When air pump is on, the nutrient will be forced up into the planters until the pump is turned off. The nutrient will then return to the reservoir through the help of gravity at a rate determined by the back flow of air through the pump and any leaks. Note: Air leaks are not necessarily bad, but will limit max height the water can be pumped while allowing for faster draining. Small amounts debris are easily flushed harmlessly to the bottom of the reservoir during the drainage cycle.
My timer is set to 2 minutes, but I let it run longer in the video to show what happens when the reservoir empties.
No check valve is required as it can't siphon through the air pump AS LONG AS the reservoir is not overfilled.
There is no overflow protection for the planters so a maximum level of nutrient will need to be established once filled with growing medium. This can easily be marked on the reservoir with the elastic band. Min/max levels should both be established this way. Cannot be seen well in video, but there's a max limit.
Put reservoir and planters in inverted chip bags to prevent algae growth. Double up if necessary. It works great. See DWC unit in finished system photo, but that's another story.
Due to the simple operation and modularity (if that's a word) of this system, it can easily be modified or expanded upon with ease to suit your space or container needs. PSI and volume are your main restrictions.
I apologize for the poor lighting in the video, but I have removed the bag so you can see the progress. Prepare to be bored! Especially by the last minute, forgot to edit that out, sorry.