Understanding pH Drift
When you install a pH loop within your plant, your priority is to receive an accurate and reliable measurement. I frequently hear from end users that they are experiencing “drift” in their pH measurements. This drift is directly affecting data accuracy and reliability. Understanding what “drift” is and the leading causes will help you maintain your pH electrodes better. Therefore, you will ensure optimal efficiency and reliable measurements!
“Drift” is the slow movement of the measured value away from the actual, expected pH of the solution. Unfortunately, several different things can cause your pH measurement to drift. By focusing on the key causes and taking preventative measures, you will save yourself time and money! Let us have a look at three main reasons for pH measurement drift and discuss how to address them.
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Plugged Liquid Junction
The vast majority of glass pH electrodes on the market utilize a liquid junction or “diaphragm” to link the internal reference system with the measured solution. There are numerous varieties of junctions available, ranging from ceramic to PTFE and even open hole styles. For that reason, the proper selection of the type junction used in your application is critical for optimal performance. A plugged junction, however, is the number one cause of pH drift. The linkage between the reference system and the measured solution is now blocked.
Ensure the electrode’s junction is clean and free of contamination by soaking the electrode in a diluted HCl or caustic solution to remove any particulates chemically. We recommend a 5% solution with a soak time of 10-15 minutes. After soaking, a thorough rinse and two-point calibration should help to bring the sensor back in line.
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Contaminated Reference Electrolyte/Poisoning
A key component of the glass combination electrode’s reference system is the reference electrolyte. The electrolyte is most commonly 3 mol/L KCl (potassium chloride). You will find it in one of three forms: gel, liquid, or polymerized. The electrolyte is a neutral solution (pH value of 7.0) and is a strong conductor. Immersed within the electrolyte is the reference electrode (typically Ag/AgCl), which provides a stable potential. Over time, the measured solution will often work its way through the liquid junction and contaminate the reference electrolyte. This contamination can shift the electrode’s zero-point and lead to pH measurement drift. Once the “poisoned” solution comes in contact with the Ag/AgCl element, failure of the electrode will occur.
“Double junction” electrodes such as the patented Silamid® reference system utilized in Knick sensors provide added protection to the Ag/AgCl element. As opposed to a bare silver wire coated in silver chloride, the Silamid reference system consists of a glass tube that is lined with silver (Ag) and then packed with silver chloride (AgCl) powder. A filter on the end of the glass tube serves as a second junction or barrier to protect against contamination, thus providing longer sensor life.
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Electrical Interference/Noise
Lastly, electrical interference on the signal between the sensor and transmitter is the third cause of drift for pH electrodes. Many pH measurement systems utilize a coaxial style cable carrying an analog (millivolt) signal back to the transmitter. When using analog sensors, you should be sure to take extreme care of this cable. You will need to protect it from any potential interference. Ground loops, signal leakage, and even external fields generated by nearby equipment have an effect. Therefore, changes in the millivolt signal on this cable, which are not caused by changes in the process, will lead to erroneous readings and drifting measurements.
Digital technology, such as Memosens, eliminates the possibility of electrical interference on the measurement. Memosens sensors are galvanically isolated and convert the analog millivolt signal to a digital signal directly in the head of the sensor. Then, the RS-485 protocol precisely carries this digital signal back to the transmitter. As a result, Memosens technology provides accurately measured values by eliminating ground loops, signal leakage, and external noise potentials.
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In summary, pH drift can cause countless hours of headaches while attempting to troubleshoot the issue at hand. By applying the proper sensor to your application, understanding the leading causes of drift, and how to address them, you will be on your way to painless, reliable, and accurate pH measurements!
