Faced with a need to monitor the accuracy or reliability of a particular instrument, engineers at some plants improvise a secondary sensor to help keep the primary sensor “honest.” These practitioners, usually without realizing it, are using second-order instrumentation, that is, instrumentation whose purpose is to measure or indicate the condition or performance of another instrument. The terminology may not be familiar, but it aptly describes an often-overlooked way of dealing with the pervasive problem of sensor error.
Measurement untrustworthiness has been around as long as there have been instruments. But there are remarkably few ways of dealing with it.
Sometimes a reading is so far off that it clearly stands out as suspect. When this happens, it often is a simple matter to repeat the measurement, recalibrate the instrument, repair or even replace it.
Even if a single reading alone doesn’;t flag that something is amiss, reliability can be improved by the time-honored technique of redundancy, either in time or space. Redundancy in time simply means repeating the measurement, while redundancy in space entails using multiple instruments in parallel. Either way, redundancy is a powerful technique, though it seldom gets the attention it deserves — perhaps because it seems so obvious.
An instructive analogy comes from computing. A modern computer works so fast that it’;s hopeless to try to verify the results of a complex series of computations by comparing them with the results of manual computations done in parallel. So, how is it possible to spot if some malfunction in the computer’;s microprocessor is skewing the results? The answer is simple: Give the same problem to two computers and then compare their results. Carrying this thinking one step further, assigning the same problem to three computers in parallel can pinpoint when there is a glitch and which computer is the faulty one. (This concept, incidentally, is not what is usually meant by the term “parallel computing,” which refers instead to dividing a computational task into sub-tasks, each assigned to a different computer, then combining the results.)
Unfortunately, many applications do not lend themselves to redundant instrumentation for reasons of cost, space, weight, inaccessibility for servicing, etc. These kinds of constraints are becoming more commonplace.
A better approach
Computing again offers insights, this time on a more sophisticated approach. Even where computational accuracy is deemed mission critical, the use of multiple computers working on the same problem in parallel is considered to be an expensive luxury, except in the case of manned spacecraft. Most other mission-critical applications instead rely upon some regimen of computer self-diagnostics: The greater the importance of the computation, the more elaborate the diagnostics.
The idea is to exercise the microprocessor via a series of test computations that are thorough enough to turn up any malfunction and whose correct results are known in advance. This technique might be considered to be a computational analog of second-order instrumentation, where the diagnostic software plays the role of a secondary sensor whose job is to monitor the performance of the primary. The analogy breaks down, however, unless the self-diagnostic computations are “multi-tasked,” i.e., performed in parallel or at least alternated with the actual “working” computations. The essence of second-order instrumentation is that the secondary instrument reports on the status of the primary in real time.
The best way to cope with the unreliability of a sensor — and especially with its degradation over time — depends upon the consequences of an inaccurate measurement and the accessibility of the instrument in question. If there is a dearth of options in general, the shortage is especially acute in those “hard” cases where the need is greatest. When the consequences of an inaccurate measurement can be catastrophic, and when it is impractical or very costly to remove an instrument from its location for servicing, second-order instrumentation often affords the best alternative.
However, instrumentation engineers seldom consider second-order instrumentation, perhaps because they are unaccustomed to the concept of a sensor for a sensor. Examples of this approach exist, even in consumer products. A household smoke detector, for instance, does not need great measurement accuracy, but it must have outstanding reliability because instrument failure can be catastrophic to the user and pose liability issues for the manufacturer. That is why most smoke detectors are equipped with LEDs to indicate that the units’; batteries have enough charge. The smoke detector, the primary or first-order sensor, has a second-order sensor to monitor its condition.
The plant context
Let’;s consider a common situation at chemical plants that uses electrochemical sensors. Sooner or later, every such device fails because the electrochemistry of the measurement process itself degrades key elements of the sensor. En route to the inevitable failure, measurement accuracy deteriorates. However, the optimum time for intervention, i.e., replacement of contaminated or consumed elements, often cannot be predicted. In such cases, the best strategy depends upon (a) the consequences of a faulty measurement; (b) the accessibility of the sensor for servicing; and (c) the economics of preemptive action, i.e., preventive maintenance before the sensor starts operating outside its nominal range.
For the least-demanding applications, the simplest strategy — preventive maintenance — works best because routine scheduled service usually is less costly than the effort, and especially the risk, of trying to squeeze more life from degraded components.
What about production or waste-treatment processes requiring sensors that are hard to access, such as those located in the flow stream of a hazardous fluid, or whose servicing requires costly shutdowns? For instance, measurements upon which regulatory compliance or batch quality depend are not always made in convenient locations. Sometimes, access to the sensor necessitates suiting up to go into a hazardous environment in 100Â°F weather, or a sensor in high-pressure service must be extracted and reinserted in cramped quarters.
In such cases, using a secondary sensor to monitor the condition of the primary sensor can be a far better strategy than preventive maintenance because preventive maintenance works only if your estimates are conservative, and the price tag for conservatism might be excessive. In contrast, second-order instrumentation allows companies to engage in predictive maintenance, which is what many maintenance departments now are striving to achieve.
This sort of thinking gave rise to a special class of electrochemical sensor/transmitters with built-in second-order sensors to monitor the deterioration process; these units flag potential problems before they can cause any real harm.
Pioneered in pH measurement, the technology applies equally well to the measurement of oxidation-reduction potential (ORP) or any other specific ion-process concentration. Many such devices have been developed and marketed.
This approach obviously is most attractive when it yields the biggest cost savings or the most dramatic risk reductions. More often than not, that turns out to be in applications where large batches of end products must be discarded or unacceptable health hazards (e.g., in waste treatment processes) result from not knowing soon enough that a sensor’;s performance had degraded past an established limit.
