Viscosity Measurement Methods
There are two most commonly used methods for testing and rating viscosity of oil:
Dynamic Viscosity is closely related to the “thickness” of an oil, and does not depend on the density of the sample. It is commonly expressed using the unit kg·m−1·s−1, or milliPascal seconds (mPa*s), also called centiPoise (cP). Water at 68°F (20°C) has a viscosity of about 1.0 cP. A dynamic viscosity test uses a controlled, measured sliding or rotating force to shear the liquid under test.
Kinematic Viscosity test results are affected by the density of the oil, and often use the acceleration of gravity or vibration as forces to shear the oil under test. The unit of measurement is millimeters2/second, also called centiStoke (cSt). Water has a density of 1 gram per cubic centimeter, and at 68°F (20°C) it has a viscosity of about 1.0 cSt.
The two different methods of testing and rating viscosity are examining the same viscosity characteristic of the liquid, but use a different applied force to move and shear the liquid, and differ only by the density of the liquid.
Translation between the two measurement systems can easily be done by the following operations.
Dynamic Viscosity (cP) / Density (g/mL) = Kinematic Viscosity (cSt) Kinematic Viscosity (cSt) * Density (g/mL) = Dynamic Viscosity (cP)
One way to conceptualize the difference is to consider that the most common way to test kinematic viscosity has been the use of a timed orifice method, such as the Saybolt apparatus or Zahn cup. In the commonly used Zahn cup test, a cup with a calibrated hole in the bottom is filled to a reference line. The kinematic viscosity is determined by the time it takes for a known quantity of the liquid to run out of the hole. If a liquid of a similar measured dynamic viscosity but greater density is tested in the apparatus, it will show a lower kinematic viscosity, because its greater mass per unit volume pushes the liquid through the orifice at a greater rate.
