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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).

Jashanjot Kaur
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  • "I have read that when BNC connector is short then voltage value should be 2.5 V", that may apply to [this](https://www.e-tinkers.com/2019/11/measure-ph-with-a-low-cost-arduino-ph-sensor-board/) 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](https://www.e-tinkers.com/2020/07/a-review-on-seeed-studio-ph-and-ec-sensor-kits-part-1/) about the Grove PH board to understand how it work and how to use it. – hcheung Mar 04 '23 at 02:37

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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.

chrisl
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  • 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. – Jashanjot Kaur Mar 03 '23 at 09:18
  • @JashanjotKaur Please add your calculation and the resulting values to your question. – chrisl Mar 03 '23 at 09:48