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Sequoia Scientific

LISST-AOBS Super-Turbidity Sensor

The LISST-AOBS Super-Turbidity sensor is a patent pending, simple, low-cost sensor to measure suspended sediment concentration (SSC).

Super-Turbidity is a patent pending technology developed by Sequoia Scientific, Inc. It involves pairing a LISST-ABS with a turbidity sensor using a weight factor, which results in a single, combined output from the two sensors. Once paired, the LISST-AOBS retains near-constant calibration for SSC over a wide grain-size range. The LISST-AOBS Super-Turbidity sensor is supplied by Sequoia as an integrated and paired turbidity and acoustic sensor with a variety of cabling and data logger options.

Sequoia expects to start taking orders for the LISST-AOBS Super-Turbidity sensor in late Q3, for delivery towards the end of Q4, 2019.

Features

  • Paired acoustic and optical technologies
  • Near-constant calibration within a factor of two for grain-sizes from 1–500 µm
  • Complete, integrated package pairing a LISST-ABS and a Turner Designs Turbidity PlusTM
  • Includes Y-cable providing power and integrated SDI-12 communication to and from both sensors
  • Tolerant to biofouling (LISST-ABS); integrated wiper (Turbidity PlusTM)

Specifications

Parameters measured

  • Suspended Sediment Concentration (SSC; mg/l)
  • NTU

Concentration range

  • 1 – 30,000 mg/l (LISST-ABS) or
  • 0 – 3,000 NTU (Turbidity PlusTM)

Technology

  • Combined optical and 8MHz acoustic backscatter technology
  • Optics per ISO 7027 Turbidity Technique
  • Mechanical wiper for optical backscatter
  • SDI-12 output
  • Sample volume (acoustic): 10mmØ x 15mmL @ 55mm from transducer

Mechanical and electrical

  • 5.72 cm (2.25″) H x 10.16 cm (4″) W x 33.65 cm (13.25”) L
  • Weight 0.7 kg (1.54 lb) in air
  • Transducer: 8 mmØ ceramic
  • Wavelength: 850 nm
  • Power supply: 9-15 VDC (12VDC nominal); 200 mA
  • Depth rating: 100 m
  • Material: ABS plastic and Delrin

Software & Downloads

  • LISST-AOBS data sheet
  • Accessories

    2 m communication and power cable for lab testing

    10, 20, 30 and 50 meter communication and external power cables with USB (for PC communication) and adapter for SDI-12 connection to generic dataloggers. The cable also includes adapters for RS232 and analog connections for using the LISST-ABS alone with a datalogger.

    X2 non-submersible datalogger (weatherproof, externally powered)
    Cable for use with the X2 datalogger. Includes cable and connectors for mating to LISST-AOBS and X2.

     

     

     

    X2-SDL submersible datalogger (60 m depth rating, internal alkaline batteries). Includes cable for LISST-AOBS connection.

     

     

     

    Submersible float kit for LISST-AOBS and X2-SDL submersible datalogger. For subsurface moorings.

    FAQ

    Why is there a problem with turbidity?

    The sensitivity (Volts/[mg/L] ) of optical turbidity sensors varies inversely with grain size. Turbidity sensors are highly sensitive to fine particles, and insensitive to large particles.

    For a constant concentration, a turbidity sensor will show a large response to fine particles (e.g. 5 µm), and – for the same concentration – a much smaller response to larger particles, e.g. 75 µm. To calibrate a turbidity sensor to suspended sediment concentration one must therefore know the grain-size, or assume that it doesn’t vary.

    Want more detail? Read this paper: Bundgaard K, Lumborg U, Nyborg L, Jakobsen BH (2019): Challenges of optical backscatter monitoring in mixed sediment environments. Terra et Aqua 154: 22-28, which you can download here (external link) – pay particular attention to Figure 2 in the paper.

    How does the LISST-AOBS solve the problem with turbidity?

    The sensitivity (Volts/[mg/L] ) of optical turbidity sensors varies inversely with grain size. Turbidity sensors are highly sensitive to fine particles, and insensitive to large particles.

