12/20/2023 0 Comments Astm d445![]() ![]() A typical monograde engine oil such as an SAE 30 will have a VI of about 95 while a multigrade 15W40 oil will have a VI of about 135. Oils with a low VI will thin out more rapidly than oils with a higher VI as temperature increases. The VI of an oil is a unitless number that gives a measure of how quickly the viscosity will change with temperature. This introduces the concept of a viscosity index or VI. Viscosity decreases with increasing temperature.ĭifferent oils thin out at different rates as the temperature increases. As temperature increases, the viscosity decreases as shown in the simplified graph below:įigure - Viscosity is highly temperature dependant. It is vitally important that the temperature at which viscosity is measured be noted, as viscosity changes with temperature. The preferred temperatures at which these measurements are made are 40✬ and 100✬. Kinematic viscosity can also be defined as the resistance to flow under the force of gravity. The kinematic viscosity is the dynamic viscosity of the fluid divided by its density at a specified temperature. Kinematic viscosity is measure using the SI unit of Stokes or specifically centiStokes (cSt) or millimeters squared per second (mm 2/s) for lubricating oils. Based on the factor of the tube (the tube factor is determined by calibrating the viscometer tube with a primary viscosity standard) and the time it takes the oil to flow between the two sensors, the viscosity can be calculated. A short distance further on is another sensor that stops the timer as the oil passes it. As the oil passes the sensor a timer is started. The oil flows down the tube under the force of gravity until it reaches an electronic sensor at the bottom of the tube. A series of viscometer tubes are immersed in the bath. The viscometer bath contains silicone oil that is kept at constant temperature (to within one twentieth of a degree). ![]() The Kinematic viscosity of an oil is determined by testing an oil sample in a kinematic viscometer at a set temperature. ![]() Since so many events can affect viscosity, care must be taken not to evaluate the viscosity as a singular value, but to consider other test results that might provide clues as to why a viscosity did or didn’t change (such as Acid Number (AN), fuel dilution, oxidation, etc.). Viscosity is one of very few tests that indicates a problem when it is too low or too high. Viscosity gives us indications of oxidation and other thickening processes from oil degradation, fuel contamination in liquid-fueled engines, addition of incorrect lube and much more. By multiplying the time measured by the tube factor a viscosity in centistoke (cSt) is calculated. The oil is started flowing and the time for the oil to pass by two regions on the tube is measured. Viscosity is performed by adding a preset volume of oil to a calibrated viscosity tube which is immersed in a temperature controlled bath (typically either 40☌ for industrial oils and other fluids, or 100☌ for engine oils). Viscosity is the most important single property of a lubricant. MLT1 - Machinery Lubrication Technician.Professional Memberships & Associations.> Membrane Patch Colorimetry - ASTM D7843.Power Industry (Steam, Gas, Aero-Derivative Turbines). ![]()
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