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Laser diffraction particle size analysis

Methodological aspects, limitations, examples

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Laser diffraction instrument

Retsch Technology, Product catalogue

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Zimmermann, 1996

Intensity vs. angle

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Scattering pattern of different particles

ISO/FDIS 13320-1, 1999

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Beckman Coulter

Measurement of Sub-mircon particles

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Light – particle interaction

Retsch Technology, Product catalogue

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Optical Models

• Fraunhofer diffraction• Particles are spheres• Particles are much

larger than the light wavelength (approx. >10 µm)

• Diffraction angle is small (only near forward scattering is considered)

• Mie-Theory• Particles are spheres• Refraction and

absorption are included

• A complex refraction-index (RI) is necessary

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ISO/FDIS 13320-1, 1999

Differences between Fraunhofer-and Mie-scattering

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ISO/FDIS 13320-1, 1999

Complex Refractive Index (RI)

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ISO/FDIS 13320-1, 1999

Influence of RI and grain size

1µm RI=1.62 - 0i- 0.01i- 0.1i- 1i

0.5 µm RI=1.62 - 0i- 0.1i

5 µm RI=1.5 - 0i- 0.1i

LÜ-Sedimentologie Laser Particle Sizer 11Beckman Coulter

Polarization Intensity Differential Scattering (PIDS)

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LS 13320

Range:0.04 – 2000 µmSample demand:Max. ~ 60 mg (Silt)Max. ~ 10g (Coarse sand)Analysis time:~ 5-7 min (full cycle)

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LS 13320 – Sample chamber

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Grain mixtures

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Starck, unveröff.

Comparison of methods

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Summary

Advantages• Rapid measurement• Large grain size range• Very good resolution• Statistical parameters for

characterisation of grain size distribution

Disadvantages• Restricted comparability

with other methods• Complex refraction

indices partly uncertain or unknown

• Matrix inversion can have more than one solution

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Praktische Durchführung

• Jede Gruppe analysiert ihre Proben

• je eine Probe der Siebanalyse

• Fraktionen der Korntrennung

• Probe der Aräometeranalyse einmal für alle Gruppen