It is a Hunter flow sensor. I bought the sensor without the control module. Hunter does not seem to be to willing to share how their sensor works, but I did some experimenting and biased the two wires out of the sensor with various voltages and measured the current. As the paddle rotates the current changes from a low state (~4 mA) to a high state (~20 mA). Here are the actual measured results.
One way I thought of solving the problem is to drive a reed relay with the 4-20 mA current, that way I could isolate the sensor from OpenSprinkler and use a separate power supply to power the flow sensor. However I am having a bit of trouble converting the current to a usable voltage to drive the relay. I was wondering if this approach seemed reasonable or if there was an easier and more elegant way the community could suggest that I am overlooking. Unfortunately, the voltage across a 250 ohm resistor does not work since it converts the 4-20 mA to 1-5 VDC and the 1 VDC is above the threshold of the turnoff voltage of the relay. Maybe what I am really after is a circuit to convert 4-20 mA to 0-5V to drive the relay powered by a 12 or 24 volt power supply. I have a separate power supply that converts 24 VAC to 5,12 or 24 VDC.