Viscosity – The Physics Hypertextbook
The more usual form of this relationship, called Newton's equation, states that the The quantity defined above is sometimes called dynamic viscosity, absolute Kinematic viscosity is a measure of the resistive flow of a fluid under the. Dynamic, absolute and kinematic viscosities - convert between CentiStokes (cSt), centipoises (cP), Equation (1) is known as the Newtons Law of Friction. The two most common types of viscosity are dynamic and kinematic. The relationship between these two properties is quite straightforward.
Toothpaste is another example of a material whose viscosity decreases under stress. Toothpaste behaves like a solid while it sits at rest inside the tube. It will not flow out spontaneously when the cap is removed, but it will flow out when you put the squeeze on it. Now it ceases to behave like a solid and starts to act like a thick liquid. You don't have to worry about it flowing off the brush as you raise it to your mouth. Shear-thinning fluids can be classified into one of three general groups.
A material that has a viscosity that decreases under shear stress but stays constant over time is said to be pseudoplastic. A material that has a viscosity that decreases under shear stress and then continues to decrease with time is said to be thixotropic.
If the transition from high viscosity nearly semisolid to low viscosity essentially liquid takes place only after the shear stress exceeds some minimum value, the material is said to be a bingham plastic.
Materials that thicken when worked or agitated are called shear-thickening fluids. An example that is often shown in science classrooms is a paste made of cornstarch and water mixed in the correct proportions.
The resulting bizarre goo behaves like a liquid when squeezed slowly and an elastic solid when squeezed rapidly. Ambitious science demonstrators have filled tanks with the stuff and then run across it. These suspensions create non-Newtonian behavior. If one were to measure the absolute viscosity of one of these commonly encountered emulsions or colloids described above with a variable shear rate absolute viscometer for example, ASTM Dthe measurement would decrease as the shear rate increases, up to a point of stabilization.
If one were to divide this stabilized absolute viscosity by the specific gravity of the fluid to estimate the kinematic viscosity, the calculated value would differ from the measured kinematic viscosity.
Again, the equations in Figure 3 apply to Newtonian fluids only, not non-Newtonian fluids described above, which is why this discrepancy occurs. Specific Gravity Effects Look at the equations in Figure 3 again. Consider the apparatus in Figure 1, the bulb that contains the sample oil, which is released when the vacuum is eliminated, then produces a head of pressure that drives the oil through the capillary tube.
Can one assume that all fluids will produce the same head of pressure? Most hydrocarbon-based lubricating oils typically have a specific gravity of 0. However, this can change over time as the oil degrades or becomes contaminated glycol, water and wear metals for examplewhich produces a differential between absolute and kinematic viscosity measurements.
Consider the data presented in Table 2. Each of the new oil scenarios is identical, and in both instances the absolute viscosity increases by 10 percent, usually the condemning limit for a change in viscosity.
In scenario A, the modest change in specific gravity results in a slight differential between measured absolute viscosity and kinematic viscosity. However, in scenario B, the differential is much greater. Here, the specific gravity increases significantly, which results in a measured increase of 1.
This is a significant difference that could lead the analyst to identify the situation as nonreportable. The error that has been made is the assumption in both scenarios that the fluids remain Newtonian. Due to the many possibilities of forming non-Newtonian fluids, the true parameter of interest for the oil analyst and lube tech should be absolute viscosity.
Kinematic Viscosity Explained
In the interest of economy, simplicity and the fact that new lubricant test procedures are commonly borrowed for used oil analysis, the kinematic viscosity of the oil is typically the measured parameter used for trending and making lube management decisions. However, in certain cases this may be introducing needless errors in determining the viscosity of an oil. The problem can be reduced to simple mathematics. The amount of error is a function of the amount of change in the unmeasured parameter, the specific gravity.
Important Conclusions One can draw the following conclusions from this discussion about viscosity measurement: Most of these devices measure absolute viscosity cP and apply an algorithm to estimate the kinematic viscosity cStoften holding the specific gravity constant. Consider trending the results from the onsite viscometer in cP. It is the parameter being measured, and it helps to differentiate the onsite trend from the trend of data produced by the laboratory with a kinematic viscometer.
It is futile and generates little value.
Difference Between Kinematic and Dynamic Viscosity
At best, look for loose correlation. Always baseline the new oil with the same viscometer you are using with the in-service oil. That is one of the reasons that emulsified water increases the rate of wear in components such as rolling element bearings, where fluid film strength is critical of course, water also causes other wear mechanisms like vaporous cavitation, rust and hydrogen embrittlement and blistering.
Viscosity is a critical fluid property, and viscosity monitoring is essential to oil analysis. Dynamic and kinematic viscosity measurement techniques can produce very different results when testing used oils.
Gaseswaterand many common liquids can be considered Newtonian in ordinary conditions and contexts. There are many non-Newtonian fluids that significantly deviate from that law in some way or other. Shear-thickening liquids, whose viscosity increases with the rate of shear strain. Shear-thinning liquids, whose viscosity decreases with the rate of shear strain.
Thixotropic liquids, that become less viscous over time when shaken, agitated, or otherwise stressed. Rheopectic dilatant liquids, that become more viscous over time when shaken, agitated, or otherwise stressed. Bingham plastics that behave as a solid at low stresses but flow as a viscous fluid at high stresses. Shear-thinning liquids are very commonly, but misleadingly, described as thixotropic.
For gases and other compressible fluidsit depends on temperature and varies very slowly with pressure.
The viscosity of some fluids may depend on other factors. A magnetorheological fluidfor example, becomes thicker when subjected to a magnetic fieldpossibly to the point of behaving like a solid. In solids[ edit ] The viscous forces that arise during fluid flow must not be confused with the elastic forces that arise in a solid in response to shear, compression or extension stresses.
While in the latter the stress is proportional to the amount of shear deformation, in a fluid it is proportional to the rate of deformation over time.