Research Paper Comparison between grain-size analyses using laser diffraction and sedimentation methods C. Di Stefano, V. Ferro*, S. Mirabile Dipartimento di Ingegneria e tecnologie Agro-Forestali, Universita ` di Palermo, Viale delle Scienze, 9018 Palerrmo, Italy article info Article history: Received 3 December 2009 Received in revised form 15 March 2010 Accepted 18 March 2010 A comparison between laser diffraction method (LDM) and the sieve-hydrometer method (SHM) was carried out for 228 soil samples representing a different texture classification sampled in a Sicilian basin. The analysis demonstrated that the sand content measured by SHM can be assumed equal to that determined by LDM technique, while the clay fraction measured by LDM was lower than that measured by the SHM. A set of equations to transform LDM results to SHM results was proposed. The influence of the LDM measure- ments of clay on the estimated percentage of silt þ very fine sand particles (particle diameter ranging from 0.002 mm to 0.1 mm), which is useful for estimating soil erodibility, was also studied. ª 2010 IAgrE. Published by Elsevier Ltd. All rights reserved. 1. Introduction Particle-size distributions (PSDs) are fundamental physical properties of soil and are typically presented as the percentage of the total dry weight of soil occupied by a given size fraction. This property is commonly used for soil classification and for the estimation of some hydraulic properties (Campbell & Shiozawa, 1992). Over recent decades, various new methods for grain-size analysis have been developed. These new methods, (electro- resistance particle counting, time of transition, laser diffrac- tion (LD), optical determination of the PSD using image analysis) (Goossens, 2008; McCave & Syvitski, 1991) generally have the advantage of covering a wide range of grain sizes, and rapidly analysing small samples. Particles of sand size (0.05e2.00 mm) are usually deter- mined using sieving. The sieve defines a particle diameter as the length of the side of a square hole through which the particle can just pass (Allen, 1990). Finer particles are usually determined by classical sedimentation methods such as hydrometer or pipette (Gee & Bauder, 1986). Sieving combined with hydrometer method (SHM) has been adopted as an international standard to determine quantitatively the PSD of soils (Allen, 1990; Cooper, Haverland, Hendricks, & Knisel, 1984). With similar pretreatment techniques, the pipette method (PM) and hydrometer method (HM) - give comparable results (Liu, Odell, Etter, & Thornburn, 1966; Walter, Hallberg, & Fenton, 1978); however the PM requires that clay and silt fractions (<0.05 mm) are separated from the sand fraction using wet sieving. Sedimentation methods are time consuming, especially for the determination of the particles having a size less than 2 mm, since they require relatively large samples (10e20 g for the pipette and 50 g for the hydrometer). They also give unreliable results for particles 1 mm because of the effect of Brownian motion on the rate of sedimentation. A particle diameter obtained by the laser diffraction method (LDM) is equivalent to that of a sphere giving the same diffraction as the particles. A laser diffraction particle size analyser “sees” the particle as a two-dimensional object and it gives its grain size as a function of the cross-sectional area of the particle. * Corresponding author. E-mail address: [email protected](V. Ferro). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/issn/15375110 biosystems engineering 106 (2010) 205 e215 1537-5110/$ e see front matter ª 2010 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2010.03.013
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Avai lab le a t www.sc iencedi rec t .com
journa l homepage : www.e lsev ie r . com/ loca te / i ssn /15375110
Research Paper
Comparison between grain-size analyses using laserdiffraction and sedimentation methods
C. Di Stefano, V. Ferro*, S. Mirabile
Dipartimento di Ingegneria e tecnologie Agro-Forestali, Universita di Palermo, Viale delle Scienze, 9018 Palerrmo, Italy
loam (group 3, 186 samples). The selected textural groups
represent homogeneous zones of the USDA triangle charac-
terised by sand content SA greater than 50% (loamy
sand þ sandy loam), quasi equal to 50% (loam) and less than
50% (silt loam, silty clay loam).
