phi Unit
phi Unit : A logarithmic unit used to measure grain sizes for sand, grit, and gravel. The 0 point of the scale is a grain size of 1 millimeter, and each increase of 1 in the phi number corresponds to a decrease in grain size by a factor of 1/2. Thus 1 phi unit is a grain size of 0.5 mm, 2 phi units is 0.25 mm, and so on; in the other direction, -1 phi unit corresponds to a grain size of 2 mm and -2 phi units to a size of 4 mm
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