Water pressure gauge on an Arduino Uno

// Water pressure gauge: a 0 to 100 psi stainless pressure transducer on a
// water line, and the pressure on a 16x2 I2C LCD in psi and bar.
//
//   A0      the transducer's SIG (yellow; red to 5 V, black to GND)
//   LCD     SDA -> A4    SCL -> A5    VCC -> 5 V    GND -> GND
//
// The transducer puts out 0.5 V at no pressure and 4.5 V at full scale, and
// it is ratiometric: 10% and 90% of its supply. The Uno's converter measures
// against the same 5 V, so the sketch works in fractions of the supply and a
// supply a little off cancels out: no 0.5 V or 4.5 V constant appears below.
//
// Each reading is the average of 32 conversions, which smooths the one-step
// jitter a converter has. One step is 0.12 psi on the 100 psi model.
//
// A house supply is 40 to 80 psi; under 20 the sketch says LOW, which is a
// burst pipe or a pump that has stopped.

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

const float FULL_SCALE_PSI = 100.0;   // the model printed on the body
const float PSI_PER_BAR = 14.5038;

LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: water pressure gauge");
  lcd.init();
  lcd.backlight();
}

float readPsi() {
  long sum = 0;
  for (int i = 0; i < 32; i++) sum += analogRead(A0);
  float fraction = sum / 32.0 / 1023.0;          // of the supply
  float psi = (fraction - 0.1) / 0.8 * FULL_SCALE_PSI;
  return psi < 0 ? 0 : psi;
}

void loop() {
  float psi = readPsi();
  float bar = psi / PSI_PER_BAR;

  lcd.setCursor(0, 0);
  lcd.print(psi, 1);
  lcd.print(" psi      ");
  lcd.setCursor(0, 1);
  lcd.print(bar, 2);
  lcd.print(" bar  ");
  lcd.print(psi < 20 ? "LOW " : "    ");

  Serial.print("psi: ");
  Serial.print(psi, 1);
  Serial.print("  bar: ");
  Serial.println(bar, 2);
  delay(250);
}