A while back I was able to get this type of flow meter to work with OpenSprinkler:
http://www.amazon.com/Liquid-Flow-Meter–Plastic-Threaded/dp/B00K0TFZN8/
The meter has a hall effect sensor inside that generates clicks which can be detected by a hardware counter pin (using a counter pin is more efficient than polling or interrupt). However, my understanding is that this type of flow meter is not inline, and it’s better to use a proper flow meter like this one:
http://www.amazon.com/Dwyer%C2%AE-Multi-Jet-Pulsed-WMT2-A-C-04-1-Economical/dp/B00D2ME6FI
I think they work the same way, but I haven’t had time to verify it yet.