Second, if your onsite instrument does not heat the oil to 40 degrees C, and most do not, you are likely determining the oil’s viscosity at the temperature of the onsite lab (typically in the 20 to 25 degrees C range), and extrapolating, again using a software algorithm to determine the viscosity at 40 degrees C. 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. Contamination and oxidation, among other things, can cause a rise in the density of used oil. 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.įirst, most onsite test equipment actually measures absolute viscosity, but calculates the kinematic viscosity by dividing absolute viscosity by density. The viscosity reported by your lube supplier and oil analysis lab is likely the kinematic viscosity. 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. What are we doing wrong?”īecause the viscosity of an oil is probably its single most important property, it makes sense to measure viscosity frequently, using onsite test equipment. However, our viscosity numbers are often up to 10 percent different from the lube suppliers. At the same time, our lube supplier is testing samples from the same system regularly. “We are using an onsite viscometer to measure viscosity at 40 degrees C.
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