ADCP, DVL & Marine Sensor FAQ
ADCP FAQ
Frequency, Range, Data Quality & Power
What's the difference between Direct-Reading (DR) and Self-Contained (SC) Configuration? Can a SC ADCP send data in real time?
DR models connect by cable to a deck unit or platform for live data. SC models are battery-powered for autonomous mooring deployments, and you normally recover the instrument to download its data. Oceantek SC models also carry onboard storage and can read out in real time when wired — the difference is deployment style, not capability.
Which ADCP frequency should I choose for my site?
Higher frequency means higher accuracy but shorter range; lower frequency means longer range with coarser resolution. As a guide: 600 kHz gives ±0.3% ±3 mm/s accuracy over roughly 50–70 m; 300 kHz gives ±0.5% ±5 mm/s over roughly 100–160 m; 75 kHz reaches deep-ocean ranges to about 650 m. Choose the frequency that covers your deepest measurement, then use cell size to balance resolution against range.
Will very clear water affect measurements? Is murkier water better?
ADCPs measure velocity from echoes off particles moving with the water. In very clear water there are too few scatterers, so range shortens and data quality drops. Extremely sediment-laden water attenuates the beam before it reaches full range. Between the two — enough particles to scatter, not so much they absorb — is where ADCPs perform best.
Do strong currents or muddy water affect accuracy? What about bubbles?
Strong currents and suspended sediment do not affect accuracy — the particles simply move with the flow. Bubbles are the real problem: an air-water interface is a strong, moving reflector, so aeration from waves, propellers or cavitation can contaminate the profile near the transducer.
How can I tell whether my data is good? Doesn't a strong echo mean good data?
Echo strength shows energy, not signal quality. Check two things: the echo level against the noise floor, and the correlation — valid velocity data sits near a correlation of 1, while noise sits near zero. In practice, a coherent profile that follows the flow pattern is good data; scattered, jumpy readings usually mean interference or hardware issues.
Is upward-looking more accurate than downward-looking?
No — orientation does not change accuracy. Point the instrument upward to measure surface and air-interface layers, downward for bottom profiles. Accuracy depends on frequency, cell size and the local environment (bubbles, sediment), not on which way the transducer faces.
Can I use the ADCP with my existing software or third-party software?
Yes. Oceantek instruments output standard formats (PD0 and ASCII-compatible), so common hydrographic and discharge packages can read the data directly.
What does "average power" in the datasheet mean? Alkaline or lithium batteries?
The datasheet figure is average power in continuous transmitting mode, without sleep. In sleep the instrument draws only a few milliwatts, so a duty-cycled mooring lasts far longer than the headline number suggests. Alkaline cells handle high temperature and pressure more safely but store roughly half the energy of lithium at the same size.
What precautions should be taken during ADCP installation?
Avoid installation near strong reflective surfaces. The transducer should be kept at least 0.1–0.3 m away from the riverbed, hull or pipe wall to prevent sidelobe interference. Use tripods, weights or special brackets to ensure stable mounting and reduce vibration. For vessel-mounted ADCPs, install them on a flat area of the hull, far away from propellers.
What may cause data loss in ADCP measurements?
Possible causes include:
- The ADCP stops acoustic transmission — check cables, connectors, communication and power supply of the deck unit;
- GPS input failure — inspect serial input and communication ports;
- Gyro input failure — similarly check serial input and communication ports.
DVL FAQ
What a Navigation DVL Really Does
Speed, bottom tracking, tracks — and the honest limits of what a Doppler Velocity Log can and can’t tell you.
Can a DVL map the seafloor
Bottom data contains some terrain information, but that is not what a DVL is for — bathymetry is multibeam sonar’s job. The DVL exists to give a vehicle accurate speed, position and (in 2.0) current data.
What raw data does a DVL output, and how does velocity become a track
The DVL outputs velocity in instrument, ship and earth coordinates, plus bottom height. With dead reckoning enabled it also reports north and east distance. The onboard algorithm filters and integrates velocity from the start position into displacement and track, exported by default in PD6 ASCII format.
