Whenever I start my RO/DI system, I drain the RO water off before the DI resin until the TDS has dropped, typically 5-10 minutes or so, then I switch it over to run through the resin. Currently I do this by hand but it would be really convenient to have timer and some solenoid valves do it automatically. I know I could do it with an Apex, but that's overkill for this application. Does anyone know of a ready-built setup to do this?