Today’s municipal and industrial water and wastewater treatment processes are under continuous pressure to provide 24/7 uptime and excellent-quality product at the lowest possible cost. At the same time, U.S. environmental standards for clean water, water treatment, and water reuse continue to tighten in order to provide the best product possible in terms of water cleanliness, quality, and conservation.
Process, instrument, and plant engineers are now tasked with ever-increasing regulatory and reporting requirements at the local, state, federal, and international levels. Every aspect of municipal water treatment and industrial plant water treatment is now and will continue to be under review for continuous process improvements, including analyzer systems and sensors that provide measurement of pH, oxidation-reduction potential (ORP), resistivity, conductivity, and specific ions.
The Problems of Continuous Measurement and Maintenance
The measurement of these parameters depends on electrochemical sensors that are designed with electrodes, which are placed in water and water-based solutions. Over time all electrodes placed in any liquid naturally wear due to exposure to the liquid, and their performance degrades. Replacement of the electrode is eventually necessary.
The problem for the technicians responsible for maintaining municipal water and industrial treatment systems is knowing exactly when to replace the analyzer sensor electrode. There are many factors that affect the electrode degradation process, including the composition of the liquid, temperature, and pressure.
Two events typically happen with maintenance staff: (1) premature replacement of the electrode and (2) waiting too long until measurement is affected. Neither is the most intelligent or best maintenance solution.
Premature replacement of sensor electrodes can be costly, in terms of purchasing too many electrodes over time as well as the labor time taken by a technician to replace each electrode. Waiting too long for replacement affects the quality of the water and may cause harm to other equipment, which then requires extra or heavier maintenance or early replacement.
The Solution — Indication of Measurement Lifetime
The best solution to minimizing the cost of maintaining analyzer sensor electrodes is a predictive maintenance solution — a technology that alerts maintenance technicians to sensor electrode wear. Sensor electrodes are replaced only as necessary, so replacement costs and labor costs are optimized, while water quality never diminishes and there is no impact on other equipment or systems that might affect their performance, maintenance, or useful lifetime. This technology allows diagnostic information about the lifetime of a pH, ORP, or specific ion measurement.
Sensors for the measurement of pH, ORP, and various specific ions use replaceable electrode cartridges specific to the measured parameter. These electrode cartridges have a measurement cell (pH glass, platinum ring, or ion selective membrane) and a reference cell.
The reference cell is designed to produce a standard potential independent of the solution it is immersed in. The highly conductive potassium chloride electrolyte in the reference cell diffuses through the liquid junction making the electrical contact between the internal silver/silver chloride wire and the process solution. The concentration of potassium chloride electrolyte also determines the potential of the silver/silver chloride electrode — the reference potential.
While these electrodes are typically trouble free, they do have a lifespan, and there are conditions that lead to failure. Diffusion through the porous liquid junction decreases the concentration of the potassium chloride inside the electrode, as the electrode ages. The decreasing concentration of potassium chloride changes the potential of the cell, which shows up as a drift in the measured value and eventually leads to a noisy, erratic reading.
Diffusion also allows chemicals in the process to infiltrate into the electrode. If these chemicals can react with the silver electrode, then the electrode will become poisoned, and a large offset voltage will be generated, destroying the accuracy of the measurement and effectively killing the electrode.
The addition of a silver/silver chloride element — called the Sentinel electrode — into the reference cell addresses this issue. The primary reference is sleeved in a glass tube, while the bare silver/silver chloride element is immersed in the potassium chloride electrolyte.
When the electrode cartridge is new, both silver elements are at the same potential. However, as the bare electrode ages or becomes poisoned, this additional element changes its potential in response to the electrolyte depletion or poisoning. The analyzer monitors the potential difference between the two elements and displays the value on a gauge as a representation of the electrodes’ remaining life.
The protected silver element is still producing the correct potential, but the potential difference with the bare electrode indicates that it is in danger of failing due to the changing environment inside the reference cell. This indication notifies the user of the potential electrode failure before the measurement actually fails.
Real-World Benefits of Predictive Maintenance
A large resin manufacturer used multiple pH electrodes on each step of its manufacturing process. Each measurement point used redundant electrodes. When one pH electrode would fail, all of the electrodes on that reactor would be replaced, assuming that if one failed, the others were probably near failure.
Monitoring of the pH is critical to the quality of the resin. If the pH is not controlled within a defined range, the batch could be lost. After retrofitting the unit with the diagnostic multistage electrode transmitters and sensors, the electrodes were only replaced when it was indicated that most of their life had been used. This dramatically reduced the manufacturer’s electrode usage and maintenance time for replacing the electrodes.
Several companies use this electrode technology to facilitate their just-in-time manufacturing, only ordering replacement pH electrodes as needed. Electrodes typically drift and die in a fairly predictable manner. Rarely is their demise due to some catastrophic failure.
So electrode life and the consequential replacement can be predicted in most of the cases if given the diagnostic technology. When the electrode is down to 10 percent of the expected lifetime, there is probably a month or so of life left in the sensor. Manufacturers order new electrodes for seamless arrival shortly before electrodes should be replaced.
With this technology, the diagnostic is displayed on the transmitter’s main screens along with the process variable, percentage 4-20 mA output, and temperature. The diagnostic value can be assigned to an optional alarm relay and/or a secondary 4-20 mA output. The mV limit value for the diagnostic is user-configurable with a default setting of 60 mV.
Steve Rupert is senior product manager for Electro-Chemical Devices Inc., a leading manufacturer of industrial process instrumentation components and systems based in Irvine, Calif. ECD makes transmitters, controllers, analyzers, sensors, and electrodes. It developed SENTINEL technology to provide “Pre-pHault” diagnostic information about the accuracy of a pH, ORP, or specific ion measurement. For more, visit www.ecdi.com.