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I caught your point that continual evaporation of that extra 1/2" that's not topped off will drift salinity up (not accounting for other salt export from creep, skimmer, spray, etc).
My point was just that the surge from the wc has to be infrequent enough so that the 1/2"+ evaporates and you get some FW top-off. In my 20g nano, daily evap is ~1/4" - so with daily SW inputs to a 1/2" high drain, the auto-top-off would never trigger! In other words, this system probably has some minimum tank size limit to work.
In fact, I guess I could get rid of FW top-off and just use your WCD but with brackish water instead of SW. The equation is something like:
salinity of WCD water = target salinity * (1 - e/(e+x)); where e equals estimated %evaporation per day and x equals the target %water overflow per day.
Having a salinity meter-controlled system would be much better, but I don't know if it's possible (and much more expensive for equipment)
Ryan
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