These are based upon study of the terminal velocity acquired by particles suspended in a viscous liquid. Sedimentation time is longest for the finest particles, so this technique is useful for sizes below 10 μm, but sub-micrometer particles cannot be reliably measured due to the effects of Brownian motion. Typical apparatus disperses the sample in liquid, then measures the density of the column at timed intervals. Other techniques determine the optical density of successive layers using visible light or x-rays.
Advantages: this technique determines particle size as a function of settling velocity.
Disadvantages: Sample must be dispersed in a liquid medium... some particles may (partially or fully) dissolve in the medium altering the size distribution, requiring careful selection of the dispersion media.
Density is highly dependent upon fluid temperature remaining constant.
X-Rays will not count carbon (organic) particles.
Many of these instruments can require a bulk sample (e.g. two to five grams).