Handheld TDS meters are much preferred over inlines due to their accuracy and their versatility. One meter can test tap water, RO only water, RO/DI, your ATO storage, the LFS, your buddies water, bottled water, the vending machine etc. Inlines are dedicated to one or two spots and cannot be used portable plus they are not truly temperature compensated so have built in errors.
Sediment and carbon filters have very little to absolutely nothing to do with TDS so you replace them every 6 months like clockwork on a set schedule. The RO membrane gets replaced when the cost of DI resin replacements exceeds the cost of a new membrane, maybe 3 years but can be calculated using the TDS meter tap water and RO only water readings and calculating the rejection rate or removal efficiency. The DI gets replaced when the RO/DI TDS exceeds 0 TDS consistently.
Good inlines such as the HM Digital TDS-3, TDS-4TM, their new AP series and the COM-100 have built in digital thermometers so can serve dual purpose, are temperature compensated and start at less than $25.
