The standard practice is to change the sediment and carbon filters every 6 months and while you have the system down disinfect it at that time. Since sediment and carbon filters have very little to absolutely nothing to do with TDS that is not an indicator of filter condition. Their sole purpose is to protect the RO membrane which is where 90-98% of the TDS is removed. TDS is total dissolved solids, things in solution. The sediment and carbon remove TSS, total suspended solids, big stuff in suspension like silt, sediment, particulates and colloidal materials, sometimes large enough to be seen with the human eye. TDS is in the 0.0001 micron range so a sediment filter will not touch it and the carbon removes chlorine and some volatiles but maybe 1-2 ppm TDS, not significant in the grand scheme of things.
Another method is to use a pressure gauge, or better yet two pressure gauges, one before the sediment filter and one after the carbon block to measure headloss or pressur drop which is evidence of filter plugging or fouling. Along with tht you need a low range chlorine test kit to monitor chlorine breakthru from exhausted or fouled carbon. carbon is made up f billions of tiny pores where the chlorine is adsorbed and when those pores are plugged the chlorine passes through and an ruin a RO membrane,
Its usually easier to stick with 6 month intervals for most users.
You change the DI resin when the finished water TDS is no longer 0 on a consistent basis. You may see a slightly elevated TDS when first starting to make wate reach time but it should pass quickly, if not your DI is exhausted.
You change the RO membrane when the rejection rate or removal efficiency is o longer in the say 95+% range which is when it starts costing you mre in replacement DI resins than the cost of a new efficient membrane would be.
To troubleshoot a RO/DI you need three TDS readings, tap water, RO only water before the DI and final RO/DI water. It takes all three readings so inline TDS meters are not as desirable as a good handheld meter since most only monitor two places, usually RO only and RO/DI so you have no idea what your rejection rate or removal efficiency is since you do not have the tap water TDS which is what is used to calculate the rejection rate along with the RO TDS. The other important tool is an inline prssure gauge to monitor the headloss. This is critial since reduced pressure at the membrane reduces its efficiency and eats DI resin faster.
Color changing DIis very unreliable in almost all cases. It can change all at once, not at all, in streaks or layers, and any number of things but almost always you are already passing TDS and weakly ionized substances way before the resin turns colors. A handheld TDS or conductivity meter is your best friend.
