pH Calibration - Comparing Electrical and CRM Methods

M

MetroBioCen

#1
Hello. In our enterprise there is the need to perform the calibration of a pH measuring system which employs smart electrodes. These pH electrodes have the electronics in the top of its body and only send a digital signal to the indicator/processor. As we can not generate this digital signal, we are not able to simulate the pH signal, so we thought to use Certified Reference Materials to solve this problem, but there is a considerations to take into account: pH electrodes degrade themselves during the measuring process, so the result of this measurement should be only valid during the calibration process So, should it be valid to acceot the result of a calibration using CRM throught time? Usually we calibrate out pH meters electrically and ALWAYS adjust them with CRM previously ti be used. We do that every day and even several times a day if an accurate measurement is need it. This is an odd case. All opinions welcomed.
 
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#2
First, I am not a pH expert, but I've had some exposure to these cals. If you use a Certified Reference Material, the length of the validity is the calibration interval. You have to review what you measured with the CRM at the beginning of the interval and what you measured at the end of the interval. How much did the readings change during the interval? And what is either the instrument tolerance limit, or your process or control limits?

Even if you were able to simulate the signal digitally, the accuracy drift of the analogue pH sensors is part of the measurement process. I would guess that the digital drift would be quite small in comparison with the drift of the sensors (I don't know - just a guess). But the goal is accurate measurement of the pH, which includes the sensors as well.

The key I think is identifying a tolerance limit and/or process/control limit (as applicable to your circumstance). Then track the as found and as left measured values in comparison with the CRM.

After you measure and adjust, by definition, the measured value will begin to change. So the actual "exact" value will only be valid at the time of the calibration (plus or minus uncertainties of the measurement). You then determine a statistical amount of drift over time, and use that to formulate what the expected value is, again, plus or minus the added uncertainties, and when to re-calibrate the system.

If you need to have a very accurate value, more accurate than the drift allows over time, you would need to re-calibrate before each use.

To accept the pH value over time, it must include some determined amount of drift as detailed above. Hope this helps.
 
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