I noticed a few posts where it seems the advice on when to clean your filter has changed. For the longest time the advice (not just from here but in general) was to clean when your pressure goes up 8-10psi. Now it seems the suggestion is to clean when the pressure goes up 20%-25% from your clean reading. In many ways this make a lot more sense than the old advice. My clean pressure is 15psi, 10 psi more is 25psi, a 66% increase, which seems like a lot.
However, the question I have has to do with sand filters. As you know sand filters when clean, filter down about 30 microns or so. As they get dirty, this gets smaller and their ability to remove smaller particles improves. It seems running them somewhat dirty is an intended/expected part design so their particle filtering is more inline with other types of filters. Sometimes we even add DE (or cellulose) to intentionally "dirty" the filter to improve its filtering.
Okay, so on to the question, does the same 20-25% guideline still hold true for sand filters? It seems, like back-washing so often would undermine their ability to filter as effectively.