The purpose of the standard “ISO 3448 Industrial liquid lubricants - ISO viscosity classification” developed by the International Organization for Standardization (ISO) is to establish a viscosity classification system for industrial liquid lubricants and related fluids. This standard covers mineral oils used as lubricants, hydraulic fluids, electrical oils and other applications. The usual method for determining kinematic viscosity is the method specified in the standard “ISO 3104 Petroleum products - Transparent and opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity”. However, this method may give anomalous results when used with non-Newtonian liquids, i.e. liquids whose viscosity coefficient varies significantly with shear rate. Meanwhile, it is also recognized that some pure chemicals and naturally occurring products not included in this classification can be used as lubricants.
The classification defines 40 viscosity grades ranging from 2 mm2/s to 3 200 mm2/s at 20 degrees. In the case of petroleum-based fluids, this covers approximately the range between kerosene and cylinder oils.
Each viscosity grade is specified to the nearest whole number to the midpoint kinematic viscosity in millimeters per second (mm40/s) at 2 degrees, with a kinematic viscosity range of plus or minus 10 percent of this value permitted. Twenty viscosity grades with appropriate limits for each are given in the table within the standard (Table I - IS20 viscosity classification).
The classification is based on the principle that the midpoint kinematic viscosity of each grade should be approximately 50 percent greater than the previous grade. Dividing each decade into six equal logarithmic steps provides such a system and permits a smooth progression from decade to decade, but the logarithmic series is rounded to provide simple numbers. The maximum deviation of the midpoint viscosities from the logarithmic series is 2,2 percent.
The classification does not imply any quality assessment and only provides information on the kinematic viscosity at the defining temperature of 40 degrees. Kinematic viscosities at other temperatures depend on the viscosity/temperature properties of the lubricants, which are usually reported as viscosity/temperature curves or stated in terms of a viscosity index (VI).
The kinematic viscosities of the same lubricants defined by this international standard but customarily used in some countries are included in the annex to this standard (Annex A). Three values of the viscosity index are used. These data are given only for general guidance in understanding or adopting this international standard.
Viscosity is a measure of the friction of a fluid to its flow. In general, there are two ways to measure the viscosity of a fluid: dynamic viscosity (absolute viscosity) and kinematic viscosity.
One way is to measure the resistance of a fluid to flow when an external force is applied. This is called dynamic viscosity. The other way is to measure the resistive flow of a fluid under the weight of gravity. This measure of fluid viscosity is called kinematic viscosity. Although it is commonly thought that these two measures of viscosity are the same, in reality, there are important differences. For several applications, kinematic viscosity is more useful than absolute or dynamic viscosity.
Temperature and pressure affect the viscosity of a liquid. The viscosity of liquids increases rapidly as temperature increases. With increasing pressure, the viscosity of the liquid molecules increases due to the increased resistance to the flow of the liquid. If the viscosity does not change with pressure, it is called a Newtonian fluid. If the viscosity changes with changes in stress or temperature, it is called a non-Newtonian fluid. Water is an example of a Newtonian fluid, while toothpaste is an example of a non-Newtonian fluid.
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