Aquarium water monitor on an Arduino Uno

// Aquarium and hydroponics water monitor: five analog water-quality probes
// on one Uno, each read with its maker's own formula, printed on Serial.
//
//   A0      TDS board (DFRobot Gravity SEN0244)            A
//   A1      pH board (PH-4502C)                            Po
//   A2      turbidity board (DFRobot Gravity SEN0189)      A, switch on A
//   A3      dissolved oxygen board (DFRobot SEN0237)       A
//   A4      EC board (DFRobot Gravity DFR0300, K = 1)      A
//   Every board's VCC (+ or V+) on 5 V and GND (- or G) on GND.
//
// The water is assumed to be at 25 C, the temperature every formula below is
// written for. With a waterproof DS18B20 in the tank, put its reading in
// waterC and the TDS, DO and EC lines compensate the way the makers' own code
// does.
//
// Each probe is read 20 times and averaged, and the voltage turned into the
// water's value:
//   TDS   EC = 133.42 V^3 - 255.86 V^2 + 857.39 V, over 1 + 0.02 (T - 25),
//         and TDS is half the EC (DFRobot's GravityTDS)
//   pH    pH = 7 + (2.5 - V) / 0.18, with the board's offset trimmer set so
//         pH 7 reads 2.5 V (the PH-4502C tutorials)
//   NTU   NTU = -1120.4 V^2 + 5742.3 V - 4352.9 (DFRobot's turbidity chart)
//   DO    DO = mV x saturation(T) / (1600 + 35 (T - 25)) (DFRobot's
//         single-point calibration, 1600 mV saturated at 25 C)
//   EC    EC = mV / 820 / 200 x 1000 mS/cm, over 1 + 0.0185 (T - 25)
//         (DFRobot_EC, uncalibrated K = 1)
// A real probe needs calibrating first; the numbers above are the makers'
// typical ones.

const float VREF = 5.0;
float waterC = 25.0;

float volts(int pin) {
  long sum = 0;
  for (int i = 0; i < 20; i++) sum += analogRead(pin);
  return sum / 20.0 * VREF / 1024.0;
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: water monitor");
}

void loop() {
  float v = volts(A0);
  float ec = 133.42 * v * v * v - 255.86 * v * v + 857.39 * v;
  float tds = ec / (1.0 + 0.02 * (waterC - 25.0)) * 0.5;

  v = volts(A1);
  float ph = 7.0 + (2.5 - v) / 0.18;

  v = volts(A2);
  float ntu = v < 2.5 ? 3000.0 : -1120.4 * v * v + 5742.3 * v - 4352.9;
  if (ntu < 0) ntu = 0;

  // Oxygen saturation of water at 25 C is 8.25 mg/L (DFRobot's DO_Table).
  float mv = volts(A3) * 1000.0;
  float oxygen = mv * 8.25 / (1600.0 + 35.0 * (waterC - 25.0));

  mv = volts(A4) * 1000.0;
  float cond = mv / 820.0 / 200.0 * 1000.0 / (1.0 + 0.0185 * (waterC - 25.0));

  Serial.print("TDS: ");
  Serial.print(tds, 0);
  Serial.print(" ppm  pH: ");
  Serial.print(ph, 2);
  Serial.print("  NTU: ");
  Serial.print(ntu, 0);
  Serial.print("  DO: ");
  Serial.print(oxygen, 2);
  Serial.print(" mg/L  EC: ");
  Serial.print(cond, 2);
  Serial.println(" mS/cm");
  delay(500);
}