To determine the efficiency of your system you need 3 numbers. Tap water TDS, RO only TDS before DI and finally RO/DI TDS. Without those 3 its impossible to tell.
Remember, prefilters and carbons have very little to absolutely no effect on TDS, they are there to protect the more expensive membrane which is the workhorse of the system. The efficiency of the membrane is what determines how long the DI lasts and how well it works.
Post your three TDS reardings here along with what membrane you have and we can help you out.
Also make sure you are getting accurate test results. Always draw your sample directly from the filter not from a storage tank or bucket. Use a squeaky clean clear glass drinking water glass with no soap residue or water spots to capture the sample and always triple rinse both the glass and the TDS meter in DI water after each sample to make sure they are clean.
To give you an example, I test my tap water first, let the TDS meter stabilize and get a reading of 835. Next I triple rinse the glass and meter probe in DI then sample the RO only, again letting the meter stabilize and get a RO TDS of 6. I do another triple rinse using DI then take a RO/DI sample, let it stabilize and get a readin gof 0.0 . I do a final rinse of the glass and meter, turn the glass upside down so it stays clean and cap the meter so the probe stays clean.
My Tap Water TDS is 835 and RO only is 6 so that means I am at 99.28% rejection or membrane removal efficiency. My RO/DI is at 0 so that means the DI is doing its job too. When I start to first see a reading of anything other than 0 on a consistent basis I change my DI cartridge since DI begins to release weakly ionized substances even before it is exhausted. I don't give it that chance.
