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  • How Accurate is the Concentration Measurement by LISST Instruments? | Sequoia Scientific
    constant of the system This is the method used for calculating suspended sediment concentration in the Sequoia product LISST 25 It is thus possible to convert a 32 element measurement light on rings to a single relevant number concentration Here is how the various sources of error work i Random Error 1 Electronic Noise The noise on the ring detectors is 1 digital count This corresponds to a noise in NEC of 0 04 µl l in 5 cm path instruments and 0 4 µl l in 0 5 cm path instruments It is an ABSOLUTE error which doesn t change with the sediment concentration ii Random Error 2 Fluctuations in Concentrations in the Physical Setup This noise is commonly misunderstood as instrument noise It is not No suspension is ever fully homogeneous This is particularly true of suspensions with large particles Statistical variation of the number of particles in the laser beam will produce fluctuation in the recorded scattering It would appear like noise It is in fact signal The cure is to average iii Bias Error 1 Bad Background A bad background file bubbles or particles in the clean water produces a bias offset in the concentrations This is potentially a serious error in measurements in low concentration suspensions because the background signal is higher than it should be Note that windows that have become dirty with age do not necessarily cause this error because that is a fixed offset value for NEC It is subtracted out in data processing iv Bias Error 2 Bias due to Gradients in Sediment Mixing Chamber Whenever particle settling velocities are of the same order or larger than the turbulent velocity in the mixing chamber a significant vertical gradient develops As an example in Sequoia s Full Path Mixing Chamber particles of 100 µm settling velocity of 8 mm s would develop such a gradient but fine particles say 10 µm settling velocity of 0 1 mm s would not This error can easily reach 50 Even particles flowing in a tube will be similarly stratified unless turbulence in the tube is high enough v Bias Error 3 Imperfection in Knowledge About the Particles Errors can arise due to shape effects For example if the particles are irregular and the inversion is done with a model for equivalent spheres Even when analyzing data that matches the inversion model spheres for spheres and irregular particles with irregular particle inversion small bias errors can arise These are difficult to quantify but research suggest they are no more than a few percent vi Bias Error 4 Error in Calibration Constant Although we try to get tight estimates of the calibration constant instruments shipped before 2011 may have errors of less than 10 percent in the calibration constant mostly due to iv Recent shift to use of fine particles 6 11 µm dust has reduced this error to about 1 vii Bias Error 5 Particles outside the measurement range Particles outside the range may leak into

    Original URL path: http://www.sequoiasci.com/article/how-accurate-is-the-concentration-measurement-by-lisst-instruments/ (2016-02-13)
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  • How to Compute the Mean Particle Diameter from a LISST Volume Distribution | Sequoia Scientific
    0 190 0 190 2 1 28 0 256 0 512 3 1 51 0 360 1 080 4 1 79 0 616 2 464 5 2 11 0 853 4 265 6 2 49 1 041 6 246 7 2 93 1 429 10 003 8 3 46 3 035 24 280 9 4 09 4 071 36 639 10 4 82 4 419 44 190 11 5 69 4 818 52 998 12 6 71 5 714 68 568 13 7 92 6 218 80 834 14 9 35 6 252 87 528 15 11 03 6 151 92 265 16 13 02 6 360 101 76 17 15 36 6 288 106 896 18 18 13 6 311 113 598 19 21 39 6 732 127 908 20 25 25 7 726 154 52 21 29 79 8 252 173 292 22 35 16 9 573 210 606 23 41 49 10 666 245 318 24 48 96 11 581 277 944 25 57 77 11 843 296 075 26 68 18 12 214 317 564 27 80 45 11 177 301 779 28 94 94 9 434 264 152 29 112 04 6 993 202 797 30 132 21 4 750 142 500 31 156 02 3 128 96 968 32 184 11 2 300 73 600 VD VC dSum 180 751 3719 339 By definition the mean particle size in terms of size class number is now dSum VD dSum VC in this example 3719 339 180 751 20 57714 We can now see that the mean particle size in terms of size class numbers is somewhere between size class 20 and 21 i e somewhere between 25 25 and 29 79 µm Now round down dSum VC to the nearest integer in this case

