Yeah, pretty much. For those of you who have inline TDS meters, watch the TDS when you first turn on the water flow. It "should" be sitting at zero, and when the water starts flowing, it will increase. I've seen as high as 30 initially, then it works its way back down to zero. I'm not an engineer, so I can't tell you WHY the RO membrane does this, or perhaps it's a function of the DI and needing to be charged. Honestly, I don't know what causes it.
Anyway, the problem is that this initial "blast" of TDS happens every time you first turn on the water flow from your RO/DI. It's referred to as TDS creep, because where else is the TDS going but into your tank/reservoir?
So a fellow reefer (Weatherson) developed an anti-TDS reservoir. Another reefer (Brian Renegar) whipped up this diagram of what's happening. It's a pretty good illustration, so I won't go into too much detail. The idea here is that initial TDS blast only happens once per filled reservoir, instead of constant, as happens when the RO/DI is connected directly to a float valve in the sump:
I've heard from
my best friend's ex-roommate's uncle's hair dresser's carpool buddy that Spectra-Pure now puts out an RO/DI that automatically dumps the first portion of filtered water down the drain before it starts producing, but I haven't verified this. Maybe someone else knows more.
Otherwise, is everyone just running their RO/DI line right into the sump/reservoir? If not, what's your setup? At this time, I manually fill my reservoir about once a week, and would love an automation idea, such as posted in the image above.