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Sequoia Scientific

LISST-ABS

Submersible Acoustic Backscatter Sediment Sensor

Sequoia Scientific, Inc. is proud to offer the first low-cost acoustic backscatter sensor designed specifically for measuring Suspended Sediment Concentration at a point. This is a new type of sediment sensor, superior to turbidity or optical backscatter sensors (OBS) for 4 reasons. See Related Articles in right column.

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

The LISST-ABS is easy to integrate into existing sensor platforms. Click here for simple version of how it works, or here to get a detailed explanation. For turbine abrasion protection, click here.

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.

Figure2_of_AppNote001

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.

The LISST-ABS is now shipping!

Contact Sequoia or our International Distributors for your country for detailed price information.

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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]

LISST-ABS Mechanical Dimensions

 

Accessories

  • 10, 20, 30 and 50 meter cables
  • FAQ

    Why a new sediment sensor?

    The LISST-ABS is superior to optical turbidity type sediment sensors for two reasons. First, the -LISST-ABS sees large grains well, whereas turbidity is least sensitive to them. Second, the calibration of the LISST-ABS is less sensitive to changes in grain size distribution than optical turbidity sensors. While compared to ADCP type sediment sensors, it holds calibration nearly constant over 30-450 microns, whereas the ADCP’s generally work in Rayleigh regime, and are more susceptible to calibration change. The LISST-ABS is designed for sediment monitoring, with particular attention to stability of acoustic power and receiving electronics. LISST-ABS instruments are shipped with identical calibrations, so that they are interchangeable.

    How is LISST-ABS better than optical turbidity sensors?

    Optical turbidity sensors do not ‘see’ large grains; their sensitivity decreases as 1/diameter. LISST-ABS sees coarse grains well. See for more

    How is LISST-ABS a better sediment sensor than an ADCP?

    The LISST-ABS is designed to measure sediments, unlike ADCP. Care is taken to ensure uniformity of acoustic power, correction for attenuation due to beam-spreading, and water and sediment attenuation. No further correction is required by a user. The LISST-ABS uses 8MHz frequency which significantly flattens its sensitivity to a wide range of sizes in the Mie-geometric scattering regime. Unlike ADCP’s, the LISST-ABS does not change transmitted acoustic power, thus further retaining stable calibration.

    Why is LISST-ABS less susceptible to bio-fouling?

    The acoustic vibration of the sensor face discourages biological growth. Also, a small amount of biological growth on the sensor does not affect performance significantly.

    What range of particle sizes does the LISST-ABS measure?

    There is no size cut-off on either the small or large size end. However, the LISST-ABS measures particles in the 30-400 micron range far better than turbidity or ADCP sensors. The plot below shows the response of the LISST-ABS as a function of grain size.

     

    RelativeResponse

    Is acoustic sensitivity, like optical turbidity, grain size dependent?

    Yes. Details are offered here.. In the Rayleigh region (particles smaller than acoustic wavelength), sensitivity changes with size, but in an opposite way from optics. Whereas optical turbidity sensors lose sensitivity for increasing grain size, acoustic sensitivity increases, till flattening out at around 30 microns for our chosen 8MHz frequency. For more

    What range of sediment concentration does the LISST-ABS measure?

    Roughly from 1mg/L to 30g/L, depending on particle size.

    For broad sediment size distributions, how do OBS and LISST-ABS compare?

    This question is answered in an application note. 8MHz Acoustics of the LISST-ABS produce a superior sensor by way of response to broad size distributions. For more…

    Why is the measurement volume at a short and fixed range?

    By choosing a short range, attenuation becomes less important, and also, very high concentrations can be measured, e.g. ~30g/L for 5-10 micron particles.

    Can I plug the LISST-ABS into my CTD?

    The LISST-ABS requires 12-volts to operate and outputs attenuation corrected backscatter via analog, RS232, and/or SDI-12. Input power and all three outputs are supplied via a single 8 pin connector. The three different signal outputs and single connector make the LISST-ABS compatible with most systems.

    Are individual LISST-ABSs interchangeable, with equal calibrations?

