Introduction
This is the first of three pH meters that I tested for Arduino. The pH meter in this article is the (same as the) DFRobot, Gravity: Analog pH Sensor / Meter Kit For Arduino. But I got my copy from Amazon.se. The control board (breakout board) is constructed for 5 volt power (Vin = 5). The pH meter in the next post can be calibrated also for Vin = 3.3.
Online manuals
I found instructions and code for this exact model posted by Electronic clinic on their own web page both on Youtube and also as a Youtube instruction.
Wiring
The wiring is very easy as the pH-meter comes with a prepared contact with only three wires, power, ground and an analog signal (see figure above).
Sketch
The simple sketch below is supplied by Electronic clinic:
#define SensorPin A2 // the pH meter Analog output is connected with the Arduino’s Analog
unsigned long int avgValue; //Store the average value of the sensor feedback
float b;
int buf[10],temp;
void setup()
{
Serial.begin(9600);
Serial.println("Ready"); //Test the serial monitor
}
void loop()
{
for(int i=0;i<10;i++) //Get 10 sample value from the sensor for smooth the value
{
buf[i]=analogRead(SensorPin);
delay(10);
}
for(int i=0;i<9;i++) //sort the analog from small to large
{
for(int j=i+1;j<10;j++)
{
if(buf[i]>buf[j])
{
temp=buf[i];
buf[i]=buf[j];
buf[j]=temp;
}
}
}
avgValue=0;
for(int i=2;i<8;i++) //take the average value of 6 center sample
avgValue+=buf[i];
float phValue=(float)avgValue*5.0/1024/6; //convert the analog into millivolt
Serial.print(" raw pH:");
Serial.print(phValue,2);
phValue=3.5*phValue; //convert the millivolt into pH value
Serial.print(" calibrated pH:");
Serial.print(phValue,2);
Serial.println(" ");
}
Seeed stduio offers a more advanced code for a similar ph_meter. This code also contains a calibration Offset parameter, and the manual also states how to set the linear equation slope.
/*
# This sample code is used to test the pH meter V1.1.
# Editor : YouYou
# Date : 2014.06.23
# Ver : 1.1
# Product: analog pH meter
# SKU : SEN0161
*/
#define SensorPin A2 //pH meter Analog output to Arduino Analog Input 0
#define Offset 41.02740741 //deviation compensate
#define LED 13
#define samplingInterval 20
#define printInterval 800
#define ArrayLenth 40 //times of collection
#define uart Serial
int pHArray[ArrayLenth]; //Store the average value of the sensor feedback
int pHArrayIndex = 0;
void setup(void)
{
pinMode(LED, OUTPUT);
uart.begin(9600);
uart.println("pH meter experiment!"); //Test the uart monitor
}
void loop(void)
{
static unsigned long samplingTime = millis();
static unsigned long printTime = millis();
static float pHValue, voltage;
if (millis() - samplingTime > samplingInterval)
{
pHArray[pHArrayIndex++] = analogRead(SensorPin);
if (pHArrayIndex == ArrayLenth)pHArrayIndex = 0;
voltage = avergearray(pHArray, ArrayLenth) * 5.0 / 1024;
pHValue = -19.18518519 * voltage + Offset;
samplingTime = millis();
}
if (millis() - printTime > printInterval) //Every 800 milliseconds, print a numerical, convert the state of the LED indicator
{
uart.print("Voltage:");
uart.print(voltage, 2);
uart.print(" pH value: ");
uart.println(pHValue, 2);
digitalWrite(LED, digitalRead(LED) ^ 1);
printTime = millis();
}
}
double avergearray(int* arr, int number) {
int i;
int max, min;
double avg;
long amount = 0;
if (number <= 0) {
uart.println("Error number for the array to avraging!/n");
return 0;
}
if (number < 5) { //less than 5, calculated directly statistics
for (i = 0; i < number; i++) {
amount += arr[i];
}
avg = amount / number;
return avg;
} else {
if (arr[0] < arr[1]) {
min = arr[0]; max = arr[1];
}
else {
min = arr[1]; max = arr[0];
}
for (i = 2; i < number; i++) {
if (arr[i] < min) {
amount += min; //arr<min
min = arr[i];
} else {
if (arr[i] > max) {
amount += max; //arr>max
max = arr[i];
} else {
amount += arr[i]; //min<=arr<=max
}
}//if
}//for
avg = (double)amount / (number - 2);
}//if
return avg;
}
Calibration
The core translation from the analog signal to initial pH is similar for all analog pH meters used with Arduino. In the first step the analog signal is translated to voltage:
voltage = signal * Vin / 1024
The DFRobot pH-meter used in this article require 5 volt as Vin, and the conversion to voltage registered by the analog port thus becomes:
voltage = signal * 5.0 / 1024
Most pH-meters also have linear responses between pH and analog voltage output (registered by the Arduino analog port), and the actual pH value is thus calculated as a linear conversion:
pH-value = votage*ScaleFac + Offset
The two sketches above use greatly different values for ScaleFac and Offset:
| Sketch | ScaleFac | Offset |
| Electronic clinic | 3.5 | 0 |
| DFRobot | -19.18518519 | 41.02740741 |
Setting ScaleFac and Offset requires solving a linear equation and retrieving the slope (ScaleFac) and intercept (Offset) in two steps:
ScaleFac = (PH2-PH1)/(V2-V1)
Offset = [(PH2+PH1)-ScaleFac*(V1+V2)]/2
This of course requires testing the pH meter against two liquids with a priori known pH.
Resources
DFRobot, Gravity: Analog pH Sensor / Meter Kit For Arduino.