Wave Gauge - NIWA DOBIE

When deployed underwater, this self-contained instrument records and analyses water waves. It can trigger other instruments and send alarms via a communications link.

DOBIE is a wave gauge with a unique ability: raw pressure data are processed onboard to produce wave statistics and spectra in real time. This makes DOBIE easy to use, for the lay user and expert alike, since usable information is downloaded directly, with no further data processing required. Because data are analysed onboard, DOBIE can be used to trigger linked instruments or raise alarms at remote stations, based on the prevailing wave conditions. Applications include monitoring wave height and telemetering an alarm signal back to a civil defence office when waves first exceed dangerous heights, and sending wave information back to a shore station on a regular basis for rebroadcasting in a boating report.

DOBIE’s calculations are robust: raw pressure data are subjected to numerous quality checks, dubious points are replaced by cubic-spline interpolation, and sensor noise is explicitly accounted for and rejected from the transformations applied to the pressure data. Because DOBIE checks its estimates of wave statistics (including significant wave height, mean spectral period, wave-orbital speed) against several real-world benchmarks, the statistics are reliable.

DOBIE has a track record in consulting, environmental monitoring and science, including benthic ecology research in estuaries, baseline monitoring for harbour design, and the study of wave generation in enclosed harbours by physicists. DOBIE represents a new generation of instrumentation: software, combined with sound hardware design, is the workhorse. The result is cost-effective and reliable delivery of wave information to the user.

Some Applications to Date

Wave Generation Study – Enclosed Harbour

DOBIEs were used by physicists to study wave generation in an enclosed harbour. The wave period was small, and the wave height and ratio of water depth to wavelength varied radically with the tide as the fetch contracted and expanded and the water level changed. The experiment provided a rigorous test of DOBIE's onboard processing ability.

Ecological Studies in an Estuary

Measurements of waves were required at nine estuarine sites simultaneously over three months in an experiment conducted by benthic ecologists. The processed data, which were downloaded every three weeks, were immediately available for inspection – no time-consuming, specialised analysis was required to compare waves at the different sites.

Control of Linked Instrument

DOBIE can be programmed with a look-up table of wave statistics to trigger linked instruments or raise alarms in real time. This unique facility was used in a sediment-dynamics research project to collect water samples over a range of wave conditions for calibrating optical backscatter sensors. The triggering was done on orbital-speed at the seabed, which is calculated by DOBIE from the raw pressure data.

Specifications

Pressure sensor Choice of Druck PMP 4000 Series (± 0.04% accuracy, 5 bar standard with 1.5, 2, 3.5, 5 and 7 bar options available) or Paroscientific DigiquartzTM (repeatability < ± 0.005% FS, 100 psi standard with 15, 25, 30 and 45 psi options available)
Resolution 12-bit 
Recommended deployment depth Down to 30 m
Power supply 8 alkaline D-cells 
Power drain <1 mA sleep between bursts and 45 mA maximum when sampling/processing
Power endurance Varies with operating mode and sampling regime; typically 30-90 days
Controller Onset Computer Corporation Model 8 TattletaleTM
Data storage 10 Mb flash (nonvolatile) memory card standard; other capacities available
Operating modes (1) pressure/depth time series, (2) pressure/depth time-average (tide gauge), (3) wave statistics, (4) wave spectrum plus wave statistics
Memory endurance (10Mb flashcard) (1) ~5,000 bursts with 1024 samples per burst; (2) ~525,000 readings; (3) ~120,000 bursts; (4) ~35,000 bursts with 128 spectral estimates
Comms RS-232 (386 PC or better required; WindowsTM 3.x or above); 10-Mb data download approximately 45 min @ 57,600 baud; LEDs set in endcap for signalling instrument status
Housing Anodised al, 350 mm long x 162 mm diameter (body) and 210 mm diameter (body plus flange)

Documentation

Users of DOBIE can click here for detailed documentation including:

  • Technical Notes
  • DOBIE Manual
  • Software Upgrades

For More Information

DOBIE is manufactured by NIWA Instrument Systems in Christchurch.
To discuss real-time control/alarm applications:
Tel: +64 3 343 7890, fax: +64 3 343 7891
instruments@niwa.co.nz