I have done lots of tests connecting Arduino to a conductivity sensor and came up with this solution.
There are lots of things that can go wrong. For example
Electrode polarization. Designs where dc voltage is applied to the electrodes.
Length of the wire from sensor to Arduino.
Length of the sensor wire to the EC shield.
I designed my own sensor and transmitter and it works just fine. You can be 50ft away from the sensor and still register steady accurate conductivity values to Arduino Uno.
You may see it here
http://webpages.charter.net/tdsmeter/index.html