# Problem at calibrating PH probe with PH meter v1.1

Hello everybody,

I'm stuck at calibrating my PH probe. I'm using the PH meter v1.1 that has only one potentiometer screw. So, I found out, that you can use this screw to adjust the PH value when placing the probe into a solution of PH 4. This is what I did. According to this guide: PH_meter_SKU__SEN0161_-DFRobot I should then place the probe in a PH 7 solution and calculate the offset to write in my program afterwards. It says, that the error should be less then 0.3. The problem is, that my error is greater than 0.3 (= 7.0 - 6.517 = 0.483) and I can't find any guide that tells me what to do in that situation. Can anybody please help me? I'm really confused because there are many types of PH meters, I was just looking on the internet for hours.

PH calibration relies heavily on the use of Known Buffer Solutions. Additionally knowing the temperature is important. I assume you have good known buffer solutions and that you rinse the probe in deionized water between going from buffer solution to buffer solution. Your referenced setup mentions 25 degrees C. When using the sensor for the first time or after prolonged storage the sensor should be active and in a PH 7 buffer solution at least 8 hours.

If you know for a fact that your buffer solutions are known you can try with a PH of 7 and see what you get, then 4 and then about 10. These are examples of calibrated buffer solutions. Remember to rinse the probe or you will contaminate the solutions. Note the error with different solutions, is the error the same?

Just as an example you mention:
"It says, that the error should be less then 0.3. The problem is, that my error is greater than 0.3 (= 7.0 - 6.517 = 0.483) and I can't find any guide that tells me what to do in that situation".

Look at this part of your code:

``````#define Offset 0.00            //deviation compensate
``````

Change 0.00 to 0.483

Now look at your code and see where Offset shows up.

Ron

Their board for signal conditioning should output as follows.
0.0 Volts = 0.0 PH
2.0 Volts = 7.0 PH
4.0 Volts = 14.0 PH

Measure the voltage between Ground and the A0 Analog Input. The pot on the board is a gain pot. The gain should be linear.

Ron

Thanks for the fast reply. I use the buffer solutions from bluelab (4.0 / 7.0) which should be fine. I just used the offset of 0.483 in my program, so that the display showed PH 7.0. After that, I cleaned the probe and put it into the PH 4 solution. The display showed 4.5 to I turned the screw until it showed 4. Actually the calibration should be fine then. But when I was done, I put the probe into the aquarium and it measured 7.2, which cannot be true. I know for a fact, that the water's PH must be around 6.2. I'm not really sure how that can be possbile because the calibration of 4 and 7 indeed worked (or I think so at least). The probe was inactive for some months, but it was kept moist and clean. I could try to put it in the PH 7 solution for 8 hours, that might be a point. But I still cannot understand why it measures 7.2, that makes no sense for me. Do I maybe have to do the calibration multiple times after that long time?

While not a water chemistry guru I worked with it quite a bit. We checked the PH meters daily using 3 buffer solutions of 4, 7 and 10. That was in accordance with our internal methods and procedures.

The pot on the printed circuit board is a gain pot so if the board output is linear it should only need set once. The analog in pin of the Arduino should see PH of 0.0 =0.0 volts, PH of 7.0 = 2.0 volts and a PH of 14 = 4.0 volts. I loaded the code on an Arduino Uno and ran it simulating DC Voltages inputs to A0 and it worked out fine. That is about 0.2857 volt per PH.

Now if all of that works out fine it only leaves the probe. I would leave it active in pure deionized water for about 8 hours. Additionally I recall a table in the probe link with what the probe outputs are before the board does signal conditioning. That's about the best I can tell you.

Ron

The specs for this particular probe state that it should be activated for 8h in 3N potassium chloride solution --??? The specs are written in pseudo-Chinese and a little difficult to interpret.

Also, some probes have refillable reference reservoirs that must be maintained. This probe states that 3N KCl is the reference solution. pH probes are strange and finicky devices.

How do you know that the aquarium water is not at pH 7.2?

Very weakly buffered solutions (probably including aquarium water) sometimes require a long time for equilibration.
S.

This topic was automatically closed 120 days after the last reply. New replies are no longer allowed.