1. Density

At first glance, density may appear to be a secondary factor that can be accepted as it is. In fact, it would be a mistake to neglect its importance for two reasons:

• the weight factor, which is essential for the automotive and aeronautics industries;

• the energy factor for low-volume hydraulic circuits.

The density of oils varies widely, from almost 800 kg/m ³ for the most fluid poly-α-olefins to almost 2 × 10 ³ kg/m ³ for perfluoroalkyl ethers.

The densities of the various oils are shown in table 1 . Suppliers' data sheets specify density at 15°C....