ASTM D6022 Standard Practice for Calculation of Permanent Shear Stability Index
6. Procedure
6.1 Calculate the degree of thickening:
DT = V0/Vb
where:
V0 5 viscosity of the unsheared oil, and
Vb 5 viscosity of the base fluid.
6.1.1 Viscosities shall be measured using the same test method at the same conditions of temperature and shear rate or shear stress and reported in the same units.
6.2 If the degree of thickening is less than 1.2, PSSI cannot be determined from these data.
NOTE 4 - Once PSSI has been determined for an additive, calculations using that PSSI can be made for oil blends where the degree of thickening is less than 1.2.
6.3 If the degree of thickening is greater than or equal to 1.2, calculate PSSI using the equation:
PSSI = 100 x (V0 - Vs)/(V0 - Vb)
where:
PSSI = Permanent Shear Stability Index,
V0 = viscosity of the unsheared oil,
Vs = viscosity of the sheared oil, and
Vb = viscosity of the base fluid.
6.3.1 Viscosities shall be measured using the same test method at the same conditions of temperature and shear rate or shear stress and reported in the same units.
NOTE 5 - If Vb is close in value to V0, that is if the degree of thickening is small, the denominator in Eq 2 approaches zero, and the precision of PSSI becomes unacceptable. A minimum degree of thickening of 1.2 was chosen to avoid meaningless calculations of PSSI.
6.3.2 For example, an additive is added to a base fluid of kinematic viscosity 10 mm2/s at 100°C, resulting in an unsheared oil kinematic viscosity of 15 mm2/s at 100°C. After a shearing test, the sheared oil has a viscosity of 13 mm2/s at 100°C. The PSSI of the additive is calculated as:
DT = 15/10 = 1.5 > 1.2
PSSI = 100 x (15 - 13)/(15 - 10) = 100 x (2/5) = 40
6.4 Re-arranging Eq 2 gives other useful relationships. The viscosity of a formulation, after shearing, could be estimated as:
Vs = V0 - (PSSI/100) x (V0 - Vb)
or
Vs = V0(1 - PSSI/100) + Vb x (PSSI/100)
and the viscosity of the unsheared oil is:
V0 = [Vs - Vb x (PSSI/100)]/[1 - (PSSI/100)]
6.4.1 For example, using an additive with a PSSI of 50 and a base oil of kinematic viscosity 8 mm2/s, it is desired to have a sheared oil of no less than 12 mm2/s. To what kinematic viscosity should the unsheared oil be blended.
V0 = [Vs - Vb x (PSSI/100)]/[1 - (PSSI/100)] = [12(8)(0.5)]/[1 - 0.5] = 16
NOTE 6 - The PSSI of the additive must be determined using the same shearing test method as the specification to be met.
