Pressure difference between the inlet and outlet on the filter (test standard ISO 3968). The initial pressure difference is a key parameter for the design of a filter. The is not a constant parameter here, instead it increases as dirt is retained in the filter element (Figure F 12).
In addition, the flow resistance of filters is also dependent on the selected filtration grade, the flow rate, the filter surface area and design as well as on the viscosity of the pressure fluid and therefore on its actual temperature (Figure F 13).
As the pressure drop is not linear, but instead corresponds approximately to a 3rd order function, a filter with a differential pressure that corresponds to half the permissible value is not around half clogged, but at approx. 80 % close to the end of its service life.
Figure F 12: Differential pressure curve of a filter element
Figure F 13: Flow resistance for varying filtration grade and oil viscosity