RPiWaterMeter
From wiki.breedveld.net
This is the way I measure water-usage with my Raspberry Pi
It works with a magnetic sensor, which can be used because of the half-metal rotor of watermeters in my country (Netherlands)
For magnetic sensor I use the LJ12A3-4-Z/B with a piece of aluminium it's easy to mount on the meter:
Shematics:
