(Cheap) conductivity sensors are difficult to find and there's considerable number of references in Public Lab and, for example, Ed Mallon in his Cave Pearl Project Blog (excellent!). I'm still struggling with finding a good, solid basic circuit and have some Coqui-ish thing that I'm working on now. However, I was wondering if there is any reason why a TDS monitor ( e.g. the one by DFRobot) couldn't be modified to collect EC data? Obviously the DFRobot TDS monitor could be temperature compensated, the input scaled appropriately, and the probe would still have the over-voltage/hydrogen problem, but it seems that at $13 it's not a bad option.