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PH MEASUREMENT IMPROVING PH MEASUREMENT PERFORMANCE


ions, contamination can also lead to the reference electrode being attacked, whereby performance deteriorates, and, if not resolved, the sensor may fail. Previously, to diagnose whether an electrode had failed, operators would have had to spend time manually checking other possibilities first. With digitalisation, diagnosing a fault is now


much easier, with features such as perpetual impedance diagnostics, which analyses the resistance and impedance between the reference and measuring electrode; and smart reference electrode monitoring, which provides early warning of electrode poisoning, enabling problems to be quickly diagnosed. Condition monitoring tells more about


Nikodem Siwek, ABB Measurement & Analytics, explains how digital advances have improved pH measurement in chemical processing


effluent discharge, calls for devices that can offer accurate and reliable performance. Being able to accurately measure levels of acidity and alkalinity in chemicals production is vital to ensuring the highest standards of product quality, safety and environmental performance. The harsh nature of many chemicals


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processes can make life particularly tough for pH sensors and can shorten their service life compared to other types of measurement instruments. It is also important for operators to be able to count on the accuracy of their pH measurement devices. Although many pH sensors still use


analogue signals, increasingly the latest and most sophisticated sensors are based on digital connectivity. Analogue sensors use a high impedance cable, making them susceptible to interference. This can restrict the length of the cable connecting the sensor to the transmitter. With digital pH sensors, these issues are removed, without compromising accuracy. In addition to


improving productivity by giving a real- time indication of process conditions, online measurement allows the pH sensor to form part of a closed loop control


he important role of pH measurement throughout chemicals processing applications, from production through to


system. Complex algorithms can be incorporated to react to changes in process conditions, adjusting dosing of chemicals to achieve the desired level. The algorithms can also increasingly predict changing pH levels, enabling corrective action to be taken automatically. Real time measurement also ensures that


the company is not breaching environmental standards, as the pH level is known at all times. Digital sensors also allow analysis and signal conditioning to be conducted in the sensor. With large quantities of pH sensors typically


being deployed in an application, being able to quickly identify a failing device can help to save both time and engineering resource. Modern digital pH instruments have built in software, making them very quick and easy to install, configure and maintain. It is now increasingly possible to dial into devices and find out everything you need without having to physically visit a device. One example of a typical fault that affects


many pH measurement applications is electrode poisoning. Most commonly caused by the substance being measured contaminating the reference electrolyte, electrode poisoning can result in erratic and unstable readings that can affect accuracy. Where the substance being measured has a high concentration of


60 JULY/AUGUST 2021 | PROCESS & CONTROL


exactly when to schedule maintenance and allows operators to prepare in advance before it impacts the process. By connecting the pH sensor to a digital transmitter, the unit’s data output from its diagnostic functions can be used to accurately determine the root cause of a pH measurement error. Operators can make use of the increasingly rich data to identify exactly what went wrong and derive strategies to prevent the same problems occurring in the future. Just like other types of measurement


instruments, pH sensors need to be calibrated at regular intervals to ensure they can offer the desired levels of accuracy. For a non- critical process application, three- to six-month intervals will typically be sufficient, while more critical or extreme applications would require a higher frequency. One of the most important aspects of


maintaining a pH sensor’s accuracy is calibration with pH buffers. All pH systems, whether analogue or digital, should always be calibrated before use. This requires the pH measurement cell to be calibrated with a solution with a known pH value. The data available from pH sensors can be


combined with that from other instruments to inform decisions across the whole plant and the wider business. A cloud-based infrastructure will help more people and systems get access to and act upon the data that is appropriate to their needs. The data can be made available on smartphones and tablets via appropriate apps, making it easier for plant personnel to interact with it. The main value of digitalisation is that it


allows a better overview of processes, ensuring decisions are made on a more informed, objective basis. Systems running analytic programs provide insights into key operations such as pH measurement and control. Analysis brings the ability to anticipate changing process trends, deal with potentially anomalous conditions as they occur, and help the organisation achieve higher-level operational and business objectives.


ABB Measurement & Analytics new.abb.com


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