Things You Should Know about Viscosity Index, Viscosity, Pressure, and Pour Point

Viscosity Index

The viscosity of most hydraulic fluids decreases approximately 75% when the temperature goes from 38⁰C (100⁰F) to 99⁰C (210⁰F), and it increase approximately 700% from 38⁰C (100⁰F) to -18⁰C (0⁰F). The viscosity of some fluids varies more than others, as shown in Fig. 2-3. Because the rate at which the viscosity of a fluid changes with temperature is critical in most systems, various means have been devised to rate them. The term that expresses the extent to which the viscosity f a fluid changes is called the VISCOSITY INDEX (VI).

Fig. 2-3. Viscosity Index Chart

The viscosity index of a hydraulic fluid is a description of another of its physical properties. It is determined by comparing the change in the viscosity of a sample fluid with the change in the viscosity of two reference oils at 100⁰F and 210⁰F. A fluid with a high VI has a smaller change in viscosity when its temperature changes than a fluid with a low VISCOSITY INDEX. At one time,  a VISCOSITY INDEX of 0 (zero) was assigned to the reference oil that showed the greatest change in viscosity, and a VISCOSITY INDEX of 100 was assigned to the reference oil that showed the smallest change in viscosity. But now, as the result of modern refining methods, the use of additives, and the availability of synthetic organic compounds, hydraulic fluids are being  produced with VISCOSITY INDEX’s as high as 160 and as low as minus several hundred. While the VISCOSITY INDEX of flow viscosity fluids is quite stable, fluids that contain VISCOSITY INDEX-improvers usually do not possess stable or permanent viscosity/temperature characteristics. Because of the large variety of improvers available, it is difficult to rate their performance. However, it is important that a hydraulic fluid have as small a change as possible in its viscosity when its temperature changes.

Viscosity and Pressure

Fig. 2-4. Effect of pressure on Viscosity Index.

An important characteristic of a hydraulic fluid is its change in viscosity when subjected to pressure. In general, increased pressure on petroleum oil fluids has a greater effect on the viscosity at high pressures than at low pressures. Increased pressure also affects high viscosity and naphthene fluids more than low viscosity and paraffin fluids. An increase in the VI of oil also takes place with an increase in pressure. The effect of pressure on the viscosity of two oils is shown in Fig. 2-4. Both oils have a nominal viscosity of 300 SSU at 100⁰F and normal atmospheric pressure.  An increase in the pressure to 10,000psi results in a 200% increase in the viscosity at 100⁰F of the paraffin-based oil that has a VISCOSITY INDEX of 94, and a 565% increase in the viscosity at 100⁰F of the naphthene-based oil that has a VISCOSITY INDEX of 15.

Pour Point

Another physical property of a hydraulic fluid is its POUR POINT, or the lowest temperature at which it will flow. The pour point should be a little lower than the operating temperature of the hydraulic system in which the fluid is to be used. Actually, the pour point is critical only if the system must operate in low temperatures.