Can you say a little about your application/project – is it something you are collecting data to supply to others or a kick-off learning project, how urgent is the project, is it outdoors with no power, does it have a cabinet, what range of water level/accuracy and placement (a well or a stream?). What range of conductance and whats the analysis you are looking to do with the data. Conductance can have simple and complex interpretations.
What is the most critical parameter you are measuring?
If you are looking to experiment with the Decagon CTD, then looking at its protocol interfaces SDI12 is the starting point,
I’d have to scout around for and SDI12 logger at this point, though I do have something I purchased but haven’t used yet. I do have a plan to make an SDI-12 logger that would interface to amongst other things the CTD.