DVDdoug:
Opto isolators are not linear (they don't work for analog).
Isolated or not you probably can't run them both at the same time, so you'll have to power them on one-at-a-time.
Right. Most opto-isolators are for switching digital signals, not power. What's 200mA for?
well i came across these and they look promising. right now my ph sensor works if there are absolutely 0 voltage leakage from the pumps inside the water im measuring. but i was thinking of adding the ec meter but im thinking about the ground loop situation. could i use one of those for each sensor and that solve the problem? the sensor supposedly only uses 20 - 40ma