A friend also sent me this and thought it might help someone ells so ill post it.
"Sediment and carbon block filters have very little to absolutely nothing to do with TDS. They are there to protect the RO membrane from sediment, silt, colloidal materials (all TSS or suspended solids, big stuff not dissolved solids or TDS) and chlorine. Its the job of the RO membrane to remove 90 to 98% of the TDS with the DI resin removing the remaining TDS.
To troubleshoot your system you need 3 TDS readings, tap water TDS, RO only TDS before the DI and final or RO/DI TDS. Take all of these readings with your handheld TDS meter and directly from the filters, not from a storage reservoir. Use a squeaky clean, clear glass drinking water glass washed by hand in very hot water so it has no soap residue, dishwasher rinse aids or hard water spots. Starting with the tap water, fill the glass and isnert the meter, waiting until the readings for both the temperature and TDS stabilize.(Hopefully you bought a good handheld such as the HM Digital TDS-3, TDS-4TM or COM-100 all of which are temperature compensated (ATC) and have digital thermometers built in. Meters like the TDS-EZ and others are not and lack the accuracy.)
Once the reading stabilizes write it down then triple rinse the glass and the meter in RO/DI or distilled water. Next, remove the line leading from the RO membrane to the DI canister at the DI end so you get RO only water(or its easy to install a DI bypass valve for testing and flushing the TDS creep water to waste on start up) and fill the glass with RO only water. Again let the meter stabilize, record the reading and triple rinse the meter and glass in RO/DI or distilled water. Reconnect the line to the DI canister and get a RO/DI sample directly from the 1/4" treated water line, not from the ATO. Record the reading do a final triple rinse and put the glass and meter away clean and ready for next time.
Your RO only TDS should be 96-98% less than your tap water TDS in a 3 month old system. Using those two numbers you can calculate your rejection rate or removal efficiency. Tap TDS-RO TDS/tap TDSx100 is the formula. So say you have a tap TDS of 250 and a RO TDS of 15, 250-15=235, 235/250=.94, .94x100=94% rejection. In this example the membrane is a little inefficient and should be changed for better water quality and longer DI life.
You can use the handheld meter to test your ATO or most anything else except your tank water as its way out of range, somewhere over 32,000 TDS. Always rinse the meter when you are done though so the probes stay clean.
There are also things you need to look at like the age of your DI resin, if it was properly packed and stored as it has a short shelf life, what your water pressure and water temperature are, what your exact measured waste ratio is, if you store the unit when not in use make sure you keep the housings full of water so the membrane and resin do not dry out etc."