Figs. 10, 11 and 12 show, for each considered textural
group, the relationship between clay percentage determined
Fig. 12 e Relationship between sand and clay fractions
obtained by LDM and by SHM for the textural group “silt
loam D silty clay loam”.
b i o s y s t em s e n g i n e e r i n g 1 0 6 ( 2 0 1 0 ) 2 0 5e2 1 5214
by the two different methods. According to these figures, for
the clay fraction the following equation can be established:
CLSHM ¼ a CLLDM (5)
where a is a coefficient equal to 2.18 for the textural group 1
(SA> 50%), 1.91 for the textural group 2 ðSAy50%Þ and equal to
1.91 for the textural group 3 (SA < 50%).
The relationship Eq. (5) calibrated for each textural group is
characterised by RMSE ¼ 3.1 for the textural group 1,
RMSE ¼ 6.4 for the textural group 2, RMSE ¼ 10 for the textural
group 3 which is lower than or similar to that obtained by Eq.
(2) (RMSE ¼ 9.27). In other words, even if in some case (Figs. 11
and 12) the number of samples used is small, for transforming
clay measurements from the LDM to the SHM a more reliable
estimate of clay percentage can be obtained using a coefficient
specific to each textural group.
As a consequence, the improved estimate of clay
percentage by Eq. (5) allows a more accurate estimate of silt
percentage by use of the following equation:
SIE ¼ 100� aCLLDM � SALDM (6)
and of the percentage of siltþ very fine sand particles fE by the
following equation:
fE ¼ 100� aCLLDM � g (7)
The calibration equations (5) and (6) by particle size
(SA > 50% and SA � 50%) presented in this paper are not
universal. As stated earlier, correlations of the LDM with the
SHM may vary for a variety of reasons related to laser
diffraction analyser used, particle shape, mineralogy, RI, etc.
Poor correlations between LDM results and SHM results may
occur if the calibration equations (5) and (6) are applied
outside the range tested here.
4. Conclusions
Taking into account the fact that LDM provides more infor-
mation and is more efficient than the SHM although the latter
is an accepted and certified method, this paper has tried to
solve the question of how similar are the results from the two
methods.
This study was developed using 228 soils sampled in Sicily,
having a variety of texture and the Fritsch A22-Economy
version laser analyser. Goossens (2008) obtained similar
results using different types of laser analyser, but the results
obtained in this paper have to be considered as being appa-
ratus specific because measurement accuracy for the LDM is
dependent on the number of detection cells.
This study showed that there was no significant difference
in the particle size distribution using different ultrasound
durations. The absence of the H2O2 pretreatment gave a small
underestimation in the clay fraction while the silt fraction can
be assumed not affected by the pretreatment. The choice of
the Fraunhofer or Mie diffraction models for the LDM gave no
appreciable differences for the soils investigated.
Analysis of all samples showed that the sand content
determined by SHM was similar to that obtained by the LDM
while the so-called “overestimation” of the clay percentage of
SHM with respect to LDM was confirmed.
Finally, a set of equations useful to refer LDM measure-
ments to the SHM results, the latter is used as an international
standard, was proposed.
The analysis demonstrated that for improving the trans-
formation of the claymeasurements from the LDM to the SHM
three textural groups (sand content greater than 50% (loamy
sand þ sandy loam), almost equal to 50% (loam) and less than
50% (silt loam, silty clay loam)) have to be distinguished. For
each textural group a more reliable estimate of clay
percentage can be obtained using a specific coefficient.
Even if for transforming the clay measurements from the
LDM to the SHM a more reliable estimate of clay percentage
can be obtained using a coefficient specific for each textural
group, the calibration equations (5) and (6) by particle size
(SA > 50% and SA � 50%) presented in this paper are not
b i o s y s t em s e ng i n e e r i n g 1 0 6 ( 2 0 1 0 ) 2 0 5e2 1 5 215
universal. Further measurements carried out contemporane-
ously by the LDM and the SHM could allow the results
obtained in this investigation to be confirmed and the
proposed scale equations to be improved.
Acknowledgements
The research was set up by Prof. V. Ferro and Dr. C. Di Stefano,
the measurements were carried out by Dr. S. Mirabile. All
authors analysed the results and contributed to the writing of
the paper.
The research was supported by a grant of Ministero del-
l’Istruzione, dell’Universita e della Ricerca scientifica, Governo
Italiano, PRIN 2007 “Monitoraggio e modellazione dei processi
erosivi a differenti scale spaziali nell’area sperimentale di
Sparacia” and Progetto FEROS.
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