250
The Chemistry and Technology of Petroleum
As a result of the various methods for viscosity determination, it is not surprising that much
effort has been spent on interconversion of the several scales, especially converting Saybolt to kinematic viscosity (ASTM D2161):
Kinematic viscosity a Saybolt s b/Saybolt s
= ¥
+
where a and b are constants.
The Saybolt universal viscosity equivalent to a given kinematic viscosity varies slightly with the
temperature at which the determination is made because the temperature of the calibrated receiving
flask used in the Saybolt method is not the same as that of the oil. Conversion factors are used to
convert kinematic viscosity from 2 to 70 cSt at 38°C (100°F) and 99°C (210°F) to equivalent Saybolt
universal viscosity in seconds. Appropriate multipliers are listed to convert kinematic viscosity
over 70 cSt. For a kinematic viscosity determined at any other temperature the equivalent Saybolt
universal value is calculated by use of the Saybolt equivalent at 38°C (100°F) and a multiplier that
varies with the temperature:
Saybolt s at 1 F 38 C cSt 4 635
00∞
∞ =
¥
(
)
.
Saybolt s at 21 F 99 C cSt 4 667
0∞
∞ =
¥
(
)
.
Various studies have also been made on the effect of temperature on viscosity since the viscosity
of petroleum, or a petroleum product, decreases as the temperature increases. The rate of change
appears to depend primarily on the nature or composition of the petroleum, but other factors, such
as volatility, may also have a minor effect. The effect of temperature on viscosity is generally represented by the equation
log n c A B log T
(
)
+ = +
where
n is the absolute viscosity
T is the temperature
A and B are constants
This equation has been sufficient for most purposes and has come into very general use. The
constants A and B vary widely with different oils, but c remains fixed at 0.6 for all oils having a viscosity over 1.5 cSt; it increases only slightly at lower viscosity (0.75 at 0.5 cSt). The
viscosity–temperature characteristics of any oil, so plotted, thus create a straight line, and the
parameters A and B are equivalent to the intercept and slope of the line. To express the viscosity and viscosity–temperature characteristics of an oil, the slope and the viscosity at one
temperature must be known; the usual practice is to select 38°C (100°F) and 99°C (210°F) as
the observation temperatures.
Suitable conversion tables are available (ASTM D341), and each table or chart is constructed in
such a way that for any given petroleum or petroleum product the viscosity–temperature points result
in a straight line over the applicable temperature range. Thus, only two viscosity measurements need
to be made at temperatures far enough apart to determine a line on the appropriate chart from which
the approximate viscosity at any other temperature can be read. The charts can be applicable only to
measurements made in the temperature range in which a given petroleum oil is a Newtonian liquid.
The oil may cease to be a simple liquid near the cloud point because of the formation of wax particles
or, near the boiling point, because of vaporization. However, the charts do not give accurate results
The Chemistry and Technology of Petroleum
As a result of the various methods for viscosity determination, it is not surprising that much
effort has been spent on interconversion of the several scales, especially converting Saybolt to kinematic viscosity (ASTM D2161):
Kinematic viscosity a Saybolt s b/Saybolt s
= ¥
+
where a and b are constants.
The Saybolt universal viscosity equivalent to a given kinematic viscosity varies slightly with the
temperature at which the determination is made because the temperature of the calibrated receiving
flask used in the Saybolt method is not the same as that of the oil. Conversion factors are used to
convert kinematic viscosity from 2 to 70 cSt at 38°C (100°F) and 99°C (210°F) to equivalent Saybolt
universal viscosity in seconds. Appropriate multipliers are listed to convert kinematic viscosity
over 70 cSt. For a kinematic viscosity determined at any other temperature the equivalent Saybolt
universal value is calculated by use of the Saybolt equivalent at 38°C (100°F) and a multiplier that
varies with the temperature:
Saybolt s at 1 F 38 C cSt 4 635
00∞
∞ =
¥
(
)
.
Saybolt s at 21 F 99 C cSt 4 667
0∞
∞ =
¥
(
)
.
Various studies have also been made on the effect of temperature on viscosity since the viscosity
of petroleum, or a petroleum product, decreases as the temperature increases. The rate of change
appears to depend primarily on the nature or composition of the petroleum, but other factors, such
as volatility, may also have a minor effect. The effect of temperature on viscosity is generally represented by the equation
log n c A B log T
(
)
+ = +
where
n is the absolute viscosity
T is the temperature
A and B are constants
This equation has been sufficient for most purposes and has come into very general use. The
constants A and B vary widely with different oils, but c remains fixed at 0.6 for all oils having a viscosity over 1.5 cSt; it increases only slightly at lower viscosity (0.75 at 0.5 cSt). The
viscosity–temperature characteristics of any oil, so plotted, thus create a straight line, and the
parameters A and B are equivalent to the intercept and slope of the line. To express the viscosity and viscosity–temperature characteristics of an oil, the slope and the viscosity at one
temperature must be known; the usual practice is to select 38°C (100°F) and 99°C (210°F) as
the observation temperatures.
Suitable conversion tables are available (ASTM D341), and each table or chart is constructed in
such a way that for any given petroleum or petroleum product the viscosity–temperature points result
in a straight line over the applicable temperature range. Thus, only two viscosity measurements need
to be made at temperatures far enough apart to determine a line on the appropriate chart from which
the approximate viscosity at any other temperature can be read. The charts can be applicable only to
measurements made in the temperature range in which a given petroleum oil is a Newtonian liquid.
The oil may cease to be a simple liquid near the cloud point because of the formation of wax particles
or, near the boiling point, because of vaporization. However, the charts do not give accurate results