For the new Nano build I have Tunze Osmolator Nano 3152. It have only one float switch (its bigger brother (Tunze Osmolator Universal 3155) have float switch and optical sensor). I wanted my ATO have little bit security and redundancy. So I ordered typical float switch from fleabay and tried to wire two float switch in series. Problem with Tunze nano is that float switch operates in reverse. So when the water level is down the float switch shuts down and when the water level is correct the float switch is on. This is very weird decision by Tunze. I do not get this design because if the float switch wire is broken the water pump starts (and can cause flood in bad case). The ATO has 3 minutes safe guard which helps to prevent this problem but in small nanos and picos even 3 minutes running can be catastrophic situation.
First thing to do was found out how to reverse operation of normal float switch.
This can be easily done by just rotating the float device upside down (not the float switch but the magnet part of the switch). Then it works like Tunzes one.
So the case of wiring two float switch in series is out of the question, because then it would not work as needed. It would then be on if one of the float switches get stuck which I tried to avoid in first place. Answer is to wire them in parallel. When float switches are wired in parallel the pump is only running when both switches are in down position. If one of the switches stuck down the pump will close down when other float switch get up.
I also noticed that it is easy add another (or more) sensors just adding little plastic stick to original float switch holder.
After some soldering and testing the system is working again but now with some redundancy. Time will tell if this any good modification. But it still have the problem if the wire brokes down. I have not yet solved that part of the problem, but now there has to be two wires to be broken down before the pump starts if the float switches are in up position.