[QUOTE=xilman;442161]Why doesn't this work?

[code]
| |
|~~~~~~~~~| |.|
|.........| |.|
|.........| |.|
|.........| |.|
|.........| |.|
|.........| |.|
|.........+--+.|
|..............|
|.........+----+
|.........|
+---------+
[/code]where ." is water and "~" is oil. The overflow pipe is raised from the bottom of the tank to collect sludge so that the system (cistern?) doesn't have to be drained and cleaned so often. The rim of the tank is markedly higher than that of the overflow pipe so that the weight of any excess pumps the water through the narrower pipe. It can be overwhelmed with a sufficiently great inflow but that's true of any device.

There is no possibility of syphoning and the lack of moving parts makes for a simple and robust solution.[/QUOTE]

That would work, but there's no U-shaped tube (I can see that the overall assembly is U-shaped).

[QUOTE=xilman;442161]Why doesn't this work?
...
There is no possibility of syphoning and the lack of moving parts makes for a simple and robust solution.[/QUOTE]The purpose of the oil is to prevent mosquitoes breeding in the free water. The overflow pipe while functional and preventing the spillage of oil adds a surface of water open to the air and to the mosquitoes.

No idea if it would work in practice but would not a a mosquito net over the cistern be the solution ? That net would have to be cleaned periodically of course.

Jacob

[QUOTE=S485122;442175]The purpose of the oil is to prevent mosquitoes breeding in the free water. The overflow pipe while functional and preventing the spillage of oil adds a surface of water open to the air and to the mosquitoes.[/QUOTE]What is to prevent the use of a check valve on the exit of the pipe? Water flows out, bugs stay out.

[QUOTE=S485122;442175]The purpose of the oil is to prevent mosquitoes breeding in the free water. The overflow pipe while functional and preventing the spillage of oil adds a surface of water open to the air and to the mosquitoes.

No idea if it would work in practice but would not a a mosquito net over the cistern be the solution ? That net would have to be cleaned periodically of course.

Jacob[/QUOTE]Put the net over the open pipe. Much smaller area to cover and the upward flow of water will tend to keep it clean.

[QUOTE=xilman;442206]Put the net over the open pipe. Much smaller area to cover and the upward flow of water will tend to keep it clean.[/QUOTE]I fiddled with the idea of the net over the overflow pipe, but abandoned it for the simpler solution. I had not thought about the cleaning effect, the set-up is almost perfect now :-)

Jacob

[B]xilman[/B]'s idea is ingenious.

Here is an all Flexible-Tube/Hose version of it which does not require adding a hole.
It is probably what [B]Mark Rose[/B] had
in mind.

Notes:

* The inside-the-tank end of the tube must be higher than the height of the mineral oil.
* Many flexible tubings are carcenigic. Make sure to choose properly.
* Initial siphon would have to be established and will be maintained as long as there is no blockage or the tank goes empty.
* The inner diameter of the tube must be large enough to relive heaviest anticipated precipitation.

Just to check in... I very much appreciate all the comments, observations and suggestions.

At the end of the day, the issue with our rain water tanks stems largely with our plumber who installed them, and the interconnecting plumbing. He got paid, and then disappeared. (Warranty on construction? Six months if you are lucky around here.)

It sounded (and looked) like it would work. Except possibly under extreme conditions.

We're now working on a solution space which works even in extreme conditions.

[QUOTE=a1call;442246]Notes:

* The inside-the-tank end of the tube must be higher than the height of the mineral oil.
* Many flexible tubings are carcenigic. Make sure to choose properly.
* Initial siphon would have to be established and will be maintained as long as there is no blockage or the tank goes empty.
* The inner diameter of the tube must be large enough to relive heaviest anticipated precipitation.[/QUOTE]So when it's not raining the inside open end is above the water level, right?

If so, won't the oil be the first into the pipe when it does rain? Result: all the oil is flushed out before the water. Either I'm missing something or that is the situation about which the original complaint was made.

[QUOTE=xilman;442255]So when it's not raining the inside open end is above the water level, right?
[/QUOTE]
No, what I meant is the inside end should be above the base of the tank at a height that is greater than the thickness of the mineral oil volume.
This is so that if the tank is near empty due to consumption the oil would be too low to fill the tube and pour out next time it rains. It is a rare occurrence prevention. It is also important as mentioned by others for the inside tube end to be above the bottom debries.
The point is to not install the inner end too close to the bottom.

[QUOTE=xilman;442255]If so, won't the oil be the first into the pipe when it does rain? Result: all the oil is flushed out before the water.[/QUOTE]

We had a 2" "u-turn" pipe for the drainage inside the top of the north-most tank. Effectively an upside "U".

So, a little bit of the oil would have flowed out first during overflow, but only a tiny percentage. And syphoning wasn't possible because air was always ready to break the syphon. (Importantly, the source of air is completely underground, and can't be accessed by mosquitoes.)

And, in fact, this worked. It turns out that the north-most tank didn't spill it's oil.

[QUOTE=xilman;442255]Either I'm missing something or that is the situation about which the original complaint was made.[/QUOTE]

The "original complaint" was our plumber relied on a 1" interconnection pipe at the bottom of the tanks to balance the levels of three tanks during rainfall. ("Water will find its level", he once told me.)

We got 4cm of rain in less than an hour; two of our three tanks didn't have a direct overflow drain pipe.

A bit like a Stupid Programmer Error (SPE), this might be called a Stupid Plumber Error.

And, perhaps more embarrassing as a customer, this was a Stupid Client Error (SCE) for accepting something which wouldn't work under load....