just to finally document incase someone at a future date is looking to see the conclusion of this thread:

Neptune Systems FS-100 flow sensor:
hall sensor output >> internal red wire >> 3.5mm plug tip
hall sensor V+ input >> internal green wire >> 3.5mm plug lower ring
hall sensor ground >> internal black wire >> 3.5mm plug Sleeve

 

Yay, it works. Now that you should get pulses for flow you can work on the K-Factor I mentioned earlier. Looking at the data sheet, what I could find anyway for the whole sensor I saw no mention of Pulses for unit of measure for example pulses per liter or gallon. That will figure into your code you write.

Ron

 

If in doubt, read the instructions.
Max.