This piece could have been titled "Exceptional Failures in Process Control." Despite my 25 years of explaining the importance of using middle signal selection, I don't see much evidence of what I have said has taken root outside of Monsanto and its spin-off Solutia, where it became a part of the culture and best practices to use middle signal of three pH electrodes for all important pH control loops. We ended up taking a view that all pH loops are important because if they are unimportant why should we go through the maintenance headaches and the risk of control system failure by installing a loop dependent upon the integrity of a single electrode. I think the main hurtle besides hardware and installation cost is the feeling that if one electrode requires so much effort, why should I add more? If the electrode life expectancy is too short, the feeling is right. We should not add more of a bad application or installation. Instead, we need to find a better design, technology, implementation, and location verified in testing via wireless pH or an alternate measurement (e.g. conductivity for concentrated acids or bases).
As an important side note, the use of three transmitters and middle signal selection on all of the important measurements (e.g. flow pressure, temperature, and level besides pH) used in the control system and safety system for a large intermediates plant has consistently eliminated false trips saving several million dollars per year.
What boggles my mind is that the risk of poor product quality and an environmental violation do not provide a wakeup call that the lifecycle cost of the measurement itself is insignificant in comparison to the risk. A simple quick ball park benefit versus cost analysis would show the absurdity of when the dollars of events likely to occur each year is more than 1000 times the dollars of the additional automation to prevent them. Unfortunately, we tend to get too focused on short term costs. Consider bioreactor batches worth millions of dollars each of a sold out pharmaceutical that are dependent pH upon control to within 0.02 pH. A second electrode is added but I am not sure it helps or just adds to the confusion. I find it almost bizarre how favorite electrodes are picked for the loop's PV and the stories that ensue about which one is best. It is a "fact of life" that electrodes will not agree in the short term due to non-ideal effects too numerous to get into here (check out my ISA book Advanced pH Measurement and Control for more info and previous blogs by searching for "pH"). The continual disagreement between two electrodes often leads to calibration adjustments chasing calibration adjustments. If left alone, the electrode that reads high today may in a couple of hours or at least by tomorrow read low. Electrodes can fail anywhere on the scale (including the most insidious failure of all type where the bad electrode signal is stuck at the pH set point). I maintain that the correct use of middle signal selection will actually reduce the long term maintenance cost by simple observation and the use of more intelligent practices eliminating unnecessary calibration and removal of electrodes.
A middle signal selection inherently ignores a single failure of any type and avoids the slowest electrode (e.g. coated electrode). This selection reduces noise and eliminates spikes without any addition of a signal lag like what you get from signal filtering. Middle signal selection also ignores an electrode with lower efficiency (shorter span) or that is drifting. Theoretically, electrodes of different "in service" time should be used to reduce the occurrence of concurrent failures. The middle electrode is the best signal on the average, but please don't use the average. I have seen some very smart attempts of computing average signals with built in intelligence on signal rejection that were out foxed by a single electrode failure scenario. You would think you could devise something smarter than the simple middle signal selection when in fact inherently it is impossible for a single failure. There can be additional intelligence for more than three electrodes or for protection against multiple concurrent failures.
To summarize, middle signal selection can improve process quality and on-stream time, reduce maintenance, and prevent environment violations by adding understanding and ignoring spurious signals, inaccurate measurements, and failures.
A prolonged deviation from the middle should be alarmed because if you don't fix the first failure or sustained error, middle signal section has a fifty-fifty chance of preventing the next failure or electrode inaccuracy. I could go on and on but I suspect you are pressed to move on. Before I go let's be frank with closing remarks in recognition of an engineer named Frank who was particularly astute at telling it the way it is.
There is an opportunity to use a statistical or first principle model based on titration curves to generate a third signal. Even if the model is wrong, it will be ignored by middle signal selection. There appears to be here mostly an upside where you will at least learn more about your process by developing a model.
I have no illusions as to whether this blog will change one person's mind enough to install middle signal selection even though it is a feature of a standard function block. I also have no expectations that enough users will see the need to take advantage of wireless measurements to eliminate the wiring installation and maintenance costs of going to three measurements. Even more unlikely is that users will end up using middle signal selection enough that it will be offered in a smart transmitter that inputs three electrodes even though electrodes are the weak link in regards to accuracy and reliability.
