I run inline monitoring coming into the system, out of the ro membrane, and out of the di canister. When I get a TDS of 1 coming from my di resin it's time to change all the filter blocks and the di resin. RO membranes should last for several years if cared for. IMHO, inline monitoring is the key to keeping your rodi working well, and your rodi water within acceptable limits. YMMV.