So my 55g drum or however big it is finally emptied over the weekend. Gave it a rinse and turned on the RO/DI to refill. I first flushed the membrane, after about 30 seconds the TDS in the post-RO (pre-DI) step came down from about 80 tds to about 15 tds. For the remainder of the fill the tds on this step hovered around 13-15. That seems not so bad, although it's not the 2 or 0-5 that Monza and Emilyb report. (Anyone else measure the TDS between the RO membrane but before the DI? What are the numbers you get?) Post-DI the tds is a solid zero but the resin is now almost halfway consumed.
So I think what's happened in my case is the constant cycling of the unit due to the top off for multiple tanks led to premature failure of the membrane. 15 tds isn't too bad but I think I'll budget for replacing the membrane again and take it as a lesson learned.
There is an interesting idea for combating "TDS creep" in the latest reefkeeping.com featured aquarium where the guy has his float valve inside a chamber inside the reservor that doesn't kick off the float valve until the reservoir is completely empty, thus allowing the RO to kick on only to fill the reservoir from empty (instead of the constant on-off to match the evaporation rate).
http://www.reefkeeping.com/issues/20...totm/index.php
Although given how long my drum lasted before refilling I think I can live with manually turning on and off my RO from now on.