The installation is a Rosemount condutivity probe connected to a Rosemount 52 transmitter. When the sensor is in line, the display is "--.--". When the probe is placed in a jar of water, there is a good reading. We checked the grounding wiring of the instrument first, no problem there. Eventually we found a 3.3 volt DC potential between the pipe and ground, and verified that one of the vessels in the flow path has cathodic protection. Any recommendations on how to isolate the probe from this voltage?