When washing sand in a Classification Tank, the particles separate based on size, predominantly (assuming they all have the same specific gravity or relative density). A range of forces are present on all of the particles. Hence, a distribution over the valves (the only position that can measured) occurs.
This figure illustrates a typical distribution of size ranges, over each of the valve stations. Please note the “sieve sizes” are the individual curves, the vertical axis is % passing (of the particular sieve size) and the horizontal axis is the actual Valve Stations.