Understanding the Viscosity Index

Usefulness for Professionals

The Viscosity Index ($VI$) is a critical metric for engineers, students, and professionals in lubrication and mechanical engineering. It quantifies how much an oil’s kinematic viscosity changes with temperature.

For Mechanical Engineers: A high $VI$ lubricant maintains its film strength and thickness across a wider temperature range, ensuring stable performance in engines, gearboxes, and hydraulic systems that experience significant thermal variation.
For Lubrication Specialists: It aids in selecting the optimal base oil and viscosity modifier additives to meet stringent industry standards and operational requirements, such as those for modern multi-grade engine oils.
For Students: Understanding $VI$ is fundamental to fluid mechanics and tribology, providing a practical application of how temperature affects fluid properties.

Step-by-Step Instructions

1. Measure Viscosity ($U$): Determine the kinematic viscosity of the test oil at $40^{\circ}\text{C}$ (centistokes or cSt).
2. Determine Reference Viscosities ($L$ and $H$): Look up the corresponding $L$ (Viscosity at $40^{\circ}\text{C}$ for $\text{VI}=0$ oil) and $H$ (Viscosity at $40^{\circ}\text{C}$ for $\text{VI}=100$ oil) values. These reference values must share the same kinematic viscosity at $100^{\circ}\text{C}$ as the test oil.
3. Input Data: Enter the three values ($U$, $L$, and $H$) into the corresponding fields in the calculator above.
4. Calculate: Click the “Calculate Viscosity Index (VI)” button to see the result.

The Viscosity Index Formula

The Viscosity Index ($VI$) is calculated using the following empirical formula:

Where the terms are defined as:

• $VI$ = Viscosity Index (unitless measure of viscosity change with temperature)
• $U$ = Kinematic viscosity of the oil being tested at $40^{\circ}\text{C}$ (cSt)
• $L$ = Kinematic viscosity at $40^{\circ}\text{C}$ of a hypothetical oil with $\text{VI}=0$ that has the same viscosity at $100^{\circ}\text{C}$ as the tested oil (cSt)
• $H$ = Kinematic viscosity at $40^{\circ}\text{C}$ of a hypothetical oil with $\text{VI}=100$ that has the same viscosity at $100^{\circ}\text{C}$ as the tested oil (cSt)