Thanks for your input on my last question, Randal. I guess I'll just get the 5 um filters from now on.
As far as the placement of the TDS meter, I was recently told by someone that it's preferable to put it between the RO and the DI. The reason I was given (which this person had gotten from someone at BRS) was that if you let your pre-filters go too long and they're no longer doing their job, Cl ions can get through and punch holes in the RO membrane. The DI media will keep the TDS in your final output down for a while, but will exhaust quickly once this process begins. So, by the time your TDS meter is indicating you have a problem, your RO membrane and DI resin are pretty much shot. Therefore, if you put the TDS meter right after the RO, you will start to see a rise as soon as this problem begins ans you can change out your filters before the damage progresses too far.
Obviously, if you change the sediment and carbon filters often enough, this is a non-issue. But is that information correct? It seems to be a reasonable explanation for what happened recently with my RO/DI unit (sudden skyrocket from 0 to 15 ppm at the final output as the DI resin completely changed colors over the course of a week or two--and the sediment and carbon filters were definitely well past their expiration dates).