dieden
Exocoetidae
Would a reservoir and a open line not be enough? Mount the open line at the desired water level... As long as the level is equal or above the open end of the line, the vacuum keeps RO water from releasing into the sump. When the water drops below this line (due to evaporation) the vacuum is broken and RO water is released until the water line raises above the designated water line again. It would, however, require the reservoir to be mounted above the sump. Kind of the same thing as a dogs water jug with an upside down jug of water feeding a bowl.