The published AQUAscat ABS concentration range is approximately 1 mg/L to 30 g/L. This is not a single fixed range for every sediment type.

The maximum measurable concentration depends on:

• – Particle diameter
• – Particle density
• – Mineral composition
• – Particle shape
• – Particle-size distribution
• – The amount of acoustic attenuation produced by the suspension

Current manufacturer information gives representative ranges of:

– Approximately 1–30,000 mg/L for 7 µm material; and
– Less than approximately 20,000 mg/L for 200 µm sand.

The sensor has no absolute lower or upper particle-size cut-off. However, its acoustic frequency is optimised to provide a comparatively consistent response to particles approximately 30–400 µm in size.

Below approximately 30 µm, acoustic response becomes more strongly dependent on particle diameter. Very fine clay or silt applications should therefore be assessed carefully, particularly where particle size changes substantially during deployment.

The digital output from the AQUAscat ABS is expressed in uncalibrated concentration units. To convert this output into suspended sediment concentration, the recorded value is multiplied by a calibration coefficient.

The coefficient can be established by:

1. Placing the sensor in a homogeneous suspension of known sediment concentration;
2. Collecting paired AQUAscat ABS and laboratory concentration data;
3. Calculating the relationship between sensor output and measured concentration; and
4. Applying the resulting coefficient within the monitoring or processing workflow.

The sensor is factory equalised using reference glass beads, providing consistent response between instruments. However, a site-specific calibration using representative sediment is recommended where the objective is accurate quantitative concentration measurement.

Representative calibration material should match the field sediment as closely as possible in:

• – Particle-size distribution
• – Particle density
• – Mineral composition
• – Particle shape
• – Organic content

The AQUAscat ABS can be installed on a fixed structure for continuous or event-based monitoring. Potential mounting locations include:

• – River-bank structures
• – Bridge piers
• – Reservoir monitoring frames
• – Hydropower intakes
• – Dredging-monitoring stations
• – Coastal platforms
• – Submerged frames and moorings

The transducer must face unobstructed water, with the sample volume clear of mounting hardware, walls and other solid boundaries. Manufacturer guidance specifies a minimum distance of approximately 150 mm from solid boundaries.

The sensor should preferably be oriented into the dominant flow. This minimises hydrodynamic disturbance around the sample volume and reduces the likelihood that the mounting structure will create a sediment wake or acoustic reflection.

Installation design should also consider:

• – Expected water-level variation
• – Debris and vegetation
• – Bed movement and burial risk
• – Access for cleaning and retrieval
• – Cable protection
• – Lightning and surge protection
• – Datalogger power requirements
• – Representativeness of the selected monitoring point

The compact AQUAscat ABS can be incorporated into profiling packages, tow bodies and underwater vehicles where a single-point sediment measurement is required.

Examples include:

• – Vertical water-column profilers
• – CTD packages
• – Towed sediment-survey systems
• – Uncrewed surface vehicles
• – Remotely operated vehicles
• – Autonomous underwater vehicles

When installed on a moving platform, the mounting arrangement should maintain clear water around the sample volume and minimise bubbles, vibration and disturbance from the platform hull or propulsion system.

The sensor should remain at least approximately 150 mm from solid surfaces. Additional clearance may be required where the mounting structure generates a wake or where acoustic reflections could enter the measurement volume.

Vehicle speed, flow direction and sensor orientation should be considered when comparing mobile measurements with stationary water samples.

The AQUAscat ABS provides multiple output formats through its underwater connector, allowing integration with many environmental dataloggers, CTDs and monitoring systems.

Analog Output

The analog output is logarithmic and is suitable for dataloggers equipped with an appropriate voltage input. Current production information describes a 0–4 V output with a scale of 0.5 V per concentration decade.

RS-232

The RS-232 interface provides digital output for connection to serial dataloggers, computers and integrated monitoring systems.

SDI-12

SDI-12 is widely used in environmental monitoring stations because it allows multiple low-power sensors to communicate with one compatible datalogger.

Power and Connection

Power and output signals are provided through a single eight-pin underwater connector. The sensor operates from a nominal 12 VDC supply, with current production information specifying a 9–18 VDC supply range.

Before ordering, the following should be confirmed:

• – Required output format
• – Datalogger make and model
• – Available supply voltage
• – Cable length
• – Topside termination
• – Deployment depth
• – Whether laboratory or field connection accessories are required

Because the acoustic sample volume is positioned approximately 55 mm in front of the transducer, the AQUAscat ABS does not rely on a clean optical window through which light must pass. This makes it comparatively tolerant of surface fouling.

Fouling tolerance does not mean that the instrument is maintenance free. Marine growth, sediment deposits, debris or physical obstruction can still affect the acoustic path or interfere with the sample volume.

A suitable maintenance programme may include:

• – Visual inspection of the transducer face
• – Removal of accumulated biological growth
• – Checking that the acoustic path remains unobstructed
• – Inspection of the connector and cable
• – Comparison with water samples or reference measurements
• – Review of baseline and signal variability
• – Checking mounting alignment after high-flow events

The AQUAscat ABS output naturally fluctuates for two principal reasons:

• – Statistical variation in acoustic scattering caused by the changing positions of particles within the sample volume; and
• – Real short-term changes in sediment concentration.

Statistical variation can be reduced through suitable averaging. Genuine environmental fluctuations should not automatically be treated as instrument noise.