Relative density
A pycnometer (from Greek: πυκνός (puknos) meaning "dense"), also called pyknometer or specific gravity bottle, is a device used to determine the density of a liquid. A pycnometer is usually made of glass, with a close-fitting ground glass stopper with a capillary tube through it, so that air bubbles may escape from the apparatus. This device enables a liquid's density to be measured accurately by reference to an appropriate working fluid, such as water or mercury, using an analytical balance.
Settling
where "w" is the settling velocity, "ρ" is density (the subscripts "p" and "f" indicate particle and fluid respectively), "g" is the acceleration due to gravity, "r" is the radius of the particle and "μ" is the dynamic viscosity of the fluid.