Contamination and oxidation, among other things, can cause a rise in the density of used oil. The measure of the absolute viscosity is correct, but because the density of the oil is only estimated, the conversion to kinematic viscosity can become overstated. First, most onsite test equipment actually measures absolute viscosity, but calculates the kinematic viscosity by dividing absolute viscosity by density. Because the onsite viscometers don’t actually measure density but rather estimate it from the oil’s spec sheets, an error can occur when translating absolute viscosity into kinematic viscosity. There are two main reasons why your onsite measurements may not correlate directly with the lab data. The viscosity reported by your lube supplier and oil analysis lab is likely the kinematic viscosity, as determined by ASTM D445. To visualize absolute viscosity, imagine the force needed to stir an oil using a metal rod. Absolute viscosity, on the other hand, determines an oil’s internal resistance to flow and shear. Kinematic viscosity measures the resistance of an oil to flow and shear under gravity, such as oil flowing through a funnel.

When it comes to viscosity, there are two determinable parameters, absolute and kinematic viscosity.

However, like all onsite equipment, it’s important to understand how these instruments work and their relative strengths and weaknesses. Because the viscosity of an oil is probably its single most important property, it makes sense to measure viscosity frequently, using onsite test equipment.

Testing viscosity of oil