i was under the impression that the water level should never be low in the reactor so please explain the high/low switch. I have not seen that on a dyi reactor before
The principle of CO2 absorption is that the rate is driven by surface area achieved by a thin liquid film on the media. The other factor is that the liquid film should be rapidly replaced (reasonable water flow) on the media (it has to do with the much slower diffusion of CO2 in water than in air).
This is the same design principle used on industrial CO2 absorbers. Our lungs work on the same general principle too.
The reactor is never really dry as the media with high surface area is constantly being wetted. The CO2 rapidly absorbs and equilibrates with the thin liquid film. The flow of water has to be fast enough to wash out the CO2 saturated water but not too fast to flood the media - which in turn reduces the air-liquid surface area. So the reactor is never truly 'dry' while the pump keeps delivering water.
On the very top - the internal nozzle is paired with a venturi to mix CO2 in the reactor dome with water and spray that out in a coarse spray.
The float switch is a neat way to regulate the delivery of CO2. It keeps the water level from inadvertently rising and choking off the media. The second purpose of the lower water layer is as an air/bubble trap - this eliminates the 7UP effect from your tank.
When the water flow stops CO2 is no longer being absorbed and removed from the reactor and the water level does not trigger the solenoid.
I hope my explanation makes sense...