Using two flow sensors connected to one Arduino.

Other Hardware Sensors
17

Delta meter with a circular buffer.

#define IRQ_A 0
#define IRQ_B 1
#define FlowA 2
#define FlowB 3

volatile unsigned int countIN = 0;
volatile unsigned int countOUT = 0;
unsigned long oldTime  = 0;

unsigned long totalPulses = 0;

unsigned int circBuf[60];
uint8_t idx = 0;

void setup()
{
  Serial.begin(57600);

  pinMode(FlowA, INPUT);
  pinMode(FlowB, INPUT);
  digitalWrite(FlowA, HIGH);
  digitalWrite(FlowB, HIGH);

  attachInterrupt(IRQ_A, CounterIN, FALLING);
  attachInterrupt(IRQ_B, CounterOUT, FALLING);
}

void loop()
{
  unsigned long now = millis();
  if(now - oldTime >= 1000)
  {
    unsigned long duration = now - oldTime;
    oldTime = now;

    // store usage of last second in circular buffer
    idx++;
    if (idx == 60) idx = 0;
    // disable interrupts while reading & reset counters.
    cli();
    circBuf[idx] = countIN - countOUT; 
    countIN = 0;
    countOUT = 0;
    sei();

    totalPulses += circBuf[idx];

    long pulsesLastMinute = 0;
    for (uint8_t i=0; i<60; i++) 
    {
      pulsesLastMinute += circBuf[i];
    }
    // LPH based upon last minute
    float LPH_M = pulsesLastMinute/10000.0 * 60;
    // LPH based upon last second
    float LPH_S = circBuf[idx]/10000.0 * 3600;
    // Total Liters since start
    float liters = totalPulses/10000.0;

    // DISPLAY USAGE 
    Serial.print(now);
    Serial.print("\tLPH_M:\t");  
    Serial.print(LPH_M, 3);
    Serial.print("\tLPH_S:\t");
    Serial.print(LPH_S, 3);
    Serial.print("\tLiters:\t");
    Serial.println(liters, 3);
  }
}

void CounterIN()
{
  countIN++;
}

void CounterOUT()
{
  countOUT++;
}