This guest post is authored by Vickie Olson, product marketing manager at Honeywell Process Solutions.
For today’s process plants, pH is an important parameter to measure in a host of demanding applications. A good example is flue gas desulfurization (FGD) control with systems using wet scrubbers in lime or limestone slurries. Proper pH can maximize sulfur oxide (SOx) removal and minimize the 
build-up of scale. Selection of the best sensor type will also enable longer life and more reliable control.
Choosing a pH sensor for use in harsh process environments requires careful consideration of numerous key factors. Extreme temperature at higher pH ranges reduces pH life due to consumption of hydrogen-sensitive ions on the glass membranes. An electrolyte (typically potassium chloride in gel or liquid form) tends to diffuse faster with temperature and high flow. In addition, abrasion removes the membrane surface over time.
Many process industry users specify a pH sensor of the combination style, meaning the device has a measuring electrode — either glass or ion-sensitive field effect transistor (ISFET) — or a reference electrode, which is usually based on silver/silver chloride. These pH sensors are considered rugged or robust compared to general-purpose sensors, since they are better able to withstand abrasive and alkaline conditions such as in FGD slurry. General-purpose pH sensors may not last a single day in sulfuric, abrasive and high-temperature environments. Depending on the style of ruggedized pH sensor and its maintenance frequency, this device can last from a few weeks to many months in operation.
Various types of reference protection on the market allow electrolyte diffusion, but reduce the infiltration of contaminants from the process fluid that can plug the junction or cause fouling of the reference. The porous junctions at the tip areas may be composed of double or multiple sections designed to slow contamination, which can result in poisoning of the silver reference material. Typically, the junction material in rugged pH sensors is composed of solid polytetrafluoroethylene (PTFE), ceramic or fibrous polyvinylidene fluoride (PVDF).
Several pH sensor designs employ wood or acrylic material containing electrolytes to slow the spread of contaminants while maintaining the required electrical connection of the reference with the measuring electrodes. An additional method to delay poisoning with solid reference designs is to locate the reference wire at the back of the sensor body, rather than hang it from the back, approaching the front of the device.
In terms of ruggedized glass measuring electrodes, the tip may have thicker glass and more hydrogen-sensitive material on the membrane. Flat glass is sometimes substituted for hemispherical or round glass on the tip to avoid breakage due to hard materials. This is less important in FGD applications, although the design is successfully used in the pulp & paper industry with heavy pulp slurry. In lime slurry, the flat tip does not have as large a measuring surface — leading to faster wear than rounded-type tips.
Most recently, ISFET measuring electrode technology was been paired with rugged reference technologies to provide a durable pH measurement solution. The advantages of ISFET include robustness, stability and precision. Sensors of this type utilize an integral automatic temperature compensator in one-piece construction, making them well suited for varying pH and temperature ranges.
The more demanding the application, the more critical it is to consider process operating conditions and expectations for a pH sensor. This is particularly important when harsh environments require frequent sensor replacement. Users employing the right device can realize savings from extended sensor service life, reduced replacement and maintenance costs, and ultimately, accurate and reliable pH measurement.
About the Author
Vickie Olson is an analytical product specialist for Honeywell Process Solutions based in Atlanta, Ga. She has been involved in process instrumentation and analysis for industrial and municipal applications for more than 25 years as a chemist, product specialist and sales manager for Honeywell, Hach, and other companies. Vickie has spoken on a variety of topics related to water analysis and control at ISA and numerous other symposiums. She earned a bachelor's degree in textile chemistry from the Georgia Institute of Technology and a master's degree in business administration from Georgia State University.
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