Great shield! I just got mine pre-assembled and am working on a controller for a freshwater aquarium. I'd like to measure a pH range from 6-7.5, but it looks like Arduino's inability to read negative voltages makes this a huge problem with only one probe. Am I correct that, out of the box, this shield can measure from pH 0-7 OR 7-14?

Do you have any suggestions how I might be able to modify the code (or augment the electronics) to read across the 7 pH boundary without adding a second probe? It feels like there's room to be clever here but my understanding of the circuit and probe operation is too crude to puzzle it out.

Any thoughts greatly appreciated!

P.S. the image scroller at the top of the page is a bit distracting when you've scrolled past it. Since many images have different heights, it keeps resizing, making the videos and comment box constantly bounce up and down. I'm dizzy! :)

Hello Andrew,

I was wondering if you got the time to program the temperature compensation in to the sketch?
I'm currently building my own Hydroponics controller with Arduino and am a bit stuck on the conversion for temperature. pH(0.0178 * sensorValue - 1.889)

Your Eagle files are a BIG help though... Thank you for making everything open source!!

With kind reagards,

Billie

"For acids you may need to subtract 7 (need to check on that)."
Did you check on that yet? What should the pH_Value math be to get acid readings to work (0-7 pH).

Hello all right?

PH electrode which I could use for this experiment.

If you can help me?

Thank you.

Great Product, but I would like to point out a couple of things in the code.

for(i = 1; i < 50; i++) {

should read

for(i = 0; i < 50; i++) {

This will take 50 readings, i 0 to 49, and still leave i at 50 for your averaging line.
The current code takes 49 samples but divides by 50.

Also the variable reading should be an unsigned int to avoid overflowing into a negative number since the maximum reading of 1024 * 50 is above the 32767 maximum for int.

Here is the code I am using, created an Analog_Sample function to average the readings since I am going to be using multiple sensors in my project.

#define PH_PIN 5
#define ARDUINO_VOLTAGE 4.99
float ph_value;

void setup() {
Serial.begin(9600);
}

void loop() {
read_ph();
Serial.print("pH= ");
Serial.println(ph_value);
delay(500);
}

///////////////////////////////////////////////////////////////
// Calculates ph_value from average sensor reading
// Sensor reads linearly from ph 7-14 from 0-4v which results
// in a value of 571.428mv per ph point.
///////////////////////////////////////////////////////////////
void read_ph() {
int ph_reading;
int ph_millivolts;
ph_reading = Analog_Sample(PH_PIN);
ph_millivolts = ((ph_reading * ARDUINO_VOLTAGE) / 1024) * 1000;
ph_value = (ph_millivolts / 571.428) + 7;
}

int Analog_Sample(int PIN) {
int i;
unsigned int reading = 0;
for(i = 0; i < 50; i++) {
reading += analogRead(PIN);
delay(10);
}
reading /= i;
return reading;
}