[bernawil]

I know pressure drops. The problem is knowing which filter is the one causing it in particular. Also, filters that are spent at different rates are a PITA. What I mean is if you are going to feed it nominally clean tap water, there's no reason to protect a catridge with equally or more expensive cartridges. Just use the RO filter and be done with it.

[tguvot]

you can put pressure guages in between or one of $10 flow meters before system.

RO membrane doesn't remove chlorine iirc or vocs. On the other side chlorine degrades membrane. "nominally clean tap water" can have enough dirt to clog membrane if you don't auto backflush it frequently