A filter medium’s efficiency is determined by the arrestance, namely the percentage of particles a filter retains. Apart from mechanical effects, the level of particle deposition is also based on a natural or artificially generated electrostatic charge of the medium. An ideal filter would retain all particulate matter whilst remaining permeable to air

Development of the arrestance during use is vital in evaluating a filter. High efficiency throughout the operating life is required, even after discharge. During use, the filter medium’s electrostatic charge reduces and consequently, the arrestance decreases. This effect can be caused by an accumulation of dust particles on the surface of the fibres or by specific aerosols such as water-vapour, oil mist or soot particles. A characteristic of Sandler media is high discharged efficiency, achieving high arrestance throughout the operating life due to reliance on mechanical deposition effects: fine fibres of less than 1 µm in diameter create a large filtration surface for optimum mechanical deposition.

Filters are classified based on the average efficiency. Coarse and fine dust filters up to grade F9 are classified according to DIN EN 779 standard; particulate air filters grade E10 and higher are classed according to EN 1822 in its latest revision. The filter is loaded with a predetermined amount of artificial testing dust until the resistance limit is reached. The arrestance or efficiency is measured several times during the test period. The efficiency of Sandler nonwovens is determined by external testing institutes, where the media are tested under standard as well as real-life conditions. 

In liquid filtration, the separation efficiency takes precedence—a filter medium’s ability to separate contaminants from a fluid. Synthetic fuel filter media, for example, separate water and particulates to prevent damage to the engine and encourage efficient combustion. Small pores of a defined size range are distributed evenly in the medium, fostering deposition solely reliant on mechanical effects and upholding the high efficiency of Sandler filter media.