Many people think that a fluid is incompressible. However, fluids are, like any material, in a certain amount compressible. In calculations the amount of compressibility of fluid is considered to be 1 volume-% per 100 bar . This means that for example when there is fluid supplied to a 200 litre oil drum which already is completely filled with fluid (see animation), the pressure increases with 100 bar for each 2 litre of extra supplied fluid. When we supply 3 litre of extra oil the pressure increases with 150 bar.
The compressibility of fluid plays a key role in for example fast hydraulic systems like servo-systems of a flight simulator. To obtain a maximum dynamic performance, the compressibility should be as less as possible. This is achieved by mounting the control valves directly on the hydraulic motor or cylinder. In that case the amount of fluid between the control valve and the motor/cylinder is as less as possible.