TECHNOLOGY: EPISODE 10 LASER DIFFRACTION SYSTEM VS HYDROMETER FOR GRAIN SIZE ANALYSIS

 LASER DIFFRACTION SYSTEM Vs HYDROMETER FOR GRAIN SIZE ANALYSIS

This Article was aimed to validate the usage of the LDS method as a reliable replacement of the hydrometer method for grain size analysis of fine-grained geo materials. The findings of this study indicated that the LDS method could be a standalone technique for particle size analysis of fine geo materials and can reproduce the hydrometer results with appreciable precision.

Using the Microtrac S3500 LDS instrument with samples prepared at a concentration of 1.4 g/l in a distilled water solution, a default refractive index value of 1.75, and without the addition of a dispersing agent or sonication, the results were proven to be satisfactory.

Based on the liquid limit of the samples, a mathematical model was provided to select the distribution method (either volume or number) to yield the closest results in comparison to the hydrometer method. Based on the provided equation; any sample with LL >= 180% will result in the number method. However, as a recommendation; this intermediate ranges to be verified by future studies. Nevertheless, the provided equation is valid for the liquid limit range used in the current study.

Further, the sieve analysis for geo materials is not complicated or time-consuming such as the Hydrometer. Therefore, and as an unsubstantiated opinion; the authors propose a combined sieve-LDS method with a correction of the passing percentage (i.e., the LDS method to be used only for the particles passing the ASTM sieve #200). The differences between the LDS and hydrometer are qualitatively insignificant based on the soil classification criteria (both methods yielded the same soil classification in USCS and AASHTO systems). Quantitatively, the LDS and hydrometer results regarding silt, clay and colloidal contents and fractal dimension are comparable to each other and highly correlated. The LDS method might be considered as a reasonable alternative to the Hydrometer with comparable results that are fast to obtain, continuous (especially for very fine particles), independent of particle density and specific gravity, and generated from the least possible quantity/concentration and efforts. These findings are applicable to the geo materials range considered in this study, and further evaluation is required to generalize the findings for geo materials with different properties.

LASER DIFFRACTION SYSTEM Vs HYDROMETER FOR GRAIN SIZE ANALYSIS

How Laser Diffraction Works 

 Laser diffraction is about the relationship between particle size and the angle and intensity of scattered light. Light scatters more intensely and at smaller angles off of large particles than small particles. Every analyzer, from the very first commercial prototype to the state of the art LA-960 utilizes this principle. In fact, the analyzer itself does not measure particle size -- it measures the angle and intensity of light scattered from the particles in your sample. That information is then passed to an algorithm designed to use Mie Scattering Theory which transforms the scattered light data into particle size information.

Both the hardware and software required to make a particle size measurement have gone through many revisions in order to improve accuracy, precision, reliability, and ease of use. The LA-960 represents the tenth generation of HORIBA laser diffraction analyzers; each different and better than the last. Read on to discover what important factors go into making this measurement routine and trustworthy. 

Laser Diffraction Principles

At the very heart of the laser diffraction technique is the relationship between light and surfaces (which can be freely interchanged with "particle" for our purposes). When light strikes a surface it is either 

·         Diffracted

·         Refracted

·         Reflected

·         Absorbed

Diffraction is also known as "edge diffraction" as that is where it occurs. Refraction occurs as light changes angle traveling through the particle.

We can obtain information about the size of a particle using the angle and intensity of scattered light. Diffracted and refracted light is useful for this purpose; absorbed and reflected light works against this purpose and must be taken into account during measurement and size calculation.

For particles larger than a certain size the vast majority of light is scattered by diffraction. The scattered light is at relatively high intensity and low angle for these larger particles. The "certain size" is determined as a multiple of the wavelength of light used for the measurement and typically approximated at 20 microns. Particles larger than this size communicate useful size information through diffraction and not refraction. This means the measurement will not benefit from the use of a refractive index to accurately interpret refracted light.

For particles smaller than 20 microns refracted light becomes increasingly important to calculate an accurate particle size. The scattered light is at relatively low intensity and wide angle for these smaller particles. The use of a refractive index and the Mie scattering theory directly affects accuracy in this size range. All HORIBA laser diffraction analyzers use the Mie scattering solution by default and allow the user to input custom refractive index values.