Definitions
• Kinematic Viscosity: The measure of a fluid's resistance to flow under the influence of gravity.
• Viscosity Index: A dimensionless number that indicates how much a lubricant's viscosity changes with temperature.
• Viscosity SUS: Saybolt Universal Viscosity (SUS) is a unit of measurement that quantifies a fluid's kinematic viscosity using a Saybolt viscometer.
Viscosity
The single most important property of a lubricant is its kinematic viscosity. It is the measure of the oil’s resistance to flow (shear stress) under certain conditions. At low temperatures, high viscosity may result in poor mechanical efficiency, difficulty in starting, and wear. At higher temperatures, low viscosity may result in overheating and wear.
Viscosity is generally measured at 40 degrees C (104°F) for industrial applications and 100 degrees C (212°F) for engine oil applications. For industrial lubricants it is classified using the ISO VG system. For engine oils it is classified according to SAE grade. The ASTM D445 test provides a specific number to compare to the recommended oil in service. Results which fall outside of the applicable grade specification are considered abnormal.
Kinematic Viscosity (ASTM D445)
ASTM D445 is a widely used test method that measures the kinematic viscosity of liquid petroleum products, lubricants, and other fluids at specified temperatures. The test involves:
- Sample Preparation: The liquid sample is placed in a calibrated glass capillary viscometer.
- Temperature Control: The viscometer and sample are maintained at a single precise temperature, such as 40°C (104°F) or 100°C (212°F), since viscosity changes with temperature.
- Flow Time Measurement: The time it takes for a specific volume of liquid to flow through the capillary is measured, which is directly proportional to its viscosity.
- Kinematic Viscosity Calculation: The kinematic viscosity is calculated by multiplying the measured flow time by a calibration constant specific to the viscometer used.
The result is reported in centistokes (cSt), which is commonly used for comparing fluid flow characteristics. The ASTM D445 test is critical for industries, like automotive, aerospace, and manufacturing, where precise fluid behavior under varying temperatures is essential for performance.
Viscosity Index (ASTM 2270)
ASTM D2270 is a test method used to measure the Viscosity Index (VI) of lubricating oils. The VI is an empirical number that indicates how much a lubricant’s viscosity changes with temperature. Higher VI values imply the lubricant maintains its viscosity better over a range of temperatures, which is crucial for applications where temperature variations occur, such as industrial machinery. This test involves:
- Viscosity Measurements: The oil's kinematic viscosity is measured at two specific temperatures, typically at 40°C (104°F) and 100°C (212°F).
- Calculation of VI: The VI is calculated using an empirical formula based on these two measurements, comparing the oil's change in viscosity due to temperature, to a reference scale.
This information is critical for evaluating the oil’s performance in terms of maintaining effective lubrication under varying thermal conditions.
Viscosity SUS (ASTM D2161)
Viscosity SUS is a unit of measurement that quantifies a fluid's kinematic viscosity. This measurement is determined by the time it takes for 60 milliliters of a fluid to flow through a calibrated opening in a Saybolt Viscometer at a specified temperature. The resulting time, measured in seconds, represents the fluid's viscosity in SUS.
ASTM D2161 is a test method for converting between kinematic viscosity (measured in centistokes, cSt) and Saybolt Universal Viscosity (measured in Saybolt Universal Seconds, SUS) for petroleum products. This method is useful since different industries or regions use different units to measure viscosity and this standard provides a reliable way to convert between them.
Here’s how ASTM D2161 works:
- Kinematic Viscosity to SUS: Given the kinematic viscosity of a fluid at a specific temperature, ASTM D2161 provides formulas/conversion tables to calculate the equivalent SUS value.
- SUS to Kinematic Viscosity: ASTM D2161 formulae/conversion tables can also be used to convert SUS to kinematic viscosity in centistokes.
This standard is essential for ensuring that viscosity measurements are consistent and comparable across different methods and units, especially in applications where precise viscosity is critical, such as in lubricants and hydraulic fluids.
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