The "sensor" is much more like a meter. If you open it up, you will see a paddle wheel inside. When the water rushes through the sensor, it spins the wheel. The wheel drives a shaft with a magnet on it, as the magnet spins the magnetic field changes from N to S to N, etc. There is a "Hall effect sensor" in the housing that is triggered by it N to S change. SO.......
We can determine how the volume of water is takes to move the wheel around 1 time. We can also determine how many wheel turn per second by counting the pulses from the wheel's hall effect sensors output. We can now find out the flow rate seeing:
Volume / time = flow rate.
The data sheet lists the calibrated pulses per volume for you (.00376 liters per pulse on average over the full flow range). So you now just have to write the code to count the pulse's per second and then use this math:
flow (lpm) = (pulses per second x .00376 liters per pulse) x 60seconds
4.73 lpm = (21 pulses per second x .00376 liters per pulse) x 60seconds