Viscosity is the one of the most important properties of an oil. Therefore, viscosity determination is a critical component of an effective analysis program. Viscosity is a measurement of the lubricant's resistance to flow.
Viscosity is typically measured at either 40 or 100 degrees Celsius. Engine lubricants are usually measured at 100 degrees while non engine lubricants are measure at 40 degrees. For some lubricants, a viscosity index is determined. The viscosity index is calculated from the viscosity results at 40 and 100 degrees. The viscosity index describes how a lubricant's viscosity changes with temperature.
Monitoring viscosity over time is an important component to a successful oil analysis program. Over time, as the lubricant becomes oxidized, the viscosity will start to increase. Other variables can also cause changes in viscosity so it is important to continually monitor this property.
Typical acceptance limits for viscosity are plus/minus 10-15 percent of the lube grade. If you have an ISO 68 lube grade, then this means the lube grade is 68 centistokes (cSt). If you are utilizing a 10 percent limit, then the acceptable range for an ISO 68 grade lubricant would be 61.2 - 74.8 cSt. Any result outside this range would be considered severe.
Viscosity is the single most important property for a lubricant to properly perform. If an oil is too thin or too thick, the lubrication film will be inadequate for the equipment. Ensuring you have the correct viscosity will optimize equipment reliability. Make sure you are testing for viscosity on a regular basis.