Reading electrochemical events in water that is electrically noisy can be a bit tricky. Pumps, heaters, chillers even overhead lights can leak electrical noise into a seemingly closed system. When using a hand held device, such as a pH meter it is automatically immune from electrical noise, this is because its battery powered and does not share a common ground with the rest of your system.
To stop electrical noise you will need to supply your circuitry with isolated power and data lines. The Atlas Scientific PWR-ISO is a simple 8 pin module that will eliminate electrical noise in your sensor application.
Unisolated power and data lines connect on the IN side.
Isolated power and data come out on the OUT side