    Original URL path: http://www.sequoiasci.com/article/how-to-compute-the-mean-particle-diameter-from-a-lisst-volume-distribution-2/ (2016-02-13)
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  • Sensitivity of Response of LISST-ABS and OBS to Size Distribution Changes | Sequoia Scientific
    so that the curves are separated The width of the log normal distribution is indicated in the legends box as ΔΦ The mean size is indicated as a Φ value on each curve Note a small ΔΦ is a narrow distribution and vice versa The amplitudes of these curves adjust to equalize total concentration area under the curve to unity Using the weighted sum method to calculate the acoustic response described on the Application Note Understanding LISST ABS we show below a contrast in how calibrations would vary for the two technologies as sediment mean size and size spread changes In the figure below with a linear axis for sizes which highlights the large grain sizes the ABS calibration is seen to be very stable for all mean sizes and size spreads In contrast the OBS response shows a strong decrease in sensitivity with increasing mean grain size and for all size spreads So this shows that for large particles 32 microns the ABS is definitely superior in holding calibration In contrast for small mean sizes i e Φ 5 or diameter 32 microns the calibration sensitivity to sizes becomes clearer in the next figure Now the size axis is logarithmic highlighting calibration for particles smaller than 32 microns Φ 5 It is seen that for particles smaller than Φ 5 both technologies show a significant variation in calibration For the smallest particles however i e Φ 6 calibration change in OBS is smaller than ABS but of course this is at the cost of severely reduced sensitivity to large grains To summarize for particles with mean diameter larger than about 32 microns and irrespective of the width of size distribution the LISST ABS calibration is not affected significantly with a change in mean grain size or size spread Only

    Original URL path: http://www.sequoiasci.com/article/abs-and-obs-calibration-stability-for-broad-particle-size-suspensions/ (2016-02-13)
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  • How The LISST-ABS Works | Sequoia Scientific
    of backscatter signal strength with sediment concentration remains nearly constant over a wide particle size range The LISST ABS calibration changes only about 30 over its mean value over size 40 500 microns This is one of the key advantages of LISST ABS over turbidity sensors whose calibration changes 600 over its mean over this same size range The explanation of this key difference between LISST ABS and 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 The LISST ABS is capable of measuring high sediment concentrations due to a key second feature the backscatter strength is measured from two adjacent range cells 5 cm apart i e the first range cell is at 5 cm from the instrument and the second range cell is at 10 cm The backscatter signal from the second range cell is weaker due to two reasons the spreading of the acoustic beam reduces signal strength and second suspended sediment causes attenuation of sound The LISST ABS instrument firmware corrects for both factors thus providing an Attenuation Corrected Backscatter ACB The attenuation correction feature permits use of LISST ABS to very high concentrations The LISST ABS is designed to work with an 11 18 VDC external power source and data storage device 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 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 Further reading

    Original URL path: http://www.sequoiasci.com/article/how-the-lisst-abs-works/ (2016-02-13)
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  • Understanding LISST-ABS | Sequoia Scientific
    phases f the last terms just add with random signs and cancel out leaving I 1 2 a 1 2 a 2 2 or if the two amplitudes are equal to a I a 2 5 In other words in the case of incoherent scattering the total intensity is the sum of individual intensities Recall the coherent case gave us 2 times higher intensity Eq 3 Strength of Acoustic Scattering Signal Because the scattering from particles adds incoherently the mean square pressure intensity from individual particles add per Eq 5 Write this as P 2 a 1 2 a 2 2 a 3 2 a 4 2 If all particles are of the same size and there are N of them P 2 N a 2 Single Size Particle Scattering The pressure not intensity scattered by a single particle is given by p f m Dρ 1 2 where we use lower case p for pressure f is a form factor analogous to scattering efficiency in optics D is grain diameter ρ is grain density and m for mass of a single particle so that for N number of particles the intensity which is the sum from Eq 5 becomes P 2 N f 2 m Dρ and combining N m into mass concentration M we have the simple result P 2 f 2 M Dρ 6 Single Size Particles If particles of a single size only are present from Eq 6 the root mean square rms pressure P is P f M Dρ 1 2 7 This implies that the pressure signal will increase as square root of concentration In a way this is wonderful because it compresses the dynamic range of concentrations An increase in concentration by a factor of 10 6 will increase pressure by only 10 3 Particle Size Distributions If particles of different sizes are present then incoherent scattering means that intensities from all different sizes add P 2 i f i 2 M i D i ρ 8 This is weird If we define a weight factor w i w i f i 2 D i ρ then Eq 8 becomes P 2 i w i M i 9 and P rms i w i M i 1 2 10 In other words when multiple size particles are present the rms signal that our transducer sees is the square root of the weighted sum of Eq 9 This gives us a way to think about the relative importance of different sizes represented by the weight factors w The weight factors of Eq 10 are shown below Fig 1 Weight factors w i for different size particles an example PSD red and the contribution of all sizes to the total scattering black The total scattering will be the square root of area under the black curve The Really Weird Part Non linear addition from multiple size classes Think of two experiments In one particles of single size are used The signal strength will follow Eq

