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DOBIE Documentation

 

The DOBIE has undergone several advancements over the years - it is no longer only a wave gauge, although this is still its primary function. We now offer various upgrade options, which add sensors and/or functionality. The sensor upgrades include:

  • Seapoint turbidity
  • Seapoint Fluorometer
  • Greenspan Conductivity & Temperature

Option and function upgradres include:

  • Extended battery versions (twice the standard battery capacity for longer deployments)
  • ZebraTech Sensor wipers (for self- cleaning of optical sensors)
  • Real-time transmitted data and external power of instruments

These options and upgrades are quoted on an individual basis. 

 

Technical Notes

98/1 At What Depth Should DOBIE be Deployed?
98/2 Hardware Control Options Explained: Conversion of Volts to Pressure, DOBIE Calibration, Sensor Noise
98/3 Task 2 (Time Series) Explained; A Warning About "Water Depth"
98/4 Task 3 (Tide Gauge) Explained
98/5 Task 4/0 (Wave Statistics) Explained; Plus an Explanation of "Reality Checks"
98/6 Task 4/1 (Wave Spectrum) Explained
98/7 Power Management 
98/8 Memory Management
98/9 More Wave Statistics
99/1 Measuring Waves in Lakes and Estuaries is Quite Tricky - Data can be Improved by Adjusting Sensor Noise Level
00/1 Conversion of Hydrostatic-Depth Spectrum to Sea-Surface-Elevation Spectrum

DOBIE Manual

Software Distribution Point

Software upgrades, fixes and special requests are posted here from time to time.

Miscellaneous

 

 

Contact

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Dr George Griffiths

Principal Scientist - Hydrology
Email Dr George Griffiths
 

Free phone within New Zealand: 0800 RING NIWA (0800 746 464)

Technical Note No. 98/1 - At What Depth Should DOBIE be Deployed?

Technical Note No. 98/1 - At What Depth Should DOBIE be Deployed?
To answer the question, we need to explain what DOBIE actually measures and how waves exert pressure at depth.
DOBIE measures pressure. In still water, the pressure experienced at depth is composed of two parts: hydrostatic pressure, which is due to the weight of the overlying water and which is proportional to the height of the water column above the observer, and atmospheric pressure, which is due to the weight of the atmosphere.

Technical Note No. 98/2 - Hardware Control Options Explained: Conversion of Volts to Pressure, DOBIE Calibration, Sensor Noise

Technical Note No. 98/2 - Hardware Control Options Explained: Conversion of Volts to Pressure, DOBIE Calibration, Sensor Noise
Conversion of Volts to Pressure
DOBIE converts volts to pressure by:

where p is pressure in pounds per square inch, V is volts output by the pressure sensor, G is gain (units of psi/volt) and O is offset (units of psi).

Technical Note No. 98/3 - Task 2 (Time Series) Explained; A Warning About "Water Depth"

Technical Note No. 98/3 - Task 2 (Time Series) Explained; A Warning About "Water Depth"
When deployed in still water (i.e. no waves), DOBIE experiences a total pressure that is composed of two parts: one part due to the weight of the overlying water (hydrostatic pressure) and another part due to the weight of the overlying atmosphere (atmospheric pressure). When waves are present there is yet another, fluctuating, pressure component that is due to the waves.

Technical Note No. 98/4 - Task 3 (Tide Gauge) Explained

Technical Note No. 98/4 - Task 3 (Tide Gauge) Explained
When deployed in still water (i.e. no waves), DOBIE experiences a total pressure that is composed of two parts: one part due to the weight of the overlying water (hydrostatic pressure) and another part due to the weight of the overlying atmosphere (atmospheric pressure). When waves are present there is yet another, fluctuating, pressure component that is due to the waves.

Technical Note No. 98/5 - Task 4/0 (Wave Statistics) Explained; Plus an Explanation of "Reality Checks"

Technical Note No. 98/5 - Task 4/0 (Wave Statistics) Explained; Plus an Explanation of "Reality Checks"
All wave statistics are calculated from burst time series of "hydrostatic water depth", h(t), which are related to raw pressure time series, p(t), by:

where g is acceleration due to gravity (9.81 m/s2), r is water density and F is a factor (6895 kg/[s2m]/psi) used to convert pressure in pounds per square inch to pressure in kg/(s2m). For this calculation, DOBIE assumes the water density to be 1025 kg/m3 (which is a typical seawater density).

Technical Note No. 98/6 - Task 4/1 (Wave Spectrum) Explained

Technical Note No.

Technical Note No. 98/7 - Power Management

Technical Note No. 98/7 - Power Management
DOBIE fitted with 256k RAM data storage has two independent power supplies: a rack of 8 D-cells (Eveready alkaline No. E95, or equivalent; ~18 Ahr) mounted in the main battery compartment and a single 9V cell (Eveready alkaline No. 522, or equivalent; ~500 mAhr) mounted in a small compartment on the back of the microprocessor housing (the black box). The 9V cell is a backup supply for securing data in RAM should the main supply fail or be drained. Note that flashcard data storage is not volatile.

Technical Note No. 98/8 - Memory Management

Technical Note No. 98/8 - Memory Management
The following equations can be used to estimate memory endurance, which is the number of bursts that will fill the data memory.

Technical Note No. 98/9 - More Wave Statistics

Technical Note No. 98/9 - More Wave Statistics
If OFFLOAD.DAT contains Task 4/0 (Wave Gauge - Wave Statistics) or Task 4/1 (Wave Gauge - Wave Statistics plus Hydrostatic Depth Spectrum) data, then the PEDP can be used to recompute the "basic" wave statistics and to compute an "extended" set of wave statistics.

Core statistics include mean water depth, standard deviation of the pressure signal, mean spectral period and spectral width. DOBIE computes these internally and they are written to OFFLOAD.DAT when data are downloaded.

Technical Note No. 99/1 - Measuring Waves in Lakes and Estuaries is Quite Tricky, data can be Improved by Adjusting Sensor Noise Level

Technical Note No. 99/1 - Measuring Waves in Lakes and Estuaries is Quite Tricky, data can be Improved by Adjusting Sensor Noise Level

It sounds paradoxical, but the most difficult task when measuring waves (Task 4/0 or Task 4/1) is knowing when there aren't any! The danger is that sensor noise - not real signal - gets transformed into estimates of wave height, which will be wrong. The "reality checks" applied by DOBIE have been described in a previous Technical Note, and these are meant to catch those bursts where sensor noise has been (incorrectly) transformed.

Technical Note No. 2000/1 - Conversion of Hydrostatic-Depth Spectrum to Sea-Surface-Elevation Spectrum

Technical Note No. 2000/1 - Conversion of Hydrostatic-Depth Spectrum to Sea-Surface-Elevation Spectrum
Since the release of version 19.5, PEDP reports beside each spectral estimate Sh(f) when processing Task 4/1 spectra.
is defined as:

where z* is the depth of DOBIE below mean water level, is the mean water depth and k is the wavenumber corresponding to f, the frequency of the spectral estimate.