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).

• hcheung: "I have read that when BNC connector is short then voltage value should be 2.5 V", that may apply to this poorly-designed PH sensor board but certainly does not apply to Grove PH Sensor board as the board is using a single 3.3v power supply with an external reference voltage of 1.8v. You may want to read my review about the Grove PH board to understand how it work and how to use it.

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:

Excel sheet with offset calculation

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.

• Jashanjot Kaur: I have done as per mentioned. But when I put sensor in 9.2 buffer it gives ph= 2.62 and voltage =2.02. Similarly in buffer 4.0 it gives some other pH value. It leads to wrong calculation of k and offset value.
• chrisl: @JashanjotKaur Please add your calculation and the resulting values to your question.