The actual range of applications of this technology turns out to be much broader, though. Process engineers have applied it to a variety of jobs, and new ones crop up all the time. Most of the inquiries about second-order electrochemical sensors have one thing in common: They come from people who have experienced financial losses or legal liabilities that might have been avoided if the inevitable degradation of a critical sensor had been caught earlier. Having already been burned, they are extremely receptive to the idea of using a sensor to monitor a sensor.
However, the majority of potential applications target replacing preventive maintenance for less sensitive duties. Often, the cost of this scheduled service is built into overall operational expenses and, consequently, these costs are never seriously examined. It’;s as if to say, “If it’;s not broken, why fix it?” But, in many instances, the simple addition of a secondary sensor can yield a tremendous return on investment, like that of adding a 2-cent LED to a $10 smoke detector.
Although the motivation for developing second-order electrochemical sensors stemmed from difficult or highly leveraged applications, this technology now is being adopted for a variety of “easy” situations, despite the availability of more traditional options. Apparently, the benefits, especially the cost-effectiveness, of monitoring the degradation of a primary sensor in real time are becoming more widely recognized.
The pH, ORP and specific ion concentrations of fluids often must be kept within a relatively narrow range for process or environmental reasons. For instance, a company may face fines or criminal prosecution when its effluent is out of compliance. The key element in a measurement system for these electrochemical parameters is the sensing electrode. Erroneous outputs from this element may compromise the integrity of the entire process.
Typically, the measurements are performed in a line, tank or vessel, and it takes time and effort to verify the accuracy of these measurements. The sensor must be removed from its operating environment and placed in a standard solution to determine its accuracy and integrity. This entails making calculations based upon the measured data that tell the operator the degree to which the measured values depart from theoretically expected ones. If the discrepancy exceeds some predetermined limit, the sensor must be serviced by first determining which element was responsible for the discrepancy, then replacing that element and recalibrating the sensor. Finally, the sensor must be reinstalled.
This awkward procedure can be simplified by using an additional sensor to monitor the natural degradation of the actual measuring sensor. The operator not only is alerted when service is required but also gets ongoing quantitative information in real time. This, of course, saves time and money in servicing the sensor but, above all, vastly increases the reliability of the primary sensor. For many applications, that translates into much greater savings, loss prevention and risk elimination for whatever process the primary sensor was installed to monitor in the first place.
A special sensor
Electrochemical sensors usually are designed with two chambers — a measurement cell and a reference cell. The measurement cell has a specific reactivity with the specimen ion(s) to be measured. In a pH sensor, for example, the electrode might consist of a hydrogen-ion-sensitive glass bulb. Variations in the relative hydrogen-ion concentration between the inside and outside of the bulb are indicated by an electrical potential difference, typically measured in millivolts.
The reference cell consists essentially of an internal element such as a metal/metal salt, an electrolyte or filling solution, and a liquid junction through which the electrolyte contacts the specimen to be measured. The reference cell serves to maintain a common and stable potential with respect to the specimen. Most problems within an electrochemical sensor occur in the reference cell.
It is easy to see why this is so. The electrolyte surrounds the reference element with an electrochemically stable environment. The liquid junction provides the conductive bridge between the reference element and the specimen fluid. Ideally, it should offer the necessary communication, yet prevent the specimen fluid from mixing with the electrolyte. In practice, however, it is impossible to prevent some mixing. When the electrolyte mixes with the specimen fluid, the defined chemistry surrounding the reference element changes and the stable electrochemical environment deteriorates. This, in turn, compromises the measurements.
The reliability of the measurement is greatly enhanced by providing a separate diagnostic feature that constantly monitors the deterioration of the liquid junction half-cell and displays the results directly on the instrument or transmits this information to a remote location, as desired. The system shows the degradation as it occurs in the front, or process, chamber of the dual-chambered liquid junction. Monitoring this cell allows detection of contamination/dilution of the reference fluid before it can compromise the measurement made in the primary chamber.
The figure at the top of this article depicts the additional chamber isolating the reference half-cell from the process. It shows the potentials e1 (of the reference half-cell) and e3 (of the measurement half-cell) in their controlled and predictable environments. As the equation indicates, when these potentials are equal, they cancel and don’;t factor in the measurement. The liquid junction acts as an ionic bridge between the reference-controlled environment and the medium that is being measured.
But it is a fluid bridge that allows some interchange of material between the reference chamber and the process; this interchange eventually contaminates or dilutes the material in the outer chamber (illustrated by the color gradient). Contamination/dilution of this chamber compromises the primary reservoir that houses the actual reference half-cell.
The system monitors the outer chamber for contamination, displays the level and alerts the user before any serious error occurs in the measurement.
Figure 2. The height of the solid bar in the display indicates the deterioration of the sensor. The bar starts to flash when reaching the limit, but before measurement accuracy suffers.
As shown in Figure 2, changes in the outer reference chamber appear as a solid bar on the upper right-hand side of the display. This makes it easy to follow the progressive contamination of the electrode (indicated by the darker color advancing through the chambers), without having to remove the sensor from its monitor/controlling function. Upon reaching full height, which indicates that a predetermined limit has been reached, the bar begins to flash, signaling the need to service the electrode. At this point, changes in the outer chambers have not yet affected the measurement, but portend deterioration that, if unchecked, could lead to erroneous readings, as shown in the last frame.
The potential of second-order instrumentation clearly goes beyond electrochemical sensors. The experience gained with these sensors should alert users to the concept and its benefits and should spur greater consideration of the approach.
Larry Berger is president of Electro-Chemical Devices Inc., Yorba Linda, Calif.