Cherryman, great animation!
I also the saw the videoclip, so we see that the force of buoyancy can overcome the static friction of the seal. I wonder if adding a lubricant could decrease the resistance further and still make it watertight?

So "in essence" you have simplified the first design I made, as instead of using a long cylinder for both chambers you connected them with a pipe, thereby making the components more accessible for normal people.

I'm not sure if this is correct, but isn't the number of valves you would need here 3, one at the red dot, one in the piston and one somewhere in the return pipe, to keep the pressure of the water column from flowing in when the piston valve is open?


Purely by guessing, since you are using the force of the water's weight to push the long column of water up, you won't probably need that much of a weight to overcome the hydrostatic pressure, since you need something like 11 meters of water just to create 1 bar, or double atmosphere.

Btw I like your design ;D
Julian