ASTM D2270 for calculating viscosity index from kinematic viscosity at 40 and 100°C
6. Procedure B - For Oils of Viscosity Index of 100 and Greater
6.1 Calculation:
6.1.1 If the kinematic viscosity of the oil at 100°C is less than or equal to 70 mm 2/s (cSt), extract the corresponding value for H from Table 1. Measured values that are not listed, but are within the range of Table 1, can be obtained by linear interpolation. The viscosity index is not defined and may not be reported for oils of kinematic viscosity of less than 2.0 mm 2/s (cSt) at 100°C.
6.1.2 If the measured kinematic viscosity at 100°C is greater than 70 mm 2/s (cSt), calculate the value of H as follows:
H = 0.1684 Y(2) + 11.85 Y - 97
where:
Y = kinematic viscosity at 100°C of the oil whose kinematic viscosity is to be calculated, mm2/s (cSt), and
H = kinematic viscosity at 40°C of an oil of 100 viscosity index having the same kinematic viscosity at 100°C as the oil whose viscosity index is to be calculated mm2/s (cSt).
6.1.3 Calculate the viscosity index, VI, of the oil as follows:
VI = [(antilog N) - 1)/0.00715] + 100
where:
N = (log H - log U)/ log Y,
or
Y(N) = H/U
where:
U = kinematic viscosity at 40°C of the oil whose viscosity index is to be calculated mm2/s (cSt).
6.1.4 Calculation Example:
(1) Measured kinematic viscosity at 40°C of the oil whoseviscosity index is to be calculated = 22.83 mm 2/s (cSt)
kinematic viscosity at 100°C of the oil whose viscosity index is to be calculated = 5.05 mm 2/s (cSt)
From Table 1 (by interpolation) H = 28.97
Substituting by Eq 8 (by logarithms):
N = [(log 28.97 - log 22.83)/log 5.05] = 0.14708
Substituting in Eq 7 and rounding to the nearest whole number:
VI = [(antilog 0.14708) - 1)/0.00715] + 100
= [(1.40307 - 1)/0.00715] + 100 = 156.37
VI = 156
(2) Measured kinematic viscosity at 40°C of the oil whose viscosity index is to be calculated = 53.47 mm2/s (cSt) kinematic viscosity at 100°C of the oil whose viscosity index is to be calculated = 7.80 mm2/s (cSt)
From Table 1, H = 57.31
Substituting in Eq 8 (by logarithms):
N = [(log 57.31 - log 53.47)/log 7.80] = 0.03376
Substituting in Eq 7 and rounding to the nearest whole number:
VI = [((antilog 0.03376) - 1)/0.00715] + 100
= [(1.08084 - 1)/0.00715] + 100 = 111
6.2 ASTM DS 39b - Viscosity Index Tables for Celsius Temperatures is based on the above calculation and may be used instead of 6.1 through 6.1.4.
