I wouldn't call it a siphon, a siphon uses suction to pull the liquid through a tube. This is just a simple drain. The pipes are connected at the bottom so they are considered to be a single container and no suction is required to bring water from the first chamber into the second chamber. As the water level equalises you are right that the drain height sets the level of water in both chambers, this is why my drain is only 1cm lower than the input.
Yes, agreed. But my point is (and I confess I'm not 100% sure about this), is that it's the exit of the drain tube that matters, not the level where it connects to the second tube. In other words, there IS suction created inside the second tube (and first tube, since they are connected to each other and effectively the same container of water), which means the level of water in the first tube will lower to match the outlet of the drain tube.
I don't mean like the first drawing. You are right, that could never exist. If I had your illustration skills, I could easily show what I mean, but I don't. If there is a siphon (and I think there is), the water level will not lower in the second tube, as your first drawing shows, but in the first tube. Think of the three pieces -- the connecting pipe at the bottom, plus the second tube, plus the outlet tube, -- as one long tube, which they effectively are when the top on the second tube is sealed. The only water level open to atmospheric pressure is in the first tube, so that is where the siphon would lower the water level.I believe you are saying that if the exit tube outlet sits 5cm below where the exit tube actually goes into the pipe that the water level inside would be in line with the outlet of the exit tube and not the top of it (5cm lower), is that right?
If this was the case then water wouldn't be able to even reach the exit tube (like in the terrible drawing above). Even if there was enough water to start a siphon it would only lower the water to just below the top of the exit tube and then break the siphon as no more water could get in. Water would then begin to rise up again until a siphon started and then the process would repeat.
In reality it is more like the terrible drawing below, the top of the exit tube sets the height of the water level in both chambers. Even if you put so much water into the first chamber that it can't escape the second fast enough, a siphon will start to bring the water level back down to the top of the exit tube before it breaks and then rises and starts a siphon again. But you shouldn't be running that much flow (at least not according to Donovan) so any water going in should just be dribbling out at the same speed it is going in.
But either way, the top of the exit tube sets the water level
Whether it creates a siphon or not has to do with the flow rate in the pipes. In this case the flow rate is so low that there is never a solid column of water which is needed to create a siphon. The water drains out of the bottom of the tube much faster than it is coming into the top so air fills most of the tube.
