viscosity - measurement of a fluid's resistance to flow. The common metric unit of absolute viscosity is the poise, which is defined as the force in dynes required to move a surface one square centimeter in area past a parallel surface at a speed of one centimeter per second, with the surfaces separated by a fluid film one centimeter thick. For convenience, the centipoise (cp) - one one-hundredth of a poise - is the unit customarily used. Laboratory measurements of viscosity normally use the force of gravity to produce flow through a capillary tube (viscometer) at a controlled temperature. This measurement is called kinematic viscosity. The unit of kinematic viscosity is the stoke, expressed in square centimeters per second. The more customary unit is the centistoke (cSt) - one one-hundredth of a stoke. Kinematic viscosity can be related to absolute viscosity by the equation:
cSt = cp ÷ fluid density.
In addition to kinematic viscosity, there are other methods for determining viscosity, including Saybolt Universal viscosity, Saybolt Furol viscosity, Engler viscosity, and Redwood viscosity. Since viscosity varies inversely with temperature, its value is meaningless unless the temperature at which it is determined is reported. See viscosity (asphalt), viscosity index, viscosity-temperature relationship, viscosity selection.