The flow sensor used here works on the principle of "Hall Effect". According to which, a voltage difference is induced in a conductor transverse to the electric current and the magnetic field perpendicular to it. Here, Hall Effect is utilized in the flow meter using a small fan/propeller shaped rotor which is placed in the path of the liquid flowing.
The liquid thus pushes against the fins of the rotor, causing it to rotate. The shaft of the rotor is connected to a hall effect sensor. It is an arrangement of a current flowing coil and a magnet connected to the shaft of the rotor. Thus a voltage/pulse is induced as this rotor rotates. In this flow meter, for every liter of liquid passing through it per minute it outputs about 4.5 pulses. This is due to the changing magnetic field caused by the magnet attached to the rotor shaft. Arduino measure the number of pulses and then calculate the flow rate in L/hr using a simple conversion formula.
The sensor comes with three wires: red (5-24VDC power), black (ground) and yellow (Hall effect pulse output).
Serial.println (" L/hour"); //Prints "L/hour"
Open Serial monitor and allow flow of some water from sensor or blow air in flow sensor to rotate the rotor.