Size analysis of silt and clay-sized sediment (i.e., particles <63 µm in diameter) can be accomplished by a variety of automated methods based on their electrical properties, transport properties (e.g., gravitational sedimentation, centrifugal sedimentation, hydrodynamic chromatography, and aerodynamic transport) and nonimaging optical methods (optical blockage technique, time-averaged light scattering, photon correlation spectroscopy) (for details, see Bunville ). The most widely used method in the earth sciences is based on the sedimentation principle and Stokes's Law. The physics of sediment settling in a fluid medium have already been discussed elsewhere (Chapters 1 and 2), and the concept is readily applicable to dilute suspensions of silt and clay. Before the development of the SediGraph, the most commonly used sedimentation techniques were those of the pipette and the hydrometer (Chapter 1), both of which used manual sampling and recording. Though effective and reasonably accurate, these techniques had the disadvantage of being highly labour intensive and time consuming, taking hours to days for a routine analysis.
The entry onto the market of the Sedi-Graph in the early 1970s was therefore welcome in that, while maintaining comparable accuracy, it offered an increased level of automation of the analysis, as well as a reduction in the time of analysis to minutes (Berezin & Voronin, 1981; Duncan & Rukavina, 1982). The first model sold was the 5000, which was followed by the 5000D, E, and ET model variations.