byte sensorInterrupt = 1;
byte sensorPin       = 1;
float calibrationFactor = 4.5; // According to docs: the sensor outputs 4.5 pulses/sec per L/min of flow
volatile byte pulseCount;
float flowRate;
unsigned int flowMilliLitres;
unsigned long totalMilliLitres;
unsigned long oldTime;
void setup()
{

  Serial.begin(9600);

  pinMode(sensorPin, INPUT);
  digitalWrite(sensorPin, HIGH);
  pulseCount        = 0;
  flowRate          = 0.0;
  flowMilliLitres   = 0;
  totalMilliLitres  = 0;
  oldTime           = 0;
  attachInterrupt(sensorInterrupt, pulseCounter, FALLING);
}
void loop()
{

   if((millis() - oldTime) > 1000)  // one counter persecond
  {
    detachInterrupt(sensorInterrupt);
    flowRate = ((1000.0 / (millis() - oldTime)) * pulseCount) / calibrationFactor;
    oldTime = millis();
    flowMilliLitres = (flowRate / 60) * 1000;
    totalMilliLitres += flowMilliLitres;

    unsigned int frac;

    Serial.print("Flow rate: ");
    Serial.print((flowRate));
    Serial.print("L/min");
    Serial.print("\t");
    Serial.print("Output Liquid Quantity: ");
    Serial.print(totalMilliLitres);
    Serial.println("mL");
    Serial.print("\t");
    Serial.print(totalMilliLitres/1000);
    Serial.print("L");

    pulseCount = 0;
    attachInterrupt(sensorInterrupt, pulseCounter, FALLING);
  }
}
void pulseCounter()
{
  pulseCount++;
}