Sequoia manufactures two instruments based on laser diffraction for monitoring sediments entering hydro turbines. These are the LISST-Hydro and LISST-Infinite. These two produce data on sediment concentration sorted into 3 size classes – fine (<75μm), medium (75 to 200 μm), and coarse (200 to 500 μm). It is believed by some that only the coarse size class damages turbines, hence this partition. Although highly accurate, these instruments are expensive and we have seen a need for lower priced instruments for smaller power plants. The LISST-ABS fills this need, being a superior match than optical turbidity sensors. It does not sort the suspended sediments into the three size classes. It only produces a single estimate, but this estimate is a good approximation to the coarse particle content. Here’s how.
We show below the relative sensitivity to different grain sizes for the 3 technologies – laser diffraction (LISST-Infinite, LISST-Hydro). the acoustic sensor LISST-ABS, and optical turbidity sensors.
This figure shows that, first the laser diffraction instruments have equal sensitivity to all sizes. This removes the need for any calibrations. The second commonly used technology is optical turbidity. It is widely known that this measurement loses sensitivity as grain size increases. In a river situation where typically a large amount of washload (particles below ~63 microns) accompanies the larger grains, the reponse of optical turbidity sensors is primarily to washload. Coarse, damaging sediments are not visible until very high concentrations relative to the fines. In short, the nature of optical turbidity sensing is fundamentally opposite of what one desires in river monitoring.
In contrast to optical turbidity, Sequoia’s acoustic backscatter sensor LISST-ABS is designed to preferentially sense the coarser particles – exactly the relevant quantity for turbine protection. The LISST-ABS ignores harmless fine particles. Granted, the relative sensitivity of LISST-ABS is not flat (i.e. equal to one); this is the fundamental nature of acoustic scattering. Even so, the sensitivity varies by only about 30% from its mean value over 30-400 micron size.
Of particular value to a low-cost application is this nearly flat response of the LISST-ABS. It means that no calibration is necessary if one can tolerate this 30% inaccuracy. Optical turbidity sensors simply can not be used without frequent calibration. For example, over this same size range, 30-400 microns, optical turbidity output would vary by a factor of 13, or ±400%.
There are other advantages of the new LISST-ABS technology. It covers a wide concentration range (1-60,000 ppm). It is less tolerant to fouling. And, if a user wishes to improve accuracy by a calibration step, Sequoia has made recalibration extremely simple.
Please contact us with your requirement. For details on LISST-ABS, click here.
[For the more sophisticated reader, this graph shows the backscatter sound pressure per unit mass concentration, The backscatter pressure is the square-root of the sound intensity, i.e. sum of squares of pressure from each size.]