Do you have a link to the flow sensor? Does it require constant current, or is it that it outputs current that’s proportional to the flow rate? If it’s the latter, there isn’t an easy way to make it work with OpenSprinkler — because in that case it’s outputting an analog signal, which needs to be fed into an analog pin. That requires changing the firmware to support such sensors. In contrast, the supported flow sensors work by sending pulses, and the frequency is proportional to the flow rate.