VISCOSITY INDEX
ASTM D 567 and D 2270
Liquids have a tendency to thin out when heated
and to thicken when cooled. However, this
response of viscosity to temperature changes is
more pronounced in some liquids than in others.
and to thicken when cooled. However, this
response of viscosity to temperature changes is
more pronounced in some liquids than in others.
Often, as with petroleum liquids, changes in
viscosity can have marked effects upon its suit-
ability for certain applications. The property of
resisting changes in viscosity due to changes in
temperature can be expressed as the viscosity
index (V.I). The viscosity index is an empirical,
unitless number. The higher the V.I. of an oil,
the less its viscosity changes with changes in
temperature.
viscosity can have marked effects upon its suit-
ability for certain applications. The property of
resisting changes in viscosity due to changes in
temperature can be expressed as the viscosity
index (V.I). The viscosity index is an empirical,
unitless number. The higher the V.I. of an oil,
the less its viscosity changes with changes in
temperature.
The Concept of Viscosity Index: One of the
things that led to the development of a viscosity
index was the eady observation that, for oils of
equal viscosities at a given temperature, a
naphthenlc oil thinned out more at higher
temperatures than did a paraffinic oil. However,
there existed no single parameter that could
express this type of response to temperature
changes.
things that led to the development of a viscosity
index was the eady observation that, for oils of
equal viscosities at a given temperature, a
naphthenlc oil thinned out more at higher
temperatures than did a paraffinic oil. However,
there existed no single parameter that could
express this type of response to temperature
changes.
The viscosity index system that was developed
to do this was based upon comparison of the
viscosity characteristics of an oil with those of
so-called "standard" oils. A naphthenic oil in a
series of grades with different viscosities at a
given temperature, and whose viscosities
changed a great deal with temperature, was
arbitrarily assigned a V.I. of zero. A paraffinic
series, whose viscosities changed less with
temperature than most of the oils that were then
available, was assigned a V.I. of 100. With ac-
curate viscosity data on these two series of oils,
the V.I. of any oil could be expressed as a
percentage factor relating the viscosities at 100°
F of the test oil, the zero-V.l, oil, and the 100-
to do this was based upon comparison of the
viscosity characteristics of an oil with those of
so-called "standard" oils. A naphthenic oil in a
series of grades with different viscosities at a
given temperature, and whose viscosities
changed a great deal with temperature, was
arbitrarily assigned a V.I. of zero. A paraffinic
series, whose viscosities changed less with
temperature than most of the oils that were then
available, was assigned a V.I. of 100. With ac-
curate viscosity data on these two series of oils,
the V.I. of any oil could be expressed as a
percentage factor relating the viscosities at 100°
F of the test oil, the zero-V.l, oil, and the 100-
V.I., oil, all of which had the same viscosity at
210° F. This Is the basis for the formula,
210° F. This Is the basis for the formula,
V.I =
X 100
L U
L H
L H
-
-
-
where L is the viscosity at 100° F of the zero-V.I.
oil, H is the viscosity at 100° F of the 100 V.I.
oil, and U is the viscosity at 100° F of the
unknown (test) oil.
oil, H is the viscosity at 100° F of the 100 V.I.
oil, and U is the viscosity at 100° F of the
unknown (test) oil.
The ASTM Standards: This viscosity index
system eventually became the ASTM standard
D 567, which has been used for years In the
petroleum industry.
system eventually became the ASTM standard
D 567, which has been used for years In the
petroleum industry.
ASTM D 567 is a satisfactory V.I. system for most
petroleum products. However, for V.l.'s above
about 125, mathematical inconsistencies arise
which become more pronounced with higher V.l.'s.
Because products with very high V.l.'s are
becoming more common, a method (ASTM D
2270) that eliminates these inconsistencies has
been developed.
petroleum products. However, for V.l.'s above
about 125, mathematical inconsistencies arise
which become more pronounced with higher V.l.'s.
Because products with very high V.l.'s are
becoming more common, a method (ASTM D
2270) that eliminates these inconsistencies has
been developed.
Calculating Viscosity Index: The viscosity index
of an oil can be calculated from tables or charts
included in the ASTM methods. For V.l.'s below
100, ASTM D 2270 and ASTM D 567 are identical,
and either method may be used. For V.l.'s above
100, ASTM D 2270 should be used. Since ASTM
D 2270 is suitable for all V.l.'s, it is the method now
preferred by Exxon. V.l.'s in future publications will
be reported as ASTM D 2270 values.
of an oil can be calculated from tables or charts
included in the ASTM methods. For V.l.'s below
100, ASTM D 2270 and ASTM D 567 are identical,
and either method may be used. For V.l.'s above
100, ASTM D 2270 should be used. Since ASTM
D 2270 is suitable for all V.l.'s, it is the method now
preferred by Exxon. V.l.'s in future publications will
be reported as ASTM D 2270 values.
The V.I. of an oil may also be determined with
reasonable accuracy by means of special
homographs or charts developed from ASTM
tables.
reasonable accuracy by means of special
homographs or charts developed from ASTM
tables.
(FIST 2-4 11/90)
A -20