This thread is already old, but as I had the same problem with floating voltage on flow meter and I managed to fix it, here is what I did.
The SN1 input is supposed to have a pull-up resistor that should prevent noise on the wire when the flow meter is in opened state.
But after reviewing the hardware design files (https://github.com/OpenSprinkler/OpenSprinkler-Hardware/tree/master/OS/3.0/AC_driver) and opening my own OpenSprinkler device, it seems the resistor is just wired in-line, and not as a pull-up resistor (that should be wired between 5V and SN1).
So I simply added a 4.7kOhms resistor between 5V and SN1, everything works perfectly now. Higher resistor values might work as well, it depends on the level of noise you have on your wires.
Do this at your own risk 😉