Introduction
This is the second of three pH meters that I tested for Arduino. The pH meter in this article is similar to DIYMORE, Gravity: Analog pH Sensor / Meter Kit For Arduino. But I got the copy used here from Amazon.se. The main difference between this pH meter and the DFRobot version in the previous post is the control board that can operate at also 3.3 Volts for the version here. The version presented here can also be set to a default calibration by using one of the two adjustable resistors on the control board.
Having found a url on the package, I think that the pH-meter is from INESA SCIENTIFIC INSTRUMENT CO., LTD. They have the same logo as the one i got, and also a pH-meter with the model id E-201-C. The reference electrolyte is given as 3.0KCl mol/L. The probe also has a refill cap, so perhaps the extra small bottle with gained powder is for adding more KCl? I need to find someone reading Chinese signs to understand.
Online manuals
I found a manual for a pH-meter from DIYMORE also from [Electronic clinic] and posted on youtube. The video covers how to calibrate the pH meter for a particular input voltage.
General cleaning and maintenace instructions are found at https://www.southforkinst.com/ph-orp-electrodes/.
Cleaning and storing
The pH electrode cannot be allowed to dry out or freeze. When it’s not in use, wash and keep the pH-meter in the 3mol KCl container. The 3mol KCL need you to prepare. The preparation method can refer to the below information:
Based on 3 mol * ( 74.55 g / 1 mol KCl) = 223.65 g KCl So to prepare a 3 mol / L solution KCl, you’d need to dissolve 223.65 g of KCl into some water, mix, and then fill it up to the 1 L mark with water.
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). The version I got have both a 3-pin port, but also separate solder pins.
Sketch
The simple sketch is supplied by Electronic clinic:
#define SensorPin A2 // the pH meter Analog output is connected with the Arduino’s Analog
#define Offset 1.3 //deviation compensate
#define ScaleFac 1.3
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*ScaleFac + Offset; //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 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 analogue 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:
ScaleFac = (PH2-PH1)/(V2-V1)
Offset = [(PH2+PH1)-ScaleFac*(V1+V2)]/2
Interactive calibration
Looking at an indepedntn Youtube presentation on pH-metering, an interactive calibration system was presented - by Jessica. That should be the right thing for our thing.