RE: Auto Start Siphon description
Is the auto-start feature simply that the spillway will begin siphoning water from the bottom when the air vent at the apex is submerged in water and will stop once the water level is below the air vent at the apex?
Not exactly, PJB. The air vent must be submerged AND the water level inside the siphon has to rise to, or somewhat above, the bottom of the siphon pipe at the apex for the siphon to start. Once water begins to flow, and the vent tube has all entering air blocked, gravity and atmospheric pressure take over and the siphon effect begins.
The vent tube is set to determine what the water level must reach for the siphon to STOP by allowing air to enter as a "vacuum break". If you set the entrance of the vent tube a couple inches below the pipe apex (and no whirlpool allows air to enter) the siphon would stop when the pond is 2" below the bottom of the apex....and no water would flow out at all.
If you set the vent tube a couple inches ABOVE the apex of the siphon pipe, the pipe will act like any normal drain pipe UNTIL the pond rises a couple inches higher and blocks the air entering the siphon pipe. Gravity will make the pipe drain slightly faster once the pond is above the top of the apex, but until air is blocked, it will not siphon.
I like setting the vent a little above the Apex.
You can also incorporate a "normal" drain into a siphon by adding 2 wye's and a valve. You connect a pipe straight through the bottom of the dam, wye'd into the riser pipes that will siphon, and a valve in the straight pipe on the back side of the dam using an frost valve access similar to the old water meter pipes.
