Filter Media Efficiency, Resistance & Quality Factor

Two constants drive this tool and they are not the same kind of number. The resistance constant comes from a model: by default it is the Davies correlation for a polypropylene web at 11% solidity, with its leading constant refitted on measured N95 filtering layers (8.3 in place of 64, O Shaughnessy and others, 2023), which gives the same web the same pressure drop as the Meltblown Pore Size tool. Davies as published would give 4.53, about eight times more, because he built the correlation on much coarser fibres. The capture constant is fitted: no model on this page predicts it, and its default describes no particular media. The capture term is a fitted single-parameter stand-in for classical single-fibre theory, which treats interception, impaction and diffusion separately and gives each a different dependence on velocity and particle size; the fitted form reproduces the exponential shape and the direction of every lever but cannot predict the most-penetrating particle size or how efficiency shifts with it, and near that size the real curve has a minimum this model does not. Fit the two constants on measured flat-sheet data at the velocity of interest and treat the model as an interpolator rather than a predictor. Electret media break the relationship entirely: a charged web achieves efficiencies far above what its structure alone would give, loses that advantage as the charge dissipates in service or on contact with oil aerosol, and a mechanical model will therefore be badly optimistic about its life and badly pessimistic about its initial performance. Pressure drop here is clean-media resistance; loading raises it through life, and the useful life of a filter is usually set by the terminal drop rather than by efficiency. Face velocity is the flat-sheet test condition and not the face velocity of a pleated element, where the media velocity is lower than the element velocity by the pleating ratio - which is precisely why elements are pleated.