Stan: A measurement is only as good as its calibration. The pH electrode has by far the greatest sensitivity and rangeability of any measurement in the process industry. The electrode can function well in a wide spectrum of process fluids and operating conditions. Here we look how to make sure the electrode can realize its full potential (pun intended).
Greg: To help us, we draw upon the expertise Jim Gray who has decades of experience in electrochemical measurements. I have known Jim personally for the last 10 years and always appreciated and enjoyed our conversations. Jim tells it the way it is (very frank and open). Jim was among the four interviewees for the Control Talk Column Series “The Secret Life of pH Electrodes” that started in Feb 2009, marking the 100th anniversary of the invention of the pH glass electrode.
Stan: The plants today are asked to do more with less staff. Technicians are really pressed to do more than what may be realistically possible. Is this affecting the quality of pH calibration?
Jim: Definitely. One of the biggest problems I see is the technician not waiting to see the total response of the electrode. A big part of this is the lack of recognition of how slow the electrode response can be particularly in reaching its final value.
There are three aspects that create a longer than expected response time. One is the desire to see a response within 1 millivolt (mv) of the final value. For a slope of 59 mv per pH unit and a 6 pH change moving between 4 and 10 pH buffers, 1 mv corresponds to seeing 99.7% of the final response (full response). Secondly, the pH electrode has a protracted response where the response rate of change slows down more than the theoretical first order response as the pH approaches its final value. A healthy electrode will take 8 or more time constants to be within 1 millivolt of the final response. Finally, glass electrode designs, aging or coatings can increase the response time by one or more orders of magnitude.
Suppliers may not be particularly cognizant of the detrimental effects of a slow electrode. A new spherical glass bulb can have a full response time of just 3 seconds while the norm is around 30 seconds. However, one manufacturer not realizing the consequences is proud to offer an electrode with a full response time of 108 seconds. Rugged glass electrodes of various formulations have a full response time that ranges from 180 to 315 seconds.
Greg: I have seen data that the full response time of conventional glass electrodes can be greater than 6000 seconds due to premature aging of the glass due to high temperature exposure or process fluid coating. Even if the electrode was accurate there is a deception where the slower response time means the electrode is acting like a huge signal filter. If the electrode lag becomes the largest time constant in the loop, the PID gain can be increased. If the electrode lag is larger than the ultimate period of the loop (e.g., larger than 4 dead times), any oscillation will be attenuated. Both of these effects give the illusion of better control whereas the opposite is true. For a static mixer the least expensive and fastest equipment for doing pH control, the process time constants is less than a second and the process dead time is only about 4 seconds for a well-designed system. This means a full response time greater than 8 seconds will enable a larger PID gain and a full response time greater than 32 seconds will attenuate oscillations. Of course, you can hope for a poorly designed injection system where the process dead time increases to over 60 seconds. The Dec 2, 2014 Control Talk Blog “Measurement Attenuation and Deception” explains the problem of a slow measurement in considerable detail. Here we find out the problem is more extensive than realized because it affects the accuracy of the measurement as well.
Stan: How are technicians dealing with the problem of a slow electrode?
Jim: The concern is that people being pressed for time don’t wait till the pH has stopped changing. As soon as the pH gets close they may use that value for calibration. This can cause more harm than good. Smart transmitters can tell you when the pH has stopped changing (e.g. less than a 0.02 pH change in 10 seconds), but this feature may not be available or fully used. If the adjustment is premature, there will be a significant negative error in the slope. For example the slope error is about 10% (-5 millivolt per pH) if the pH value is 15 millivolts short of each full response for 4 and 7 pH buffer solutions.
To adjust the slope, two buffer solutions must be used. To verify the slope and offset adjustments are correct, the electrode should be inserted in the opposite buffer at least one more time. Buffers should be kept in closed containers in a cool place and not used after the expiration date or reused. For example, if fresh 4 and 10 pH buffers are used the pH electrode would be rinsed and inserted in the 4 pH buffer and an offset adjustment made. The electrode would then be rinsed and inserted in a 10 pH buffer and a slope adjustment made. These adjustments are only made when the full response is seen. The Auto Cal routine of a smart transmitter will automatically detect when the response is complete and will turn-off solution temperature compensation. The electrode is finally rinsed and reinserted into the 4 pH buffer to see how close the pH is to the buffer value. For this check the solution temperature compensation must be manually turned off.