These oils have viscosities of 37, 56 and 77 cSt but are not officially ISO viscosity grades.Īlthough this numbering system may appear arbitrary, each subsequent grade is approximately a 50% increase on the previous grade. There are some intermediate grades in common usage which are not ISO approved. According to the ISO, 10% leeway is allowed either way, so any industrial oil with a viscosity between 90 and 110 cSt would be considered an ISO 100, and even the same brand and grade might vary slightly from batch to batch. When measuring the viscosity of an ISO oil, do not expect an ISO 100 to have a viscosity of exactly 100 centistokes. Some ISO 68 oils can be used in high speed, low load gearboxes and some ISO 320 oils can be used in compressors with very high discharge temperatures. There is no exact cut-off point where gear oils become hydraulic oils, but ISO 150 is a good approximation. Generally, the lower viscosity oils are hydraulic fluids and the higher viscosity oils are gear fluids. For example, Caltex Meropa 460 is an industrial gear circulating oil with a viscosity of 460 cSt. The beauty of this system is that the name of the oil states its viscosity. This means that an ISO 320 oil has a kinematic viscosity of 320 cSt at 40 degrees C. These numbers refer to the kinematic viscosity of the oil in centistokes at 40 degrees C. The numbers associated with the ISO VG are as follows: ISO VISCOSITY GRADE (ISO VG) 2 The International Standards Organisation, Viscosity Grade (ISO VG) is a grading system that is generally used to describe industrial oils ie oils used in stationary plant (pumps, turbines, gearboxes, compressors etc). The different systems are convertible but only for measurements made at the same temperature. Engler Degrees (Europe), Redwood Seconds (UK), and Saybolt Universal Seconds (USA). Although centistoke units will be used throughout this article, different units are used in other parts of the world e.g. Oil grades are usually described in kinematic viscosity (normally at 40 degrees C). The two viscosity measurements are related to one another by the density of the fluid: nD / p = nKĭynamic viscosity is of little concern when describing an oil's viscosity grade. Dynamic viscosity nD = 1 centipoise (cP) = 1 mPa.S Dynamic viscosity is measured in centipoise and one centipoise equals one millipascal second. Kinematic viscosity is not the only viscosity measurement that can be made there is also dynamic viscosity (sometimes called absolute viscosity) which is a measurement of a fluid's resistance to shear at a specified temperature. The type of oil under consideration and its properties determine which temperature is employed, although 40 degrees C is used most commonly. Two industry temperature standards are used when measuring kinematic viscosity, namely 40 degrees C and 100 degrees C. It is important to note that as temperature increases, viscosity decreases, so one must always state the temperature at which viscosity is measured, otherwise the reading will be meaningless. Kinematic Viscosity nK = 1 Centistoke (cSt) = 1 mm 2 /s The symbol for viscosity is denoted by the Greek letter 'eta'. Kinematic viscosity is measured in centistokes (cSt) and one centistoke equals one millimetre squared per second. A fluid's resistance to flow is known as kinematic viscosity and this is the measurement that is of greatest concern to people using lubricants. It is a measure of the oil's thickness thick oil has a high viscosity while thin oil has a low viscosity. This is defined as the oil's resistance to flow at a specified temperature. The most important property of an oil is its viscosity. The classifications, which include ISO, SAE, API, CCMC, SABS, JAMA and ISLS, are each followed by a series of numbers and letters detailing either the viscosity of the oil or its performance properties. When purchasing a can, drum or tankerful of oil, it is important to realise that a number of international classification systems are used to describe the product and its uses. Oil classifications either describe viscosity or performance. This article was first published in 2001.
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