conjunction with a computer) as a data logger during instrument calibration. The standard instrument is supplied with 3 frequency data channels.
Additional counter channels or a multiplexed A/D for voltage-output sensors can easily be added. For CTD work in water less than a few hundred meters, the SBE 31 can eliminate the requirement for a regular CTD Underwater Unit.
The temperature, conductivity, and pressure sensors send their frequency outputs directly to the SBE 31 for encoding. A multi-conductor cable with an individual coax for each sensor is required. A computer is required to take the data from the IEEE-488 or RS-232C output port, store it on disk, and convert it to engineering units for display and/or plotting. Indicator lights are provided to show when data is being properly encoded without error and when the computer interface is in the Transmit or Receive mode. Output buffers accumulate data while the computer may be temporarily occupied with other tasks.
Hybrid period counting is used, so the measurement period is always the same (regardless of sensor frequency) but the high-resolution advantages or period-counting are realized. Each channel is digitized 24 times per second. Any sensor frequency between 1687.5 Hz and 98,304 Hz will be unambiguously encoded (there are no wrap-arounds to keep track of). Other frequency ranges are available as options. The SBE 31 computes the sensor frequencies and presents the binary equivalents of these frequencies to the output port. While the scan rate is fixed at 24 per second, the SBE 31 can average-together the data from up to 24 scans. The effective scan rate is set by a command from the computer and can be any rate determined by the formula:
Effective Scan Rate (scans per second) = 24/N
where N is any integer between 1 and 24 inclusive
If the effective scan rate is reduced by increasing N, the system resolution improves. Operating at a lower effective scan rate results in fewer bytes of data at the output port, a feature which proves useful when less capable computers are used to process and log the data.
Although almost any computer may be used with the SBE 31, IBM-PC compatibles are convenient because they run Sea-Bird's SEASOFT© data acquisition software. Supplied without charge to users of Sea-Bird equipment, SEASOFT provides CRT tabulation and graphing, multi-pen plotting, and floppy/hard disk archiving of full-speed CTD data.
A thumbwheel switch is used to select any sensor frequency or A/D channel information for display with an 8-digit LED front panel array. No computer is necessary to obtain this display. Under software control, the LED array can also be used to display temperature, conductivity, or depth as computed by the controlling computer.
SPECIFICATIONS
Number of Data Channels:
Allowable Frequency Range and Waveform:
1687.5 to 98,304 Hz standard, 0.5 to 10 volt peak-to-peak sine or square wave. Other ranges available.
Resolution:
| at 24 Scans per Second | at 1 Scan per Second |
| 0.006 Hz at 1687.5 Hz | 0.00024 Hz at 1687.5 Hz |
| 0.035 Hz at 10,000 Hz | 0.0014 Hz at 10,000 Hz |
| 0.341 Hz at 98,304 Hz | 0.0142 Hz at 98,304 Hz |
Accuracy:
The 27,648,000 Hz reference crystal (± 2ppm, 0 to 50°C) is divided by 4, to obtain a 6,912,000 Hz reference frequency yielding count accuracies as follows:
Outputs:
Power and Size:
The SBE 31 requires less than 100 watts at 115 or 230 VAC 50/60 Hz, and is supplied in a standard 19-inch rack-mount configuration -- 13.3 cm high x 43.2 cm deep (5.25 x 17 in.).
ADDITIONAL INFORMATION / LINKS:
Documentation -- manual, photos, technical papers, application notes, etc.
Specifications are subject to change without notice.
Sea-Bird Home Phone: (+1) 425-643-9866 Fax: (+1) 425-643-9954 E-mail: seabird@seabird.com
