SEDIMENT PARTICLE ANALYSIS: WHICH METHOD IS "BEST"?

Geoscientists have long employed a variety of methods to describe the size distribution of sediments. Over the years newer procedures have been developed and criticisms have been directed toward some of the older methods (e.g., the hydrometer and pipette) alleging that these yield both incorrect and imprecise results. While there is little argument that laser diffraction and X-ray particle analyzers do provide greater precision, controversy still surrounds the question as to whether these, or any of the newer methods (electroresistance particle size analyzers, settling tubes, image analyzers, etc.) produce information that is more accurate. The dilemma arises from the fact that different methods use different criteria to measure “size.” A Ro-tap® sieve shaker, or a Sonic Sifter, measures the particle’s “intermediate” diameter; X-ray particle analyzers (SediGraph®) and Hydrophotometer measure the “spherical sedimentation diameter” while electroresistance particle analyzers measure the “spherical volume diameter.” Settling tubes (and the hydrometer and pipette) are Stoke’s Law based methods, which provide a measure of the particle’s “sedimentation diameter.” Advantages do exist when newer methods are used in place of the combined sieve-hydrometer or sieve-pipette techniques, but these accrue mainly in the speed with which samples are analyzed. All methods have problems and limitations associated with them (e.g., cost, calibration difficulties, maximum-minimum size restrictions, etc.) and there is yet no totally “ideal” instrument. Even for those for which precision and apparent accuracy are well documented, the small size of the sample used calls into question whether the precision and accuracy would hold for a truly representative sample of larger size. To demonstrate actual differences that do result using different methods, samples from a number of different environments were analyzed by five methods (Settling Tube-SediGraph, Laser Diffraction, Sonic Sifter, Sieve-Hydrometer, Sieve-Pipette, and Sieve-Precision Pipette (Andreasen Pipette). Different results were obtained with the different methods but the bottom line is simply that if the researcher is consistent with the methodology used, then any of the methods will provide information useable for most geological applications.