③ SUS or SSU (Saybolt Universal Seconds Viscosity) The measured temperatures are 20℃, 50℃, and 100℃ and mostly used in Europe.
It is measured by dividing the flow time of sample oil of 200cc by the ratio of the flow time of water at 20℃.
The measured temperatures are 40℃ and 100℃ according to the ISO (International Standard Organization) Viscosity Classification, which are used universally. S and one hundredth of it is expressed in centistroke (cSt). Strokes refer to the kinematic viscosity expressed in C. Generally the viscosity of lubricants means the kinematic viscosity expressed in cSt and the viscosity expressed in units other than cSt is as follows: But synthetic base oils are relatively very resistant to external factors because of the same molecular size and structure (high binding energy), resulting in slowing the viscosity change. In case of mineral base oils, the molecular size and structure change by external factors over the period of use, leading to a viscosity change in the end. Meanwhile synthetic base oils have the same molecular structure and size. The size and structure of mineral-base oils are so diverse that viscosity is determined by the average molecular size. Kinematic viscosity is a relative indicator of the optimal degree of fluidity which indicates how well the substance flows, so the smaller the value is the better the fluidity is.Īnd the viscosity of lube base oils is defined by the molecular size as shown in Figure below and the bigger the molecular size is, the higher the viscosity is. In other words, the fluid with high viscosity requires much more forces.Ībsolute viscosity gives the absolute size of the degree of stickiness that resists the direction of the substance’s motion in the state of a flowing fluid. As shown in Figure below, if the upper slab is moved to the right side when a fluid is filled between two slabs, the magnitude of force needed to move the slab differs depending on the type of fluid. Viscosity is defined as a fluid’s internal resistance to flow. Viscosity is the most important property governing the functions of a fluid and a measure of “sticky property” that represents the fluidity.