particle size

(geology) The general dimensions of the particles or mineral grains in a rock or sediment based on the premise that the particles are spheres; commonly measured by sieving, by calculating setting velocities, or by determining areas of microscopic images.
(metallurgy) The average and controlling lineal dimension of an individual particle of metal powder as determined by suitable screens or other methods of analysis.

geology While the range of sizes of soil, rock, and similar particles varies through a continuum, it is convenient to classify the sizes, and hence the particles, using common names in a defined way. Various schemes have been used, all broadly geometric to focus on relative size. The finest particles are clay (a term misleadingly associated in vernacular usage with the clumps formed by the highly adhesive fine particles). A clay is usually defined as particles less than 2 μm in diameter. Sand forms the central band of sizes, from various minima around 50 μm invariably to a maximum of 2 mm, qualified progressively from very fine up to very coarse. Silt is between clay and sand, while coarser than sand comes a variety of terms: gravel, pebbles, cobbles, then ultimately boulders. The phi scale, in contrast, avoids names, merely uses numbers to represent size, increasing values for decreasing size; the size is the negative of the logarithm to the base 2 of the size in millimetres, written with the Greek letter following. Thus 2φ represents a size up to 2^-2 × 1 mm = 250 μm, and -2φ represents a size up to 2² × 1 mm = 4 mm.

Table 38, organized in decreasing size, shows limiting size for each label used in the various schemes; for instance, the label ‘cobble’ is limited to 200 mm in the British scheme but 256 mm (-8φ) in the Uddan-Wentworth and US Department of Agriculture schemes, and effectively unlimited in the International scheme, where it is the largest labelled particle. Within any one scheme, the label applies to all sizes down to the figure for the next label below, e.g. the cobble of U-W must exceed 64 mm else it would be a pebble, while in the International and British schemes it would remain a pebble if >60 mm. The left-hand column gives the US sieve numbers for gradations of sand within the USDA scheme. (Note that the U-W scheme is based on the SI but employs binary subdivisions, to produce a true geometric scale; the International scheme, endeavouring to be decimal, uses the square root of 10 as its ostensible multiplier step, but compromises the geometric pattern by using alternate multipliers of ³/₁ and ¹⁰/₃ to achieve an overall decimal progression.)

1. In evaluating the efficiency of a filter for removing particles from an air stream, the minimum particle diameter in microns that will be removed by the filter.
2. In paints, the diameter of a pigment or latex particle; usually expressed in mils or microns.