Sequoia LISST-ABS

This is a low-cost sensor designed specifically for measuring Suspended Sediment Concentration at a point. It represents an advance over turbidity sensors. See Related Articles in right column. It is suitable for use in environmental applications, as well as in industry, such as hydropower.

• The 8MHz acoustic sensor ‘sees’ all size grains, and unlike turbidity, it sees coarse grains very well. [see article on river profile.]
• The LISST-ABS calibration is far less sensitive to grain size changes than turbidity sensors, changing only ~ ± 30% over 30-400 microns
• Operates over  a >4-decade working range in concentration!
• Tolerates fouling.

The LISST-ABS is easy to integrate into existing sensor platforms.

 

Features

•  Output: Backscatter signal strength in decibels as volts or digital value  (formula provided for conversion to concentration)
•  Sample Volume Location: 5.5 cm from sensor face.
•  Operating Frequency: 8MHz
•  Tolerant to biofouling
•  Calibration over 30-400 micron sizes: flat to within ±30%; [compare with ±400% for optical turbidity sensors]
•  Calibration for fine particles below 30 microns: response follows d^1.5
•  Analog, SDI-12 and Digital (RS232) outputs available on the underwater connector

 

The LISST-ABS is a single-point monitoring sensor. It uses a new and novel technique to internally compensate for geometric beam spreading and attenuation, yielding de-attenuated backscatter signal strength. It is held pointing into flow for low drag.

Why acoustics? Why single-point, and why 8 MHz?

Acoustic sensors benefit from their much higher tolerance to fouling than optical sensors, which is an obvious advantage. Single-point use makes them suitable for monitoring wherever turbidity type sensors are in use. The choice of 8MHz frequency is made to ensure nearly flat response to particles in the size range 30-400 microns.

For example, LISST-ABS maintains calibration over a size range 30 to 400 microns within ~ ±30% from its mean value (thin black line). In contrast, sensitivity of optical turbidity sensors changes by ~ ±400% over the same size range (see graph below).

The LISST-ABS also covers a >4-decade wide concentration range, from 1mg/L up to 30g/L for 5-10 micron sediments, and similar to fine sand, see Figure below.

This wide range makes a single sensor usable in low turbidity regimes to extremely high turbidity regimes.

With the much improved grain-size tolerance, the wide working range of concentration, and the tolerance to fouling, the LISST-ABS is just a superior new technology instrument for the sedimentologist’s toolbox.

 

Specifications

Technical:

• 8 MHz
• Sample Volume: 10 dia x 15 L (mm) (located 5.5 cm in front of sensor)
• Resolution: 0.5 % of reading
• Outputs:
  • 0-4 V analog (logarithmic, 0.5 V/decade covering 100 g/l)
  • SDI-12
  • RS-232
• Range: 1 mg/L to 30 g/L (7 micron dust) or <20 g/L (200 micron sand)
• Calibration: Recommended with sediment samples

 

Mechanical and Electrical:

• Sensor Dia.: 2.00 in (5.08 cm)
• Length: 13.25 in (33.65 cm)
• Weight: 1 lb. (0.5 Kg) in air; 0.5 lb. (0.22Kg) buoyant in water.
• Transducer: 8 mm dia, ceramic
• Power supply : 9 to 18 VDC, 100 mA
• Max. Depth: 100 m (check with factory for deeper rating)
• Material: ABS Plastic
• Connector type: Impulse MCBH-8-MP-SS
• Power on LED: Green, blink on update
• Sample update rate: 1Hz [average of 1000 measurements]