Describes the increase in viscosity of mineral oils given increasing pressure. This increase gets bigger as the nominal viscosity increases (= viscosity/pressure characteristic at +50°C or recently +40°C). At the same time, the viscosity/pressure characteristic also changes when facing temperature changes: the influence of the pressure is reduced as the temperature increases (Figure V 28).
The viscosity/pressure characteristic must therefore be particularly observed if a system has to work at low temperatures. The compressibility (volume reduction) –– and therefore the stiffness of a column of oil changes: as the pressure and temperature rises, it increases more rapidly than at lower temperatures. This affects the efficiency (Figure V 29).
Figure V 28: Viscosity/pressure/temperature behaviour of mineral oil
Figure V 29: Influence of viscosity on the efficiency of a system