This indicator is used to characterise the quality of lubricants and hydraulic fluids. It takes into account the influence of temperature on the kinematic viscosity of the fluid, which decreases as the temperature increases and vice versa. The viscosity of lubricants and hydraulic fluids is a very important factor.
The viscosity index is particularly used in the oil industry.
If the lubricant is too viscous, the process will have to provide much more energy to set the parts in motion, generating excessive energy consumption with reduced yields. If the oil is too fluid, the quality of the lubrication is reduced with a risk of wear of the moving parts. Precise control of the viscosity index ensures the quality and stability of the oil for a given use.
We can cite the following example concerning engine lubrication: lubricating oils for motor vehicles must reduce friction between the components when the engine is cold when starting (ambient temperature) and during operation (200° to 300°C). The best oils corresponding to the highest viscosity index will have a “constant” viscosity according to their temperature range. The viscosity index scale uses the following reference temperatures:
- 100° Fahrenheit (40° C)
- 210° Fahrenheit (100° C)
Originally this scale was graduated from 0 to 100 VI (high quality oil). But nowadays, with the improvement of manufacturing processes and additives, we can find oils with a much higher index. Nowadays, the index of some synthetic oils can exceed 400 VI.
During the manufacturing process of lubricating oils, the MIVI 9731 Thermoset, certified explosion-proof (ATEX), provides all the guarantees of an excellent continuous control of the viscosity index, a factor of optimal quality of the final product.