    Yes, their response is equalized at factory for interchangeability.

    What calibration process must I use in order to measure mass concentrations?

    The calibration requires measuring LISST-ABS output for one fixed sediment concentration. We provide a procedure to automatically convert the measurement to calibration factors.

    How is LISST-ABS data converted to sediment concentration?

    The ABS digital outputs are in Uncalibrated Concentration (UC) units. To convert UC to calibrated concentration, multiply UC by a coefficient determined by measurement of a known concentration. The LISST-ABS software provides a simple procedure for measuring the coefficient.

    What if I forget to calibrate my LISST-ABS before use?

    If you save the raw data sent from the LISST-ABS, you can always apply a corrected calibration later.

    Can LISST-ABS measure attenuation in water?

    The LISST-ABS measures sediment attenuation but only at high sediment concentration values. It is not recommended as an attenuation measuring device at sediment concentrations below several g/L.

    How does attenuation coefficient of particles vary with grain size?

    There is a simple formula published by Thorne and Hanes (Cont. Shelf Res. v 22, pp 603-632; 2002).

    What causes fluctuations in output of LISST-ABS

    There are two factors that contribute to a noise-like fluctuation in the output of the LISST-ABS. The first is Rayleigh statistics of scattering due to the random positions of particles in the acoustic sample volume, and the second is real fluctuations in sediment concentration. The first cannot be avoided, it can be managed by adequate averaging; the second is part of the measurement, and should be treated as signal, not noise.

    The LISST-ABS manages Rayleigh noise by averaging a large number of pings (1,000) each second. If particles move about a wavelength between pings, then successive signal strengths become statistically independent, and noise is reduced by the square root of number of pings. If they move less, the noise reduction is less also as successive samples are not statistically independent. Since the sound wavelength at 8MHz is ~0.2mm, full benefit of a 1,000 pings happens at velocities about 20cm/s or higher.

    As for turbulent fluctuations of sediment concentration, these are, of course, signal. Such fluctuations occur in nature almost everywhere – from laboratory mixing chambers to rivers and seas. So, don’t be distressed to see fluctuations in your 1Hz data.

    How does particle scattering coefficient vary with grain size for LISST-ABS?

    The scattering coefficient can be calculate by a formula provided by Thorne and Hanes2002, see citation above.

    Is the ABS measurement degraded by unknown attenuation?

    No. The instrument measures the in-situ attenuation and applies a correction to produce the ACB.

    Does the near-field effect affect calibration?

    The backscattering signal strength from two range-bins differs by the attenuation between them – geometric and water absorption. Thus, near-field effects, which are geometrical, cancel out.

    How is attenuation measured and compensated?

    Attenuation is measured from the difference in backscatter strength from two adjacent range bins. This measured attenuation is the sum of contributions from beam-spread, water absorption, and sediment scattering. The first two factors remain dominant at low sediment concentrations. Their combined attenuation is measured in low sediment concentration tests, and is stored. When attenuation measured in field data exceeds this value, that is attributed to sediment attenuation. The stored Attenuation Corrected Backscatter (ACB) is de-attenuated for both these effects.

    Why is LISST-ABS called a LISST?

    LISST was originally an acronym for our laser particle sizing sensors. LISST now is just a Sequoia trade name. So, we carried it into acoustics.

    What is ACB? How is it measured?

    ACB stands for attenuation corrected backscatter. As explained above, attenuation is estimated in situ for each sample. The raw acoustic signals are then compensated for this attenuation, producing ACB.

    What is the size of sample volume?

    The sensor volume is approx. 4 cm long x 5mm in diameter.

    Which sound velocity is used? Fresh or saltwater?

    We use sound velocity in salt water. People ask these questions because they are thinking of long-range ADCP where changes in sound velocity affect the location of cells. For example, a 1% change in sound velocity would displace a measurement volume that was earlier at 100m to 101 m. Since our measurement volume is so close to the sensor, the change in location is proportionally small, and negligible. It is also not the dominant cause of fluctuations in signal strength (which is due to variable numbers of particles in the sample volume at different pings; the pings are 1msec apart, and 1000 pings are averaged.)