Campbell conductivity probe on Mayfly?

I don’t know of anyone who has used that particular sensor with a Mayfly, but it could be theoretically possible. That sensor works by sending it an excitation voltage and then measuring the differential voltage it returns. The Mayfly has an auxiliary A/D converter that allows you to do differential measurements. You’d also have to buy the A547 interface adapter since the sensor doesn’t connect directly to the logger. One issue is that the sensor is designed to use an AC excitation instead of DC, so it might not perform properly with a DC excitation. You could add some more circuitry and get fancy with your code if you wanted to generate an AC signal with the Mayfly, but that’s not going to be easy. And then there’s all the calibration equations needed to take the measured voltages from the conductivity sensor and the thermistor to turn them into actual units of EC and temperature. There’s some info and example CRBasic code in the manual if you want to try adapting that to Arduino code. Otherwise, I think the easier option would be to use a a different conductivity sensor that has an easier output like serial or SDI-12.