Yes we thought also about two sensors in the upper tank and also two for the lower.
Our first idea was to use float sensors and also pressure switchs
I'm not sure to understand well, you mean two float sensors in the upper tank and can you be more precise one the term : "a fill sensor " ?
Finally I can use two of them in the upper tank,
one will be ON when there will be too much water,stopping the pump
the other will be OFF when there won't be enough water, starting the pump
that will keep a constant level except if we have a water leak and in this case we will have the third sensor which will be OFF when the minimum level of water is reached in the second tank stopping the system.
Now for the components, we have to supply them with a voltage as 5 V and get back this one through the analogic inputs ?
These three float sensors http://uk.rs-online.com/web/p/level-sensors-switches/0519242/
are fitted ?