![]() The VI is a number without a unit that characterizes the viscosity vs. In addition to determining the kinematic viscosity, the other viscosity-related property of lubricating oil is the viscosity index (VI). These instruments are designed to allow for much faster analysis times, while providing similar accuracy to the manual method outlined in ASTM D445. To provide adequate throughput, these labs usually use automatic viscometers, based on ASTM D445. Many of the larger commercial labs typically test 500 or more oil samples per day. This is of particular importance considering the method requires at least 10 minutes of heating in the oil bath, and at least five minutes to flow through the capillary tube. While ASTM D445 is an accurate, reliable method of determining the viscosity of an oil, many commercial oil analysis labs modify this method to allow for faster sample analysis. The time taken for the oil to flow under gravity back through the narrow capillary section is used to determine its kinematic viscosity (Figure 1). ASTM D445 requires that the oil sample be drawn in a capillary viscometer tube, which is immersed in an oil-heating bath at the prescribed test temperature. Most commercial oil analysis labs use a variation of ASTM D445 to determine the kinematic viscosity of used oil samples at 40☌, 100☌ or both. The specification comes from a combination of design parameters such as bearing loads, machining tolerances and the required rate of heat removal.īecause of its importance, viscosity is one of the fundamental properties that should always be measured in any used oil program. Designers of machinery specify a viscosity or “viscosity grade” which must be used in their equipment. Among other factors, it determines the dynamic load that may be supported, what the lubrication film thickness will be, and what the flow rate will be under a given pressure. ![]() ![]() The viscosity of a lubricating oil is perhaps its most important physical property.
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