Using two flow sensors connected to one Arduino.

Using Arduino Sensors
13 
#define IRQ_A 0
#define IRQ_B 1
#define FlowA 2
#define FlowB 3

volatile unsigned long countIN = 0;
volatile unsigned long countOUT = 0;
unsigned long oldTime  = 0;
float flowRate;

volatile byte pulseCountIN;
volatile byte pulseCountOUT;
volatile byte pulseCount;

float liters;

void setup()
{
  Serial.begin(57600);
  
  pulseCountIN = 0;
  pulseCountOUT = 0;
  pulseCount = 0;
  flowRate = 0.0;

  pinMode(FlowA, INPUT);
  pinMode(FlowB, INPUT);
  digitalWrite(FlowA, HIGH);
  digitalWrite(FlowB, HIGH);
  
  liters = 0.0;

  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;

    liters = (countIN - countOUT); 
    float litersPerMinute = (1000 * liters)/duration; 
    Serial.println(litersPerMinute, 3);    
    int LPM = litersPerMinute;           
    
    pulseCount = pulseCountIN - pulseCountOUT;
       

    flowRate = ((1000.0 / (millis() - oldTime)) * pulseCount);
    unsigned int frac = (flowRate - int(flowRate))*10;
    Serial.print("pulseCount: ");
    Serial.print(pulseCount, DEC);
    Serial.println(" ");

        
    pulseCountIN = 0;
    pulseCountOUT = 0;
    
    attachInterrupt(IRQ_A, CounterIN, FALLING);
    attachInterrupt(IRQ_B, CounterOUT, FALLING);  
  }
}

void CounterIN()
{
  countIN++;
  pulseCountIN++;
}

void CounterOUT()
{
  countOUT++;
  pulseCountOUT++;
}

This is my last version of the sketch, and now it works! Thanks so much for your help :smiley: I will keep developing this sketch to work more properly. I added two more volatile bytes, because the first sketch did not work.