stregoi:
1- All my sensors are in the air, so i should expect a "0" reading. but i'am getting really random reading from the serial monitor every cycle ! (4 of my sensors are not connected to the multiplexer, does make any interferance ?)
What are your sensors and how are they providing a low impedance path to ground (0V) when 'in the air'? It sound like you might mean 'unconnected' which would allow your inputs to float and thus give fairly random results. INPUTS THAT ARE NOT CONNECTED TO A CURRENT SOURCE OR CURRENT SINK ACT AS ANTENNAS THAT PICK UP NOISE.