    Original URL path: http://www.sequoiasci.com/article/understanding-lisst-abs/ (2016-02-13)
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  • LISST-ABS for Protecting Hydro Turbines from Abrasion | Sequoia Scientific
    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

    Original URL path: http://www.sequoiasci.com/article/lisst-abs-for-protecting-hydro-turbines-from-abrasion/ (2016-02-13)
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  • LISST-ABS vs Turbidity in the Field | Sequoia Scientific
    Elwha a free flowing river again For a time lapse of the dam removal click here Fig 1 The Elwha dam before removal the sediment plume that followed removal and the monitoring station LISST ABS is 2 nd from left click on images to see larger The LISST ABS single point acoustic backscatter sediment sensor was installed at the USGS gage below the site of the Elwha dam photo below The sensor was programed to deliver acoustic estimates of sediment concentration at 15 minute intervals Simultaneously 2 optical turbidity meters and an ISCO water sampler provided independent estimates of suspended sediment concentration Fig 2 Intercomparison of LISST ABS Blk and two turbidity sensors Also shown are physical samples and the river stage Blu click to enlarge Preliminary Data courtesy of USGS The data show general agreement between the 3 sensors Only the LISST ABS and Analite sensors could cover the full dynamic range of sediment concentration the DTS 12 saturated part way Some mismatch in the early part of the time series between all sensors and physical samples is seen in the first of two large peaks Following the first main peak all sensors show good agreement Some of the disagreement between acoustic and optical backscatter data may well be attributed to changes in particle size distributions PSD We show 2 example PSD s The PSD on left R24 unlabeled is from the last sample at end of first small peak and the second is the physical sample used as calibration point B24 unlabeled Fig 3 The PSD for the R24 data point left and B24 right The changes in PSD shown in Fig 3 affect the calibrations of OBS and ABS significantly and are the likely the cause of disparity in Figure 2 In reality the calibration for both

    Original URL path: http://www.sequoiasci.com/article/lisst-abs-and-turbidity-inter-comparison/ (2016-02-13)
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  • Comparing Optical Turbidity With LISST-ABS | Sequoia Scientific
    0 2 micron in size scatter light following geometric optics as against Mie regime Here ka 1 In contrast the LISST ABS operates partly in the Rayleigh zone ka 1 but from 30 400 microns it is in the transition zone where ka 1 In the zone the scattering strength is relatively independent of grain size see here scroll down This flat response region makes the LISST ABS a far superior sensor although not as good as our LISST 100X or other laser diffraction devices The figure below illustrates this This figure is a composite of actual field data from the Cowlitz River in Washington State USA and one panel that is based on calculation For field data see here The first panel is the optical transmission profile left axis is depth in m This is like turbidity It sees no structure that the LISST SL revealed The second panel shows the vertical profile of suspended sediment concentration measured by LISST SL in mass concentration mg L and verified with P 61 samples Obviously turbidity first panel sees none of the structure of this second panel Now using the size distribution profile from LISST SL data we computed what the LISST ABS response would be So this is a theoretical estimate This is seen in the third panel The LISST ABS sees the structure quite well Does the LISST ABS have constant calibration for all size distributions Not quite The fourth and last panel shows the ratio of concentration to the ABS output a calibration There is variability though contained in a small zone This is the key improvement of suspended sediment measurements with the LISST ABS As time progresses we will be looking forward to a field comparison Tell us what you find More in Library Library Articles Technical

    Original URL path: http://www.sequoiasci.com/article/comparing-optical-turbidity-with-lisst-abs/ (2016-02-13)
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