The two ways of measuring and reporting viscosity, kinematic and absolute (also known as dynamic), often cause confusion in the minds of those who work with them on a less-than-regular basis. In this column, I'll explain the differences between them, and give a few tips on how to apply them to lubricating fluids.
Absolute Viscosity
Absolute viscosity is defined as a fluid's resistance to shear, or the fluid's resistance to deform when subjected to a force. Put more simply, the thicker the fluid, the more energy that is needed to get it to flow. Kinematic viscosity, strictly speaking, is defined as the ratio of absolute viscosity to density.
Density is a mass-derived property, and because mass and weight are, for practical purposes, proportional anywhere on the surface of Earth, kinematic viscosity is often interpreted as the fluid's resistance to flow under forces of gravity.
I like to think of kinematic viscosity as a specialized case of absolute viscosity. The shear forces introduced by gravity are actually very small compared to the shear forces introduced by the mechanical interaction of machine components.
Let's illustrate this difference with an example. Suppose you have a jar of honey and a jar of water sitting on the table. The jars are fixed to the table, so they can't move. If you put a spoon into each jar and begin stirring, you are introducing shear forces into the fluid.
Note that these forces are not created by gravity, so what you are doing is an absolute viscosity test. And obviously the fluid with the greater resistance to the stirring is the honey, so we can conclude that the absolute viscosity of honey is greater than that of water. Now take those jars, unfix them from the table, and tip them on their sides.
The fluids are both going to flow out of the jars, and in this case, the forces causing the flow are introduced by gravity. So we have just performed a kinematic viscosity test, and have shown too that the kinematic viscosity of honey is greater than that of water, because it has a greater resistance to flow out of the jar.
The viscosity of a fluid, either kinematic or absolute, varies according to the temperature at which it is measured. Thus it is imperative to report the temperature at which the viscosity is measured. The units of measurement are reported in Table 1.
Table 1. Units of Measurement of Viscosity
Kinematic Viscosity
As the strict definition of kinematic states, absolute viscosity and kinematic viscosity are directly convertible if the density of the fluid is known. This relationship may be expressed as:
Absolute Viscosity = Kinematic Viscosity x Density
The appropriate SI units must be used to employ this formula correctly.
So far, we've shown that the absolute and kinematic viscosity tests both prove that honey is more viscous than water. Let's take a look at another example.
Using the same two jars fixed to the table, fill one with honey and the other with mayonnaise. Now do the absolute viscosity test by stirring the fluids. The test will show that honey is the more viscous fluid.
Perform the kinematic viscosity test by turning the jars on their sides, and the test will show that the mayonnaise is now the more viscous fluid (the honey flows out faster than the mayonnaise). What is firstly the explanation of the different results, and secondly the significance, at least as far as machine lubricants are concerned?
Differences
To explain the different results we need to know about the Newtonian properties of a fluid. If one relates the viscosity of a fluid to the amount of shear it is experiencing, some fluids show a viscosity which is independent of the amount of shear forced being applied.
These are called Newtonian fluids, of which honey is a good example. Some fluids have a viscosity profile which varies according to the amount of shear being experienced. These are called non-Newtonian fluids, and mayonnaise is the example of this.
When the shear rates are low (the kinematic viscosity test) the non-Newtonian fluid demonstrates a high viscosity. When the fluid is being sheared more energetically, as with the absolute viscosity test, the viscosity of the non-Newtonian fluid decreases (Figure 1).
Why It's Important
So what is the significance of this to lubricating fluids?
1. Most lubricating oils (see exceptions below) demonstrate near-Newtonian properties. So whether we measure and trend the kinematic or absolute viscosity makes little difference.
2. The oils which demonstrate significantly more non-Newtonian properties are:
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Improved lubricants (oils which have viscosity index-improver additives)
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Oil emulsions, which include moderate contamination by solids and/or liquids and air entrainment, all of which can create emulsions.
3. Given that the mechanically introduced shear force, not gravity, influences the flow of lubricating fluid in the machine, it is fair to say that the absolute viscosity test is the better means of determining the viscosity. However, it is also fair to assume that whatever is going to influence a change in the absolute viscosity is also likely going to influence a change in the kinematic viscosity.
As long as we are measuring and trending one measurement method (with suitable reproducibility), we should be able to get good data patterns. Even if we are doing kinematic viscosity measurements, and we know intuitively that they are not correct as far as the machine is concerned, they are still trendable. So stick with one method. To put a tongue-in-cheek spin on this: it is better to be consistently wrong than occasionally right.
4. Kinematic viscosity is the commonly used method of measuring and reporting viscosity in used-lubricant analysis, at least as far as most commercial labs are concerned. As explained in the previous paragraph, it is probably not the best method, but for historical and ease-of-use reasons it has become the dominant method.
5. Most commercial laboratories will use an autoviscometer to measure kinematic viscosity. Most on-site laboratory instruments will measure absolute viscosity, yet report it as kinematic viscosity by using an assumption of the density of the fluid and performing the appropriate calculation.
This is generally not a problem getting trendable results, but it is important to make sure that the viscosity determinations are always made at the same temperature. This can be room temperature, but always make sure that the oils have time to reach room temperature in a climate-controlled environment before the test is performed.
And don't try to compare the results of your commercial oil lab and your on-site lab results too minutely; they are going to be different, but that's because they are measuring different things. There should be some correlation, but they are not going to equate exactly.
Viscosity is generally regarded as the most important property of a lubricant. So being able to measure it and understand is important too. I hope this has made the understanding of the subject a little clearer.
References
1. NASA. 2007.
2. Wikipedia. 2007. http://en.wikipedia.org/wiki/Viscosity. 7/29/07.
