PH sensorCalibration Issue - Same Voltage for pH 4.0 and pH 7.0

Projects Programming
1

Hi everyone,

I’m working on a project with a pH sensor and an Arduino. During calibration, the voltages measured for the pH 7.0 and pH 4.0 buffer solutions are the same (e.g., around 2.48V). Because of this, I can’t calculate the correct slope and offset.
This is the code I’m using :
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

// Initialiser l'écran LCD I2C
LiquidCrystal_I2C lcd(0x27, 16, 2); // Adresse I2C : 0x27, 16 colonnes, 2 lignes

#define SensorPin A0 // Broche analogique connectée au capteur de pH

float slope = -5.70; // Valeur initiale de la pente (sera ajustée lors de la calibration)
float offset = 21.34 + 0.7; // Valeur initiale de l'offset (sera ajustée lors de la calibration)

unsigned long int avgValue; // Stocke la valeur moyenne du capteur
int buf[10], temp;

void setup() {
pinMode(SensorPin, INPUT);

// Initialiser l'écran LCD I2C
lcd.init();
lcd.backlight(); // Activer le rétroéclairage
lcd.clear();
lcd.setCursor(0, 0);
lcd.print("Welcome To");
lcd.setCursor(0, 1);
lcd.print(" pH Meter");
delay(2000);
lcd.clear();

Serial.begin(9600); // Activer le moniteur série pour la calibration

// Effectuer la calibration
calibrate();
}

void loop() {
// Lire 10 échantillons pour lisser la valeur
for (int i = 0; i < 10; i++) {
buf[i] = analogRead(SensorPin);
Serial.print("Raw Value: ");
Serial.println(buf[i]);
delay(10);
}

// Trier les valeurs pour éliminer les valeurs aberrantes
for (int i = 0; i < 9; i++) {
for (int j = i + 1; j < 10; j++) {
if (buf[i] > buf[j]) {
temp = buf[i];
buf[i] = buf[j];
buf[j] = temp;
}
}
}

// Calculer la moyenne des 6 valeurs centrales
avgValue = 0;
for (int i = 2; i < 8; i++) avgValue += buf[i];
float voltage = (float)avgValue * 5.0 / 1024 / 6; // Convertir en tension (0-5V)
Serial.print("Voltage: ");
Serial.println(voltage);

// Calculer le pH à partir de la formule calibrée
float phValue = (slope * voltage) + offset;
Serial.print("Calculated pH: ");
Serial.println(phValue);

// Afficher la valeur du pH sur l'écran LCD
lcd.setCursor(0, 0);
lcd.print(" pH Value: ");
lcd.print(phValue, 2); // Afficher avec 2 décimales
lcd.print(" ");

// Afficher la description qualitative du pH
lcd.setCursor(0, 1);
if (phValue < 4) lcd.print(" Very Acidic ");
else if (phValue >= 4 && phValue < 5) lcd.print(" Acidic ");
else if (phValue >= 5 && phValue < 7) lcd.print(" Acidic-ish ");
else if (phValue >= 7 && phValue < 8) lcd.print(" Neutral ");
else if (phValue >= 8 && phValue < 10) lcd.print("Alkaline-ish ");
else if (phValue >= 10 && phValue < 11) lcd.print(" Alkaline ");
else if (phValue >= 11) lcd.print("Very alkaline");

delay(1000); // Mettre à jour toutes les secondes
}

// Fonction de calibration dynamique
void calibrate() {
Serial.println("Place the sensor in pH 7.0 buffer solution and press any key...");
while (!Serial.available()) {} // Attendre que l'utilisateur appuie sur une touche
Serial.read(); // Vider le tampon série

// Mesurer la tension pour pH 7.0
float voltage_pH7 = readVoltage();
Serial.print("Voltage at pH 7.0: ");
Serial.println(voltage_pH7);

Serial.println("Place the sensor in pH 4.0 buffer solution and press any key...");
while (!Serial.available()) {}
Serial.read();

// Mesurer la tension pour pH 4.0
float voltage_pH4 = readVoltage();
Serial.print("Voltage at pH 4.0: ");
Serial.println(voltage_pH4);

// Calculer la pente et l'offset
slope = (7.0 - 4.0) / (voltage_pH7 - voltage_pH4);
offset = 7.0 - (slope * voltage_pH7);

Serial.print("Calibration complete! Slope: ");
Serial.print(slope);
Serial.print(", Offset: ");
Serial.println(offset);
}

// Fonction pour lire la tension stabilisée
float readVoltage() {
unsigned long int avgValue = 0;
int buf[10], temp;

// Lire 10 échantillons pour lisser la valeur
for (int i = 0; i < 10; i++) {
buf[i] = analogRead(SensorPin);
delay(10);
}

// Trier les valeurs pour éliminer les valeurs aberrantes
for (int i = 0; i < 9; i++) {
for (int j = i + 1; j < 10; j++) {
if (buf[i] > buf[j]) {
temp = buf[i];
buf[i] = buf[j];
buf[j] = temp;
}
}
}

// Calculer la moyenne des 6 valeurs centrales
for (int i = 2; i < 8; i++) avgValue += buf[i];
return (float)avgValue * 5.0 / 1024 / 6; // Convertir en tension (0-5V)
}

I’ve already checked:

  • The connections (everything seems fine).
  • The buffer solutions (they’re fresh and high-quality).
  • The sensor is fully submerged and rinsed with distilled water between measurements.

Does anyone have an idea what might be causing this? Could it be a problem with the code, the sensor, or something else?

Thanks for your help!

2

Please edit your post to add code tags and to the follow the forum rules. Post links to all the components, especially the pH sensor and calibration solutions, and post a wiring diagram.

See the "How to get the best out of this forum" post for guidelines (linked at the head of every forum category).

3

First, read this post and learn how to properly post your code with Code Tags:

Second:

If you're using a bare pH sensor connected directly to an Arduino analog input, I doubt you'll ever get it to work satisfactorily. Those sensors have extremely high output impedance and require a special amplifier with high input impedance. Very carefully electrical design and circuit layout are required to get them to work well.

4

Thank you very much for your reply. I truly appreciate your help. I will make sure to follow this instruction next time. This is my first time, so I sincerely thank you

5

In the IDE, click on Edit, then Copy for Forum, that will copy your code for pasting on this forum. Then come back here and do a simple paste. That will paste you code in a new window with scroll bars.