Viscosity Blending Calculator | Refutas Method (ASTM D7152)
Viscosity Blending Calculator
Blend Components
Component A
Component B
Total fraction must equal 100%
Blended Results
Final Viscosity
--- cSt
Statistics
Total Fraction
0%
Viscosity Blending Number (VBN)
---
Technical Notes
Variable Definitions
- Ξ½: Kinematic viscosity of a component at blending temperature (cSt = mmΒ²/s).
- xi: Component fraction (%) on the selected basis (mass recommended).
- VBN: Viscosity Blending Number (Refutas index, dimensionless).
- VBNmix: Weighted blend index, VBNmix = Ξ£(xiΒ·VBNi)/100.
- Ξ½mix: Final blended viscosity (cSt) from inverse Refutas relationship.
Why Refutas Method?
- Viscosity blending is non-linear. A 50/50 mix of 100 cSt and 10 cSt oil does not result in 55 cSt (result is typically much lower).
- This tool uses the ASTM D7152 Refutas indices to linearize the viscosity-temperature relationship.
Formulas
-
VBN Calculation:
VBN = 14.534 Γ ln( ln(Ξ½ + 0.8) ) + 10.975(Ξ½ in cSt) - Blending: The VBN of the mix is the weighted average of component VBNs based on mass fraction.
-
Inverse:
Ξ½ = exp( exp( (VBN β 10.975)/14.534 ) ) β 0.8
Assumptions
- Temperature: All component viscosities must be at the same temperature.
- Miscibility: Assumes fluids are fully miscible and no chemical reaction occurs.
- Basis: Weight (Mass) fraction is the standard engineering basis. Volume fraction is an approximation assuming similar densities.
-
Validity: Refutas requires
ln(Ξ½ + 0.8) > 0βΞ½ > 0.2 cSt. This calculator enforces Ξ½ β₯ 0.2 cSt. - References: ASTM D7152 (Refutas Viscosity Blending Index/VBN). Use product datasheets for viscosity values.