Thanks VFR, you are right about highlighting the EASA procedures I don't know if the school were I freelance will end up buying this CO detector or not.
So far I got this for me and it happened to be there for that picture.
There were some flights on C152 and Arrow that I ended up with a strange inner headache and my logic was to suspect small dosis of CO, which with this detector I managed to prove. The sticker one will give you strong warning but the electronic will inform you sooner.
So far the AC were inspected and fixed (small leaks) and since then those headaches are gone.
There must be more CO detectors at cheaper prices, so far this is working fine.
We always talk/read about big doses of CO but there's nothing about continuous contamination of small doses... I believe that in the long run it affects us.