Do soft, muddy bottoms affect DVL accuracy?
Yes. A hard bottom gives a strong, stationary echo, so bottom-track speed is accurate. Soft mud absorbs acoustic energy and can creep with the current, weakening the echo and adding bias to the measurement.
What matters most when installing a DVL?
Align the instrument’s forward axis with the platform’s travel direction, then verify alignment with a GPS speed comparison before use. Keep platform tilt within specification — up to 45°, though real installations usually run from a few degrees to about 20°. Beyond that, beams can lose bottom lock.
Single-Point Current Meter FAQ
One Clean Velocity Reading, at Depth
How the OCEAN-SPCM differs from an ADCP, what its “point” really measures, and how to install it right the first time.
What is the difference between a single-point current meter and an ADCP?
Both use the same acoustic Doppler principle, but they answer different questions. A single-point current meter (OCEAN-SPCM) reports the average velocity of one cell — typically a 1–3 m depth window — at a chosen depth, and is built for deep-water moorings (rated to 6000 m). An ADCP profiles the whole water column in layers at once.
How deep can a single-point current meter measure?
Range is set by frequency, as with any Doppler instrument — a 600 kHz unit reaches roughly 50 m from the transducer. Turbid water extends range; very clear water shortens it. In the deep ocean, mount several units at the depths you need to cover and interpolate between them.
What exactly is the "point" it measures?
Macroscopically, it refers to the flow velocity at a specific depth point (for example, a point at 5000 meters deep in the ocean); at the equipment level, the range of the ‘point’ is determined by the layer thickness. If the layer thickness is 1 meter, it represents the average flow velocity within 1 meter of that point; if the layer thickness is 2 meters, it represents the average flow velocity within 2 meters (i.e., the average flow velocity within a volume).
How should the current meter be installed? Any precautions?
Keep the four transducers free of ropes, brackets and frame members — anything blocking the beams. Hang it as close to vertical as possible; internal tilt sensors compensate, but large tilt degrades accuracy. It attaches directly to the mooring cable; no special frame is needed.
How long will the battery last?
Working draw is roughly 300–350 mW and sleep draw 1–2 mW. Endurance = battery capacity ÷ (working power × duty cycle + sleep power × duty cycle), and we recommend planning with a 1.5× safety margin.
Self-Contained Hydrophone FAQ
Passive Listening Is Simple. Clean Recordings Are Not.
Range, sensitivity, gain and platform noise — the questions that decide whether a deployment returns usable audio or unusable noise.
How does a self-contained hydrophone work
A piezoelectric ceramic converts underwater pressure changes into an electrical signal, which the unit samples and records onboard. Think of it as an underwater voice recorder that listens passively — it emits nothing and stores everything internally.
How far away can it hear?
There is no single range — it depends on the source level, frequency and the hydrophone’s sensitivity. Default sensitivity is −166 dB (re 1 V/µPa), adjustable with gain. Ship noise is typically detectable tens of kilometres away; powerful sources such as explosions can be heard 50–100 km or more. Low frequencies travel much farther than high frequencies.
Does the platform's own noise affect recordings?
Significantly. Thrusters, generators and hull flow can swamp the ambient signals you want to record. Mount the hydrophone away from noise sources, or deploy it from a quiet platform (such as a CTD frame) for ambient-noise work.
How do sensitivity and gain work together?
They add. At −166 dB sensitivity, +18 dB gain gives an effective −148 dB; −24 dB gain gives −190 dB. Use high gain for faint signals and low gain in noisy environments to keep strong sources from clipping.
Can several self-contained hydrophones be time-synchronized for arrays?
Not on current self-contained models. Synchronization would require the units to stay powered and “disciplined” before deployment, which drains the battery, and the cost is significant for the single-point use case. The supported approach is single-unit logging with RTC calibration before deployment. Synchronized multi-channel acquisition belongs to towed-array systems.