The temperature must be defined to interpret the viscosity reading. Likewise, reporting viscosity for trending purposes without a reference to temperature is nonsensical. Viscosity is not a dimensional measurement, so calling highly viscous oil thick and less viscous oil thin is misleading. Sometimes, viscosity is erroneously referred to as thickness (or weight). Generally speaking, viscosity is a fluid’s resistance to flow (shear stress) at a given temperature. Given the importance of viscosity analysis coupled with the increasing popularity of onsite oil analysis instruments used to screen and supplement offsite laboratory oil analysis, it is essential that oil analysts clearly understand the difference between absolute and kinematic viscosity measurements. By contrast, most onsite viscometers measure absolute viscosity, but are typically programmed to estimate and report kinematic viscosity, so that the viscosity measurements reported reflect kinematic numbers reported by most labs and lube oil suppliers. Most used oil analysis laboratories measure and report kinematic viscosity. The two are easily confused, but are significantly different. Viscosity can be measured and reported as dynamic (absolute) viscosity or as kinematic viscosity. However, there is more to viscosity than meets the eye. Likewise, there is no property more critical to effective component lubrication than base oil viscosity. Of all the tests employed for used oil analysis, none provides better test repeatability or consistency than viscosity.
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