user48700:  #include <Wire.h> //I2C lib
#include <LCD.h>
#include <LiquidCrystal_I2C.h>
 // LCD lib
LiquidCrystal_I2C lcd(0x3F, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);

const int pot_pin = A0;
const int pump_pwm= 3;

int press_pin =A1;

//flowmeter parameters
int flowPin = 2;  // input pin on arduino D2

double flowRate;   // value intented to calculate
double flowR; // flow value in lt
double totalFlow; // total output of flow from system

byte sensorInterrupt = 0; // interrupt 0 on D2 pin Arduino Nano

volatile int count; ////integer needs to be set as volatile to ensure it updates correctly during the interrupt process. 

int pot_init= 0;
int pump_init= 0;
int percentValue =0;



void setup() {



  lcd.begin(20,4);  // A4 - A5 connection SDA - SCL
  lcd.backlight();
  Serial.begin(9600); 

  pinMode( flowPin,INPUT); // Set D2 pin as an input

  attachInterrupt(sensorInterrupt,Flow,RISING); // Configures interrupt 0 ( pin D2 on Arduino Nano ) to run function "Flow" 
}

void flow_control(void) {

  count = 0; // reset counter so it could start counting from 0

  interrupts(); // enables interrupts on arduino nano
  delay(1000); // wait 1000 msec
  noInterrupts(); // disable interrupts on arduino nano


  //calculation for flowmeter 

  flowR = (count*8.93);   // 112 pulse/lt 423.84 pulse /gallon 
  flowRate= flowR*60;     // convert seconds to minutes, new unit is ml/minutes
  flowRate= flowRate/1000; // convert ml to liters, new unit is lt/minutes

  totalFlow += flowR;

  // Print the flow rate for this second in litres / minute
    Serial.print("Flow rate: ");
    Serial.print(int(flowRate));  // Print the integer part of the variable
    Serial.print("L/min");
    Serial.print("\t");       // Print tab space

    // Print the cumulative total of litres flowed since starting
    Serial.print("Output Liquid Quantity: ");        
    Serial.print(totalFlow/1000);
    Serial.println("L");

    lcd.setCursor(0,2);
  lcd.print("Flow: ");
    lcd.setCursor(8,2);
  lcd.print(flowRate);
  lcd.print("lt/min");

  lcd.setCursor(0,3);
  lcd.print("Total: "); 
  lcd.setCursor(8,2);
  lcd.print(totalFlow/1000);
  lcd.print("lt");




}

void Flow(void) 
{
  count++; // every time this function is called, increment  "count" by 1
}

void loop() {

  pressure_cal(); 
  pump_control();
  flow_control();
  lcd_control();
}



void pressure_cal(void) {

  float sensorVoltage = analogRead(press_pin);   // sensor voltage A0
  float psi = ((sensorVoltage-102)/204)*25;  // Offset    0 PSI= 0.5V=102 unit, 50 PSI= 2.5, 100 PSI= 4.5V, 1 PSI= 0.04V

  // calibration 
  float bar = psi*(0.0689475729);           // Conversion PSI to BAR


  lcd.setCursor (0,1);
  lcd.print (psi);
  lcd.print (" PSI");

  lcd.setCursor ( 10,1);
  lcd.print(bar);
  lcd.print( " BAR");

  //lcd.setCursor(17,1);
  //lcd.print(sensorVoltage);

  Serial.println (sensorVoltage);
  Serial.println(bar);
  Serial.println (psi);

  delay (100);
}

void pump_control(void)

{

  // read the analog in value:
  pot_init = analogRead(pot_pin);
  // map it to the range of the analog out:
  pump_init = map(pot_init, 0, 1023, 50, 230);  //  duty cycle between %20 - %90: speed control , duty cycle between %0 - %20: turned off  , duty cycle between %90 - %100: full speed
  // map pump speed percent of full scale
  percentValue = map (pump_init, 50, 230,0,100);
  // change the analog out value:
  analogWrite(pump_pwm, pump_init);

  // print the results to the Serial Monitor:
  Serial.print("\t Speed Input = ");
  Serial.print(pot_init);
  Serial.print("\t Speed Output = ");
  Serial.print(pump_init);
  Serial.print("\t Pump Speed Percentage = ");
  Serial.println(percentValue);

  lcd.setCursor(2,0);         
  lcd.print("Speed: ");
  lcd.setCursor(8,0);         
  lcd.print("%");         
  lcd.setCursor(9,0);         
  lcd.print(percentValue); 
  lcd.print("     ");         

 //  delay after the last reading:
  delay(2);


}

I try to write code including flow sensor, pressure sensor and pump. I wrote the code. But When I use interrupt , code stop. I used functions in " void loop()" . I moved the interrupt code ( flow_control) between void setup and void loop. But I can see outputs in serial monitor but I cannot see on lcd. Could you help me fix the problem ? 

user359: A lot of peripheral libraries requires interrupts to be enabled, especially when they use timing or the I2C/SPI pins. Instead of disabling interrupts for the majority of the sketch, what you want to do is isolate disabling the interrupts to just reading and resetting the count variable:
noInterrupts(); // disable interrupts on arduino nano
local_count = count;
count=0;
interrupts(); // enables interrupts on arduino nano

Also all that processing takes some time which will skew the timing so it's better to use timestamp and millis:
unsigned long previous_read;
if(millis() -  previous_read > 1000){
    noInterrupts(); // disable interrupts on arduino nano
    int local_count = count;
    count=0;
    interrupts(); // enables interrupts on arduino nano
    previous_read += 1000;

