The centistoke (0.01 of a stoke) is used to describe kinematic viscosity of a fluid, which also may be given in Saybolt Universal Seconds (SUS). The conversion between these units is affected by the temperature of the fluid system. Kinematic viscosity is now the preferred units, but Saybolt Universal Seconds may still be used for petroleum and other hydrocarbons.


Kinematic viscosity is the absolute viscosity of a fluid divided by its density at the temperature of measurement. It is a measure of the fluid's resistance to flow under gravity, as specified in ASTM D 445.


Saybolt Universal Seconds refers to the efflux time of 60 mL of a fluid through a Saybolt Universal viscometer at a given temperature, as described in ASTM D 88.


temperature factor for SUS =

= 1 + (((temperature in degrees Fahrenheit) – 100) * 0.000064)



• Different sources give different values for the factor expressed as 0.000064 above (one is 0.000061 and another is 0.000034).

• I am not sure the temperature range for which the equation is valid, but it works over the range of 100°F to 210°F.


The same kinematic viscosity in centistokes will be seen by a fluid:

(1) with a viscosity of (X) SUS at 100°F

(2) with a viscosity of (X * (temperature factor)) SUS at a higher temperature


The conversion between centistokes and SUS at 100°F is 4.635:


value for fluid in SUS at 100°F =

= (centistokes) * 4.635


value for fluid in SUS at higher temperature =

= (centistokes) * 4.635 * (temperature factor)


Significance of SUS:

• In the range of 1 to 100 SUS at 100°F, the lower the SUS the higher the potential for an aspirated hydrocarbon to cause a chemical pneumonitis.


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