Calibration of pH sensor

Question

I am working with pH sensor (https://wiki.seeedstudio.com/Grove-PH-Sensor-kit/ ) and arduino. The board provided with sensor is not having any potentiometer to set the voltage value when dipped in known buffer solution. I have read that when BNC connector is short then voltage value should be 2.5 V. But when I do the same voltage comes out to be 1.9 V. Also sensor gives wrong readings when put in known buffer solutions of 4.0pH, 7.0pH and 9.2 pH. I am not getting how to calibrate the sensor without having potentiometer on board. I have already done as per mentioned on website (in link given above).

Answer 1

You probably need to do that in software. That is supported by step 7 in the chapter Software on your linked site.

In step 4 we see the proposed demo code, which contains the calculation of pH based on the read voltage:

pHValue = -19.18518519 * voltage + Offset;

The first number is probably k, so the calculation is

pHValue = k * voltage + Offset;

For the calibration you need to find the values for k and Offset. This is done with 2 buffer solutions with known pH. Step 7 shows the formulas for calculating these values based on the known pH values (removed the dollar signs):

k= (PH2-PH1)/(V2-V1)
Offset=[(PH2+PH1)-k*(V1+V2)]/2

And they even provide a link to an Excel sheet, which helps you with the calculation: https://files.seeedstudio.com/wiki/Grove-PH_Sensor_kit/Calibration_for_k&offset.xlsx

Here is an image of that sheet:

In column B behind PH1 and PH2 you can input the known pH values for your buffer solutions. In column E behind V1 and V2 you can input the resulting sensor voltages (! voltages, not wrongly calculated pH values) for each solution. Then you will get your values of k and Offset in the cells B4 and B5.

Input those values into the calculation for phValue in the code and you should get correct values.