Let's take these one at a time. Again, I think there is some sound advice in here, but it is mixed with poor practice.
And what happens when the water evaporates in the sump? The difference between sucking a little air and sucking just air is about 1/4". It depends on the return section, but that's about 1.25 cups of water in the SCA sump. I won't get into every nitty gritty detail here, this is just bad advice even as a fall back plan.
It is a lot more than 1/4". The impeller in this pump is pretty large. I suspect a full gallon of water or possibly more would be able to evaporate before it is actually dry. This is also not a running state that is expected to last for any length of time.
It is called an emergency drain because its job is to handle the full flow of the return pump should the primary drain become clogged. I refer to that situation as an emergency, but the name isn't important. If your second drain can't handle the full flow of your return pump, you can get a flood when the primary drain clogs. See my previous post about why having your DT "absorb" the extra volume from your sump is a bad idea. The second drain is not needed to tune the siphon, it is however part of the drain system. You can run your tank off of just one siphon if you were able to tune it perfectly and never have the lines foul. The second drain takes up any extra flow that your siphon isn't handling. As the water level in your sump changes due to evap the level in your tank will also change slightly. Rather than needing to constantly adjust your siphon, the second drain handles the difference.
And again, I DON'T call it an emergency drain since I don't think an emergency drain is needed, and I prove that with this video. The video clearly shows a flood is not possible. And again, the 2nd drain IS needed to tune the siphon. Have you ever tried to maintain a full siphon without an open channel for any length of time? I have. "If you were able to tune it perfectly" - you can stop there, you can't.
This is not true:
As the water level in your sump changes due to evap the level in your tank will also change slightly.
As long as the siphon is maintained, so will the water level. This system can fluctuate as much as 3 gallons without any effect on the water level in the DT or the overflow compartments.
I agree, you want the second drain higher than the siphon. The difference in height between them represents the amount of water that can flux within the overflow your system can handle. If it is too small, you'll have a hard time finding that happy medium. Too big and you wind up draining a lot of extra water to your sump when the pump is off. That may not be a big deal if your sump is properly sized for the back flow.
I don't understand the flux logic. If the siphon is running, the levels are the same, all the time. The level within the overflow effects 2 things.
1: noise level from water cascading over the edge of the overflow
2: amount of extra water that will drain back to sump when the pump is off. The higher the full siphon, the less that will drain. This can be mitigated with proper system design. ie: my system can handle the entire volume of the overflow back to the sump. Keep in mind your standpipes should not be glued in (for maintenance reasons) they should be just set in place (I prefer friction fit) so they are not water tight at the bottom and eventually could leak back in the sump. This volume needs to be accounted for in the system design.
*****
The scenario shown in the video works with the system shown. One can decide of this setup will translate into your particular scenario. Rimless tanks, wavemakers, lower volumes, etc, etc are for those that have these variations to test and ensure are appropriate as each setup is a little different.
I have heard a lot of what you say, which is, quite frankly, what prompted me to make the video. It is worth 1000's of words.
One thing I do want to point out is I do not have an ATO set up on this tank. I add water daily at the moment. I have an ATO system I am designing (using the Tunze Osmolator) that will allow the tank to run as safely and flood free as it does now while also adding the convenience of an ATO. Just adding an ATO to this setup depending on how much the ATO holds could result in a flood (but probably won't). Keep in mind though, most ATOs have an auto-shutdown if they pump too much water (if the pump runs too long). My Tunze Osmolator pumps maybe 1/2 gallon before it will shut down (permanently until I reset it). JBJs also have this feature.