    //process flowrate based on local_count

}


48700: <pre><code> #include &lt;Wire.h&gt; //I2C lib
#include &lt;LCD.h&gt;
#include &lt;LiquidCrystal_I2C.h&gt;
 // LCD lib
LiquidCrystal_I2C lcd(0x3F, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);

const int pot_pin = A0;
const int pump_pwm= 3;

int press_pin =A1;

//flowmeter parameters
int flowPin = 2;  // input pin on arduino D2

double flowRate;   // value intented to calculate
double flowR; // flow value in lt
double totalFlow; // total output of flow from system

byte sensorInterrupt = 0; // interrupt 0 on D2 pin Arduino Nano

volatile int count; ////integer needs to be set as volatile to ensure it updates correctly during the interrupt process. 

int pot_init= 0;
int pump_init= 0;
int percentValue =0;



void setup() {



  lcd.begin(20,4);  // A4 - A5 connection SDA - SCL
  lcd.backlight();
  Serial.begin(9600); 

  pinMode( flowPin,INPUT); // Set D2 pin as an input

  attachInterrupt(sensorInterrupt,Flow,RISING); // Configures interrupt 0 ( pin D2 on Arduino Nano ) to run function "Flow" 
}

void flow_control(void) {

  count = 0; // reset counter so it could start counting from 0

  interrupts(); // enables interrupts on arduino nano
  delay(1000); // wait 1000 msec
  noInterrupts(); // disable interrupts on arduino nano


  //calculation for flowmeter 

  flowR = (count*8.93);   // 112 pulse/lt 423.84 pulse /gallon 
  flowRate= flowR*60;     // convert seconds to minutes, new unit is ml/minutes
  flowRate= flowRate/1000; // convert ml to liters, new unit is lt/minutes

  totalFlow += flowR;

  // Print the flow rate for this second in litres / minute
    Serial.print("Flow rate: ");
    Serial.print(int(flowRate));  // Print the integer part of the variable
    Serial.print("L/min");
    Serial.print("\t");       // Print tab space

    // Print the cumulative total of litres flowed since starting
    Serial.print("Output Liquid Quantity: ");        
    Serial.print(totalFlow/1000);
    Serial.println("L");

    lcd.setCursor(0,2);
  lcd.print("Flow: ");
    lcd.setCursor(8,2);
  lcd.print(flowRate);
  lcd.print("lt/min");

  lcd.setCursor(0,3);
  lcd.print("Total: "); 
  lcd.setCursor(8,2);
  lcd.print(totalFlow/1000);
  lcd.print("lt");




}

void Flow(void) 
{
  count++; // every time this function is called, increment  "count" by 1
}

void loop() {

  pressure_cal(); 
  pump_control();
  flow_control();
  lcd_control();
}



void pressure_cal(void) {

  float sensorVoltage = analogRead(press_pin);   // sensor voltage A0
  float psi = ((sensorVoltage-102)/204)*25;  // Offset    0 PSI= 0.5V=102 unit, 50 PSI= 2.5, 100 PSI= 4.5V, 1 PSI= 0.04V

  // calibration 
  float bar = psi*(0.0689475729);           // Conversion PSI to BAR


  lcd.setCursor (0,1);
  lcd.print (psi);
  lcd.print (" PSI");

  lcd.setCursor ( 10,1);
  lcd.print(bar);
  lcd.print( " BAR");

  //lcd.setCursor(17,1);
  //lcd.print(sensorVoltage);

  Serial.println (sensorVoltage);
  Serial.println(bar);
  Serial.println (psi);

  delay (100);
}

void pump_control(void)

{

  // read the analog in value:
  pot_init = analogRead(pot_pin);
  // map it to the range of the analog out:
  pump_init = map(pot_init, 0, 1023, 50, 230);  //  duty cycle between %20 - %90: speed control , duty cycle between %0 - %20: turned off  , duty cycle between %90 - %100: full speed
  // map pump speed percent of full scale
  percentValue = map (pump_init, 50, 230,0,100);
  // change the analog out value:
  analogWrite(pump_pwm, pump_init);

  // print the results to the Serial Monitor:
  Serial.print("\t Speed Input = ");
  Serial.print(pot_init);
  Serial.print("\t Speed Output = ");
  Serial.print(pump_init);
  Serial.print("\t Pump Speed Percentage = ");
  Serial.println(percentValue);

  lcd.setCursor(2,0);         
  lcd.print("Speed: ");
  lcd.setCursor(8,0);         
  lcd.print("%");         
  lcd.setCursor(9,0);         
  lcd.print(percentValue); 
  lcd.print("     ");         

 //  delay after the last reading:
  delay(2);


}
</code></pre>

<p>I try to write code including flow sensor, pressure sensor and pump. I wrote the code. But When I use interrupt , code stop. I used functions in " void loop()" . I moved the interrupt code ( flow_control) between void setup and void loop. But I can see outputs in serial monitor but I cannot see on lcd. Could you help me fix the problem ? </p>
 359: <p>A lot of peripheral libraries requires interrupts to be enabled, especially when they use timing or the I2C/SPI pins. Instead of disabling interrupts for the majority of the sketch, what you want to do is isolate disabling the interrupts to <em>just reading and resetting</em> the count variable:</p>

<pre><code>noInterrupts(); // disable interrupts on arduino nano
local_count = count;
count=0;
interrupts(); // enables interrupts on arduino nano
</code></pre>

<p>Also all that processing takes some time which will skew the timing so it's better to use timestamp and millis:</p>

<p>unsigned long previous_read;</p>

<pre><code>if(millis() -  previous_read &gt; 1000){
    noInterrupts(); // disable interrupts on arduino nano
    int local_count = count;
    count=0;
    interrupts(); // enables interrupts on arduino nano
    previous_read += 1000;

    //process flowrate based on local_count

}
</code></pre>
