Updated: 4 March 2016.
INTRODUCTION: Sequoia’s Acoustic Backscatter Sensor, LISST-ABS is designed to measure suspended sediment concentration at a point. Unlike ADCP’s which may record similar backscatter strength at multiple points (‘ranges’) along acoustic beams, the LISST-ABS only reports backscatter signal strength at a single point about 5 cm from the transducer. The instrument presents the strength of this backscatter to the underwater connector as (i) analog voltage, (ii) RS-232 digital data, and (iii) SDI-12. The instrument does not include a battery or data logger.
WHY 8MHz: The LISST-ABS employs a high frequency of 8MHz. This choice is made so that the calibration of backscatter signal strength with sediment concentration remains nearly constant over a wide particle size range, ~40-500 microns. Over this size range, LISST-ABS calibration changes only about ~±30%. In contrast, over this same size range, optical turbidity sensors would change calibration by ~ ±600% over its mean value. This is one of the key advantages of LISST-ABS over turbidity sensors, . The explanation of this key difference between LISST-ABS and optical turbidity sensors is described further in this Application Note. All LISST-ABS instruments are factory calibrated with equal sensitivity so that they may be interchanged.
WHY Short Range: The LISST-ABS measures backscatter from a sample volume that is approx. 5cm from the transducer. This has two advantages. First, the short path in water means that water attenuation is insignificant (click here for data on water attenuation vs frequency); and further, changes in attenuation with changes in water temperature are even less significant. This leaves calibration unaffected by water temperature. The second advantage is that sediment attenuation over the short path is also small. The small attenuation means that measurements can be made at high concentrations.
HIGH CONCENTRATION: Attenuation due to sediment is measured internally by the instrument. This permits a correction to the backscattering signal strength. The correction permits operation at higher sediment concentrations.
DATA Formats: The LISST-ABS outputs data in 3 formats. In the serial (RS-232) as well as in SDI-12 formats, the output is in uncalibrated concentration, presented as mg/L. In the third, analog voltage format, the analog voltage is scaled to the attenuation-corrected-backscatter (ACB), i.e. the analog voltage is logarithmic. The conversion to calibrated units is described in the User’s Manual.
[This updates an earlier version which only referred to conversion of ACB to concentration.]
To get an in-depth understanding, click here.
To see how the Response of LISST-ABS changes with grain size distribution, click here.