here is a pretty easy auto topoff unit.
Construction
First of all, you need a food safe container with a "snap-on" lid. The largest I could find was a 4 litre container, but you may be able to get bigger.
I then drilled a hole on one side to take a 22mm 90 degree tank connector from B&Q. This was siliconed in place to ensure a water tight seal (VERY important!).
I then measured the distance from the connector to the surface of the water in the sump and cut a piece of 22mm plastic pipe. This pipe is inserted into the 22mm tank connector. The pipe is sufficiently tight fit to be waterproof, so is not glued into place. This means that the pipe can be removed and shortened if needed.
Before you use it, you need to make sure that air cannot enter the container by any route other than the end of the pipe. Otherwise the water will just leak into the tank.
How to use
The container is then filled with RO water. By placing my finger on the end of the pipe and carefully lowering it into the sump I can ensure that the pipe is below the surface of the water before removing my finger. The container rests on the edge of the sump. When the water level drops, air enters the container and water fills the sump!
Parts list
Parts required:
Container - £5.99
Tank connector - £1.80
22mm pipe - already had a short piece, but about £4 for a 2 metre length
Silicone sealant - already had this, but would cost about £5 for aquarium safe sealant.
Total cost:ca. £8 to £17
Improvements
A valve on the pipe would mean that I wouldn't need to place my finger on the end of the pipe, but have not had time to find a suitable plastic 22mm valve yet. A valve would also mean that I wouldn't have to wait until the container is empty before I refilled it!