    The LISST-ABS has a nearly constant sensitivity (Volts/Concentration) for particles larger than about 30 µm, out to about 500 µm. Below about 30 µm, LISST-ABS response is also size dependent: relatively insensitive to fine particles, and sensitive to large particles – the opposite of optical turbidity.

    The combination of acoustics and optics, when done properly, compensates these opposite tendencies of change with grain size, and produces a system that is substantially (but not totally) independent of grain size variations in this range.

    What are the benefits of the LISST-AOBS?

    Because of the near-constant calibration for the LISST-AOBS Super-Turbidity sensor, the measurements of suspended sediment concentration are far more accurate, over a wide grain size range, than either turbidity (optical backscatter) or LISST-ABS (acoustic backscatter) individually.

    Due to size dependence, individual calibration of the turbidity and LISST-ABS sensors is normally required when operating alone. Because of the fixed calibration achieved by the combination of ‘paired’ LISST-ABS and turbidity sensors, the need for calibration of either sensor is greatly reduced. This is a major convenience to experimentalists!

    Can I combine past data that I already have?

    YES!

    If you have concurrent, historical LISST-ABS and turbidity data you can combine them into one. A single patent pending calibration method finds a critical weight factor that then is used to combine past data. The step involves obtaining one measurement of sensitivity of the optical turbidity sensor at a given grain size (30 µm). A weight factor is found as the ratio of LISST-ABS output and turbidity sensor output. This weight factor then allows the weighted sum of the two instrument outputs, thus correcting past data also.

    How does it work?

    From the calibration of the turbidity, the response (Volts/[g/L]) is measured for 30 µm particles. A weight factor is computed using this value. The two outputs – LISST-ABS and turbidity – are combined with this weight factor applied to the turbidity data, resulting in the Super-Turbidity data. The figure below shows the sensitivity of the optical turbidity and LISST-ABS, working alone, and after weighted addition.

    AOBS response

    The COMBINED response (black) from the LISST-AOBS as a function of grain-size, compared with the response from OBS (red) and LISST-ABS (Blue).

    How well does it work?

    The figure below shows the sensitivity of a LISST-ABS and a turbidity sensor for a wide range of narrow grain sizes (in legend) and powders from PTI. Combining the two produces the LISST-AOBS Super-Turbidity Meter output. It is evidently that the combined results are substantially improved over either technology. If not combined, the individual technologies produce errors of a factor of ~100. When combined, the errors are within a factor of ~2.

    ABS and OBS responses to changing concentrations and grain-sizes

    AOBS vs concentration

    LISST-AOBS vs actual sediment concentration for a range of sediment grain-sizes.

    Is the LISST-AOBS a method or an instrument?

    Both!

    Sequoia has applied for a patent on the methodology and method whereby one can ‘pair’ concurrent data from a LISST-ABS and any turbidity sensor. This will give the user the combined output, the AOBS data.

    Sequoia also offers an instrumentation package, called the LISST-AOBS. This package is made up of a standard LISST-ABS and a Turner Designs Turbidity Plus(TM) turbidity sensor. The instrumentation package is delivered as a turnkey system with the two sensors fully paired by Sequoia, and includes a bracket and Y-cable for powering and combining the signal from the two sensors into a unified SDI-12 output. Nexsens data loggers are available as accessories from Sequoia.

    Does Sequoia offer to pair other turbidity sensors with the LISST-ABS?

    No.

    Users can purchase the complete, paired LISST-AOBS instrumentation package, consisting of a LISST-ABS integrated with a Turner Designs TurbidityPlus(TM) sensor from Sequoia. This is the only combined package Sequoia will offer for sale.

    Users who wish to use another turbidity sensor must provide their own cabling, wiring, data logger(s), and do the pairing between their LISST-ABS and turbidity sensor themselves.

    How do I do the pairing?

    You don’t need to do the pairing if you buy the complete instrumentation package from Sequoia.

    If you have your own OBS you want to pair with a LISST-ABS, Sequoia offers for sale a vial of particles designed for pairing, including written instructions.