To ensure a reliable and constant operation of a hydraulic system and its components, the hydraulic fluid must have and maintain over time a degree of contamination and chemical-physical characteristics within the values established by the project.

The contamination of hydraulic fluids is due to several factors: initial contamination of the new fluid, particles present in the environment that are able to penetrate the system through filling caps, air filters, gaskets, residues of assembly operations, particles deriving from wear of the components and condensation. Failure to check the level of contamination of hydraulic fluids can lead to damage and, in severe cases, to block of the systems.

It is, therefore, clear that quantifying and classifying the particulate and the other pollutants present in the fluids is an essential "tool" to avoid the occurrence of wear, corrosion and degradation of the systems.