In other words, with more fluid velocity is through a flow restriction, the higher the dP. This is the reason some oil filters lose a lot of efficiency as the dP across the media goes up. Some filters slough off already captured debris pretty badly as the filter loads and the dP increases (ie, Boss and XP), and some don't (OG Ultra and Royal Purple). If you see a filter with a high ISO 4548-12 efficiency, by definition of the ISO efficiency, that filter is not sloughing off much debris as the dP increases. We saw this in Ascent's ISO 4548-12 testing thread in this forum.That's not been my professional experience supporting fuel filter development. Real world efficiency decreases with what's called "face velocity." More velocity through the filter, less efficiency.
This also means that the filter that's good at retaining captured debris will be much less sensitive to changes in fluid flow rate (ie, dP fluctuations and spikes). In an ISO 4548-12 test, they would be less sensitive to what the established test flow rate is, because they are a low debris sloucher with dP increase from either flow rate or loading. This is another good reason to use filters with high ISO 4548-12 efficiecy ratings. Oil filters see dP fluxuations all the time from changing oil viscosity due to oil temperature changes, and also from engine RPM fluxuations while the car is being driven around.
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