The forum recommendation of a 25% change in pressure is just that, a recommendation. In my own pool, if I waited for a 25% increase it would take over a year for that to happen since my DE filter is very much oversized in relation to the pool volume being filtered. I have literally gone one full year without cleaning my filter or backwashing and the filter pressure has not changed more than 25%. My normal mode of operation is to tear down and clean the filter twice per year (I do not backwash). Doing so keeps the Quad cartridges clean and I use precisely one 10lb bag of DE with each cleaning. If I back washed as the pool store recommends, then my DE costs would be a lot higher and my filter quad cartridges would be under more frequent stress. Remember that for a DE filter (grid or cartridge style), backwashing is not the normal flow pattern and puts a lot of stress on the filter medium. This reduces the life of the grids or the cartridges are is most often responsible for torn fabric or blown out cartridges.
