Why do I need an RODI reservoir?

[DogFishMD]

I've been ruminating about the best way to get a new ~150 gallon tank started. I plan to use RODI for auto top-off and saltwater mixing, and would ideally like to have a fancy salt mixing station with the capacity for at least 1 month of salt water to support ~10% weekly water changes. The idea is to not have to make saltwater more often than once monthly.
If I assume a total system volume of ~150 gallons (e.g. the Waterbox 150.4 Reef) I would need ~60 gallons of mixed saltwater monthly. Most solutions I've seen involve an RODI reservoir of equivalent size so I would need 2 x 60 gallon containers and the footprint for a system like this is simply too large for the space I have.
So my epiphany today was why do I need an RODI reservoir at all? If I have a reasonably sized RODI unit, I think I could "T" the output and run one line to the auto top-off controlled by gravity with a float/optical switch with solenoid shut-off backup and should easily be able to keep up with daily evaporation. The other T line could run directly into the saltwater reservoir and when more water is needed, I could top off the salt reservoir with RODI, add the correct amount of salt based on the volume of added water, and should be able to mix up the needed amount of saltwater monthly that would then be available for auto or manual water replacements. A relatively inexpensive in-line digital flow meter or Apex could determine the actual volume of each freshwater bolus and the appropriate amount of salt could then be added to maintain proper salinity. An even simpler option would be to top off to a known volume and subtract the starting volume to determine the amount of added RODI. Seems to me this cuts the footprint of a system in half, still provides for adequate daily RODI top-off, and reduces space requirements to that needed only for the largest saltwater reservoir I can manage.
Sorry for the lengthy post but I have not seen similar solutions. I think this will work. What say you in the community? Am I on the right track or have I not thought this through correctly?
Thanks to all!

 

[Fish Styx]

You'll burn through membranes like lightening due to creep just topping off from the RODI unit. A smaller system would be better, where you make water more frequently than your desired one month schedule; but, you will still go through membranes faster for the same reason (making smaller volumes at a time).

 

[DogFishMD]

Thank you so much for the great input!
I did not understand TDS creep and now I see why that’s a problem.
There appear to be some relatively expensive solutions that automatically purge the membrane with zero TDS water after each use so there are no residual dissolved solids in the feed line when you start the next production cycle. There do not seem to be other straightforward ways to automate RODI production that would allow purging of the line to avoid TDS creep. if anyone has additional suggestions, I would love to hear them!

 

[Snoopy 67]

I use a 30 gallon drum for ato & water for changes but I have a smaller tank, 100 Gallons.
I would look for a 55 gallon container & use that for both water changes & ATO.
You would wind up making RO water every other week.

 

[Zero_Cool]

Reasons for a larger RODI storage container include emergency RODI water needs and longer RODI membrane lifespan.

If you need to run a large water change and only have a smaller amount of saltwater made it will take time to produce the RODI needed for mixing more saltwater.

As mentioned above, frequent on/off of the RODI unit leads to rapidly declining membrane performance. Making a larger amount of RODI at one time is much better for the unit.