Absolute oil viscosity versus Kinematic oil viscosity
What is motor oil viscosity?
Viscosity is usually defined as a fluid’s resistance to flow. But there’s more to it than that; there’s absolute or “dynamic” viscosity and there’s kinematic viscosity
Absolute viscosity is different than kinematic viscosity
Kinematic viscosity measures how a motor oil flows under
gravity. You may be familiar with the method used to measure a paint’s viscosity. That method uses a Zahn cup. The cup is dipped into a container of oil and lifted and timed until the flow stops.
However, for testing motor oil kinematic viscosity, the testing lab uses a capillary tube viscometer. The oil sample is placed into a glass capillary U-tube and the sample is drawn through the tube using suction until it reaches the start position indicated on the tube’s side. Then the suction is then released and the sample flows back through the tube under gravity. The narrow capillary section of the tube controls the oil’s flow rate; more viscous grades of oil take longer to flow than thinner grades of oil. This procedure is described in ASTM D445 and ISO 3104.
Absolute or dynamic viscosity
To measure absolute viscosity, the fluid is stirred and the amount of force required to stir the fluid is measured. As you might guess, it takes more force to stir a 75W gear oil than a standard automatic transmission fluid. That’s increase in force is due to the high internal friction of the gear oil compared to the much lower internal friction of the transmission fluid.
So absolute/dynamic friction measurements are used to determine a motor oil’s cold cranking ability and it’s pumpability. In other words, its “W” rating.
Posted on by Rick Muscoplat
©, 2020 Rick Muscoplat