In many water treatment and chemical processes,
it is a requirement to keep track of the pH of the water or product stream. In DeLoach Industries equipment such as degasification systems, or odor control scrubbers, pH measurement is critical to control the chemical reactions happening within the treatment system. PH is an indication of the acidic vs alkaline nature of the fluid. An acidic fluid will have a greater concentration of H+ hydrogen ions, while an alkaline fluid will have a greater concentration of OH- hydroxide ions. This electrochemical nature is used in the construction, reading, and maintenance of electronic pH probes.
PH probes are generally glass and will contain a reference element, and a sensing element. When the pH probe is immersed in the fluid to be measured, the electrical potential difference between the sensing element and the reference element is amplified by electronics and the resulting voltage is used in a calculation to determine pH from differential electron potential. As a pH probe remains in service, ion exchange will slowly change the electrical potential of the sensing element, the reference element, or both. This happens because the hydrogen ions are small enough to travel through the glass sensor body and cause reference potential shifts over time. This is normal behavior for all pH probes and is the reason why pH probes must be periodically calibrated.
Calibration is a process where a pH probe is immersed in a series of standardized stable pH solutions called “buffers”. The standard set of buffers includes a pH 4.0 acidic buffer, a pH 7.0 neutral buffer, and a pH 10.0 alkaline buffer. These buffer solutions are chemically designed to hold a stable pH and are used as a reference for the internal calculations that are done by the pH amplifier or transmitter that interprets the reading taken by the pH probe. As the reference voltage vs actual pH for a mature probe changes, the known buffer solution provides a benchmark for the calculation. Each pH instrumentation manufacturer will have a slightly different method for performing a calibration, but in general, the system will have you step through the buffer solutions while an automated routine makes note of the expected voltage vs calibration voltage at each step. The computation algorithm will use this drift information to re-scale the calculation to re-establish accuracy.
Over time, as a probe goes through multiple recalibration cycles, there will be a point where the reference or sensing element(s) have absorbed or donated enough H+ ions that it can no longer be calibrated. The time until this point is reached will vary depending on the process. In general, the more neutral the solution being measured, the longer a probe can be expected to provide accurate results. If the solution being measured is either highly acidic or highly alkaline, the ions in the reference element or sensing element will become depleted or neutralized more quickly. When this point is reached, the pH system will usually that it is time to replace the probe completely or to replace the reference element
PH is a very important chemical measurement that is critical to the operation of a multitude of systems in the field of water treatment and elsewhere. Because pH measurements are often part of a critical process control path, it is important to understand the calibration needs of your system and get a feeling for how often calibration is necessary for your process. An out-of-calibration pH probe can read with an error of 25% or more while appearing to be functioning correctly if calibrations are not done. I personally have seen an unmanaged pH excursion completely destroy a biological odor control system worth over $100,000. In general, I will suggest that new systems have their probes calibrated every month. Over time, the relationship between millivolts developed at the probe, and the pH computed can be tracked to get a more specific feel for how quickly the pH probes are drifting in a specific installation.
Modern PLC-controlled automated process systems can often make plant control seem easy and “hands-off.” With automated systems, it is important not to let the high level of automation lead to complacency in maintenance or training. The importance of regular pH probe calibration is a perfect example of a detail that is easy to miss in an automated process that can lead to bad outcomes. With pH-controlled systems, remember to plan for regular probe recalibration from the very beginning. It is helpful to locate pH probes inaccessible locations and use quick-disconnect fittings where possible to make the calibration process as pain-free as possible for plant operators. It is also advisable to develop a log system to keep operators accountable. The damages from an uncalibrated out-of-specification probe can very quickly overcome any savings gained by skimping on maintenance.
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