YF-S201 Hall Effect Water Flow Meter / Sensor

This sensor sits in line with your water line and contains a pinwheel sensor to measure how much liquid has moved through it. There's an integrated magnetic hall effect sensor that outputs an electrical pulse with every revolution. The hall effect sensor is sealed from the water pipe and allows the sensor to stay safe and dry.
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 calculate water flow. 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!
We have as example Arduino sketch that can be used to quickly test the sensor, it will calculate the approximate flow of water in liters/hour. arduino code is at the bottom.
The pulse signal is a simple square wave so its quite easy to log and convert into liters per minute using the following formula.
Pulse frequency (Hz) / 7.5 = flow rate in L/min.
Features:

• Model: YF-S201
• Sensor Type: Hall effect
• Working Voltage: 5 to 18V DC (min tested working voltage 4.5V)
• Max current draw: 15mA @ 5V
• Output Type: 5V TTL
• Working Flow Rate: 1 to 30 Liters/Minute
• Working Temperature range: -25 to +80?
• Working Humidity Range: 35%-80% RH
• Accuracy: ±10%
• Maximum water pressure: 2.0 MPa
• Output duty cycle: 50% +-10%
• Output rise time: 0.04us
• Output fall time: 0.18us
• Flow rate pulse characteristics: Frequency (Hz) = 7.5 * Flow rate (L/min)
• Pulses per Liter: 450
• Durability: minimum 300,000 cycles
• Cable length: 15cm
• 1/2" nominal pipe connections, 0.78" outer diameter, 1/2" of thread
• Size: 2.5" x 1.4" x 1.4"

Connection details:

• Red wire : +5V
• Black wire : GND
• Yellow wire : PWM output.
• Green temperature wire

Features:
1, the product looks light and flexible, small size, easy to install.
2, the impeller inside is stainless steel shaft.
3, the use of seals on the structure of the force never leak.
4, Hall sensor components imported from Germany, and potting package, to prevent water, never aging.
5, all raw materials are in line with ROHS testing standards
6: contains temperature sensor.

product description:
Water flow sensor mainly by the plastic body, the flow of rotor components and Hall sensors. and contains temperature sensor.
It is installed in the water inlet end of the water heater, used to detect the influent flow, when the water through the water flow rotor components,
The magnetic rotor rotates and the speed varies with the flow rate, and the Hall sensor outputs the corresponding pulse
Signal, feedback to the controller, by the controller to determine the size of the water flow, to control.

Precautions for use:
It is strictly forbidden to violent impact and erosion of chemical substances.
Do not throw or hit. 
Medium temperature should not exceed 120 degrees

the output waveform:  square wave
wires:
1 red IN connected to the cathode
2 yellow OUT signal output line
3 black GND connected to negative pole
4 green R temperature sensor

Arduino code
/* YF-S201 Hall Effect Water Flow Meter / Sensor Read Water Flow Meter and output reading in litres/hour*/volatile int flow_frequency; // Measures flow meter pulsesunsigned int l_hour; // Calculated litres/hour unsigned char flowmeter = 2; // Flow Meter Pin numberunsigned long currentTime;unsigned long cloopTime;void flow () // Interruot function{ flow_frequency++;} void setup(){ pinMode(flowmeter, INPUT); Serial.begin(9600); attachInterrupt(0, flow, RISING); // Setup Interrupt // see http://arduino.cc/en/Reference/attachInterrupt sei(); // Enable interrupts currentTime = millis(); cloopTime = currentTime;} void loop () { currentTime = millis(); // Every second, calculate and print litres/hour if(currentTime >= (cloopTime + 1000)) { cloopTime = currentTime; // Updates cloopTime // Pulse frequency (Hz) = 7.5Q, Q is flow rate in L/min. (Results in +/- 3% range) l_hour = (flow_frequency * 60 / 7.5); // (Pulse frequency x 60 min) / 7.5Q = flow rate in L/hour flow_frequency = 0; // Reset Counter Serial.print(l_hour, DEC); // Print litres/hour Serial.println(" L/hour"); }}