I found an easy, inexpensive way to make the Hunter flow sensor compatible with opensprinkler, even though it may not be optimal. I found this board on the ebay:24V 4-20mA Current Detection Limit Value Control Switch Alarm Converter Module. The reason it is not optimal is that the relay should be a reed relay for reliability. I biased the sensor with 12 vdc with a separate 24vac to dc converter. I’ll attach a picture of the setup. I’m interested if anyone has a better approach.