Water meter on an Arduino Uno

// Water meter: a YF-S201 flow sensor and a 16x2 LCD on an I2C backpack.
//
//   pin 2    the sensor's SIG (yellow); VCC and GND from the rails
//   A4, A5   the LCD backpack's SDA and SCL (address 0x27)
//
// The wheel in the sensor pulses SIG 7.5 times a second for each liter a
// minute, so 450 pulses make a liter. An interrupt counts the falling edges;
// once a second the sketch takes the count, and
//
//   flow (tenths of L/min) = pulses x 10 / 7.5 = pulses x 4 / 3
//   total (ml)             = all pulses x 1000 / 450 = all pulses x 20 / 9
//
// in whole numbers, since this board has no floating point to spare.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

const int SENSOR = 2;

LiquidCrystal_I2C lcd(0x27, 16, 2);

volatile unsigned long pulses = 0;
unsigned long counted = 0;
unsigned long lastSecond = 0;

void onPulse() {
  pulses++;
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: water meter");
  pinMode(SENSOR, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(SENSOR), onPulse, FALLING);
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("Water meter");
  lastSecond = millis();
}

void loop() {
  if (millis() - lastSecond < 1000) return;
  lastSecond += 1000;
  noInterrupts();
  unsigned long all = pulses;
  interrupts();
  unsigned long thisSecond = all - counted;
  counted = all;

  unsigned long tenths = thisSecond * 4 / 3;
  unsigned long ml = all * 20 / 9;

  lcd.setCursor(0, 0);
  lcd.print("Flow ");
  lcd.print(tenths / 10);
  lcd.print('.');
  lcd.print(tenths % 10);
  lcd.print(" L/min   ");
  lcd.setCursor(0, 1);
  lcd.print("Total ");
  lcd.print(ml / 1000);
  lcd.print('.');
  unsigned long frac = (ml % 1000) / 10;
  if (frac < 10) lcd.print('0');
  lcd.print(frac);
  lcd.print(" L    ");

  Serial.print("flow ");
  Serial.print(tenths / 10);
  Serial.print('.');
  Serial.print(tenths % 10);
  Serial.print(" L/min, total ");
  Serial.print(ml);
  Serial.println(" ml");
}