The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #88, and was written by Greg.
Biopharmaceutical batches are often worth more than a million dollars. The product quality of these batches is sensitive to changes of 0.1 pH and the steam sterilization of electrodes reduces the life and accuracy of a sensor that under the best of conditions is the weakest link in a control system. Yet people looking at project costs will balk at using three electrodes because of hardware and installation costs, despite the fact that middle selection inherently protects against a single electrode failure of any type and reduces unnecessary maintenance. A million-dollar batch is put at risk by trying to save a few thousand dollars. Large continuous plants may have production streams worth several hundred thousand dollars per hour, with product quality dependent upon pH. Millions of dollars of production are routinely put at risk by the use of single electrodes for pH measurement.
The only way I can explain this is a focus on up-front costs, a lack of understanding of Tip #55, and instrumentation limitations. People may think pH electrodes are as accurate, stable, and reliable as a transmitter, when in reality electrode life can vary from hours to months, and drift can vary from 0.1 pH per month to 0.1 pH per hour.
Human nature often wants to believe the best. We see the best-case scenario (e.g., lab conditions) from advertisements and sales presentations. With just one electrode, we want to believe the reading is correct. With two electrodes, we pick a favorite and discount the other. With three, it is possible to develop realistic understanding of measurement capabilities. At Monsanto and Solutia, pH control loops had three electrodes and middle signal selection.
Middle value selection also inherently protects against the effects of a single electrode failure of any type. If an electrode goes high or low due to a reference electrode offset, one of the other two electrodes will be selected. If a glass electrode becomes sluggish, one of the other two electrodes will be selected for a pH excursion. If a glass electrode, wire, or termination is broken, one of the other two electrodes will be selected even if the broken electrode causes a pH reading that is at the setpoint (e.g., 7 pH) because when the process does change, one of the other two electrodes will be chosen.
Middle value selection will also reduce noise and spikes in the pH measurement. Unless the noise and spikes are exactly the same in two or more electrodes, the largest excursion will be ignored by selection of one of the other two electrodes. No lag is introduced into the measurement response as it is when a signal filter is added to attenuate noise.
Concept: The use of three measurements with middle signal selection will inherently ignore a single measurement failure of any type and spikes and errors in a single measurement. The middle value is the referee to detect and quantify problems such as offset and drift in the reference electrode and a loss of span or speed of the glass electrode. The knowledge gained can reduce maintenance costs. The benefit is considerable for pH measurements because the electrodes are vulnerable to offsets, coatings, failures, spikes, and noise and because batches or product streams can be worth a lot of money. The concept can be extended to any measurement where the benefits obtained provide an acceptable return on investment.
Details: Use three measurements and middle signal selection to be able to ignore problems with a single measurement. If pH is important enough to control, use middle signal selection to improve reliability, accuracy, and troubleshooting. A time delta that is fixed or is increasing between the response of an electrode and the middle value for a disturbance or setpoint change is indicative of a slow and low efficiency glass electrode, respectively. For a slow electrode, the fixed time delta is the time constant. A slow glass electrode can be caused by coating and/or aging of the glass (Tip #86). Offset and drift in the reference electrode can be caused by junction equilibration, coating, or plugging or by internal electrolyte poisoning (Tip #87).
Use the understanding gained to reduce unnecessary calibration adjustments and provide predictive rather than reactive maintenance based on trends in sensor offset, efficiency, and lag time. The reduction in life cycle costs for more efficient maintenance often provides a sufficient return on investment without even getting into process benefits. Use three measurements, and use middle signal selection for any measurement that can be the probable cause of a bad batch, poor product quality, environmental violations, or a plant shutdown. The process benefits can more than pay for the additional instrumentation. Use three measurements and use middle signal selection to prevent false trips in Safety Instrumented Systems (SIS) and in critical loops that are responsible for preventing abnormal conditions getting to the point where a trip by the SIS is required.
Watch-outs: Experienced configuration and process control engineers may try to do something better than simple middle value selection. To protect against a second or third failure, take advantage of this expertise but make sure the customized technique does not interfere with middle signal selection to protect against the first failure. When a measurement failure is detected, the measurement sensor should be replaced immediately to restore the inherent protection offered by middle signal selection.
Exceptions: If process conditions are so harsh that the life expectancy of the best pH electrode is only a few days, try to find a different measurement such as conductivity or density. If an alternate means of measurement does not exist, consider using an automated retractable sensor assembly to periodically insert a single pH electrode just long enough to get a measurement. Upon insertion, wait for the electrode to stabilize before updating the pH for control. When the electrode is automatically withdrawn, the electrode should be automatically flushed and buffer calibrated. The offset and response time from the buffer calibration should be historized and analyzed to determine the health of the electrode. An enhanced PID developed for wireless (Tip #100) can be used to deal with the large time intervals between periodic insertions and subsequent updates of pH.
Insight: The use of three measurements and middle signal selection can inherently protect against any type of sensor failure and ignore noise and errors in one measurement.
Rule of Thumb: Take three measurements and use middle signal selection for pH and for any other measurement that can be a probable cause of a significant loss in process efficiency or capacity.
About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.
Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.