Order up better measurements

Second-order instruments can provide improved accuracy and reliability for electrochemical and other sensors.

By Larry Berger

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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.

Crucial measurements
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.

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