VISCOSITY

ASTM D445 - Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids

 

 

OVERVIEW: The Society of Automotive Engineers (SAE) established clear standards that define the viscosity grade of engine oil (SAE J300).In order to claim a viscosity grade , the oil must  meet all viscosity test requirements which define that grade.  One test used to define the viscosity of engine oil is its kinematic viscosity at 100°C measured in mm2/s or centistokes (cSt). The standard for a 5W-30, for example, is 9.3 to 12.5 cSt. The standards for other viscosity grades are shown in Table 1Viscosities in the red zone shown on the bar graph  for a 5W-30  indicate that the film thickness of the oil is too thin or too thick for the viscosity grade claimed.  When the film thickness is low, the engine may experience more metal to metal contact and higher levels of wear.  When it is too thick, it may not flow fast enough or to some parts of the engine also leading to higher levels of wear and less efficient engine operation.

 

In simplest terms, viscosity is a measure that speaks to a liquid's resistance to flow. Fluids with a high viscosity are thick and flow slowly. Low viscosity fluids are thin and flow quickly. A common example of each are seen when comparing molasses with water. Whereas molasses is thick and flows slowly (high viscosity) when poured, water is "thin" and flows quickly when poured (low viscosity).

 

TABLE 1

SAE Viscosity Chart (High Temp)
100° C (210° F)

SAE Viscosity

Kinematic (cSt)

Min

Kinematic (sCt) Max

20

5.6

< 9.3

30

9.3

< 12.5

40

12.5

< 16.3

50

16.3

< 21.9

60

21.9

< 26.1

 

    

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