Understanding the concept of kinematic viscosity or KV is essential in making the right lubrication decisions. A lubricant’s kinematic viscosity gives insight into its suitability for use for your machines and equipment. Understanding the different measures of viscosity also helps in understanding viscosity testing and oil analysis.
What is Kinematic Viscosity (KV)?
Kinematic viscosity is a fluid’s internal resistance to flow under gravitational forces. You can determine an oil’s KV using a standard method that involves a measuring tool called a viscometer.
At a specific temperature, a fixed volume of oil is poured into the viscometer and allowed to flow by force of gravity. The time it takes to reach a certain level in the viscometer is measured and used to calculate the KV. The unit of measure for kinematic viscosity is square millimeters per second or centistoke (cSt).
The ISO (International Organization of Standardization) 3448 is the standard method of determination of kinematic viscosity. In this standard, the basis of measuring and comparing different KV values is at the temperature of 40℃ or 104°F.
Kinematic Viscosity vs. Dynamic (Absolute) Viscosity
A concept related to KV is DV or dynamic viscosity. You may know dynamic viscosity as absolute viscosity. Unlike kinematic viscosity, DV does not take gravitational force into consideration. DV is the internal resistance of a fluid to external and controlled forces.
To measure DV, the common standard used is the ASTM (American Society for Testing and Materials) D2983, D6080, etc. This standard method uses the Brookfield method which employs a rotating probe that measures the resistance of an oil to the force that is created as the probe rotates. The unit of measure for DV is centipoise (cP).
In general, DV is related to KV with this equation:
DV = KV x SG
Where:
- DV is the dynamic viscosity of a fluid in centipoise, cP.
- KV is the kinematic viscosity of a fluid in centistoke, cSt.
- SG is the specific gravity or the ratio of the fluid’s density to that of water at 4℃.
Understanding Viscosity in Lubrication
The relationship between dynamic viscosity and kinematic viscosity depends on the type of fluid. The relationship in the equation above only applies to Newtonian fluids or fluids that have constant viscosity and follow the laws of fluid dynamics initially stated by Sir Isaac Newton. Fluids that act otherwise are non-Newtonian.
Most oils are non-Newtonian fluids when used as lubricants because of the following factors:
Presence of Additives
Lubricants contain additives to improve their stability against changing temperatures. These additives also help prevent oxidation which can reduce lubricant performance over time. These additives can cause the DV and KV relationship to deviate from the above equation.
Contamination
When water, soot, and other contaminants enter machines, they react and form emulsion and suspension with the lubricating oil. Emulsions and suspensions are non-Newtonian fluids that have undefined KV and DV relationships.
Formation of Byproducts
Over a lubricant’s lifetime, exposure to heat and pressure can cause the formation of byproducts that accumulate over time. These thermal and oxidation byproducts also form suspensions with the lubricating oil and cause it to behave like a non-Newtonian fluid.
Change in SG
The specific gravity of lubricating oil changes with use over time. As lubricants are used, some components are lost through evaporation, and some are added through contamination. The relationship between DV and KV is thus affected by the changes that happen in the oil’s density.
What is the implication of all these factors, you ask? If lubricants do not behave as Newtonian fluids, their kinematic viscosities are less accurate to determine by measuring their DV. For example, the DV and KV values you measure would be different from those predicted by the Newtonian relationship of DV and KV.
Kinematic Viscosity in Lubrication Management
Understanding how your oil analysis methods measure viscosity leads to a better understanding of its results. If your tests use kinematic methods, you would need to account for the specific gravity of your lubricant to get the absolute or dynamic viscosity.
Also, kinematic viscosity results are highly dependent on the methods, tools or testing equipment, temperature conditions, etc. On-site and laboratory viscosity results are not likely to give the same values. Thus, trending results and historical plotting of data are the best ways to make use of viscosity measurements.
Using software like Redlist’s Lubrication Management software helps you record, track, analyze, and report on-site and laboratory kinematic viscosity readings. With computerized recording and analysis, you get timely and accurate results. From this, you get instant viscosity trending for fast and results-based decision-making. Want to learn more about how Redlist can support your lubrication management program? Schedule a free demo today!
