Awesome thanks for the quick reply. There's also this liquid flow meter which is less expensive, but more accurate. http://www.adafruit.com/product/828
It sounds like the same thing. The description seems like it will help with the math.
The sensor comes with three wires: red (5-24VDC power), black (ground) and yellow (Hall effect pulse output). By counting the pulses from the output of the sensor, you can easily track fluid movement: each pulse is approximately 2.25 milliliters. Note this isn't a precision sensor, and the pulse rate does vary a bit depending on the flow rate, fluid pressure and sensor orientation. It will need careful calibration if better than 10% precision is required. However, its great for basic measurement tasks!