Figure 1. DolpHin unit has lasted far longer than units its replaced in an acid-gas scrubbing system.

To remove hydrochloric acid and meet air pollution regulations, Eastman uses a scrubber with a 20% sodium-hydroxide solution. According to the company, the DolpHin sensors’ pH glass formulation increased sensor life to six months compared to three to four weeks, at best, for previous sensors. Also, pH measurement equipment and operations costs dropped by almost 90%. Eastman Chemical was successful with the DolpHin series because these fit the company’s maintenance style and simply lasted longer.

Tough conditions

Sensors must stand up not only to harsh chemicals but also often to heat. Rosemount’s Joseph notes that customers have been requesting sensors that last longer at elevated temperatures and that withstand even higher temperatures. To fill this need, in July 2006 the company launched the PERpHX, a refillable sensor with high performance electrolyte rated to 293°F (Figure 2).

Figure 2. PERpHX unit can withstand temperatures up to 293°F and boasts fast calibration.

FMC Biopolymers, Rockland, Me., uses this device in producing carrageenan from imported seaweed. To maintain pH of 7.5 to 8 in the batch tanks used for processing, the plant controls acid make-up using a triple-redundant pH measurement system in the recirculation line. One benefit of controlling pH is less frequent filter cleaning and replacement.

Rosemount’s sensors have been installed there since mid-May, Joseph says. The “x brand” sensor previously used typically lasted only four days, the local FMC engineer responsible for instrumentation told him. Joseph adds that engineer said, “We have tested everybody’s pH probe over the last 15 years — competitors x, y and z … We usually trash them within 24 hours.” It’s not clear why FMC’s process is hard on pH sensors, Joseph explains. But he notes the process temperature is approximately 200°F and the process liquid is “thick” and contains organics. He adds that the cleaning cycles use strong acids and bases, which also can be problematic.

An added PERpHX benefit is its fast calibration. The x-brand sensor required two minutes for calibration versus 10 to 15 seconds for a new PERpHX and 45 to 60 seconds for an older unit, Joseph recalls the engineer saying.

Polychemie, at its Pearlington, Miss. facility, employs the TB557 pH sensor from ABB Automation, Norwalk, Conn., in an agitated reactor. The company, which manufactures coagulant and solution flocculants, tried several types of pH sensors before ABB’s, but they quickly degraded in the harsh, abrasive operating environment, notes Kyle Becker, an engineer at the site. According to ABB, its probe’s hemispherical glass electrode, which is rated for a true 0 to 14 pH range at temperatures to 284°F, is rugged enough to withstand abrasion and strong caustic.

Uncertainty about uncertainty

Measurement problems also can arise because of the reference electrode. “Chemical attack of the glass and poisoning of the reference can be major concerns,” notes McMillan on his blog (www.modelingandcontrol.com).

“That (reference) electrode is put into a customer’s process where there are chemicals that will poison the function of the electrode. Anything that is going to come in from the process — sulfides, salts, various process chemicals — can alter the chemical environment and can add electrical potential and introduce drift,” Invensys’ Connelly explains. It’s this poisoning that could create analytical uncertainty, he adds.

The general lack of attention paid to this uncertainty troubles David B. Mills, author of the recent book “The Consumer Guide to Industrial pH and ORP Instrumentation” published by Cooperhill and Pointer Inc., who is a systems manager at Proctor & Gamble Co.’s Ivorydale Technical Center in Cincinnati, Ohio, and principal of Mills Process Consulting, Cincinnati. “From a standpoint of pH measurements products, there’s a lot of information provided,” explains this chemical engineer. “You can get the slope, the zero point, the equipotential point, but if you just want to know how much uncertainty in the pH measurement there is, most suppliers don’t tell you.” Why? “I don’t think they’ve been pressured to provide that from end users.”

McMillan also raises the issue of the lack of specified accuracies. In the preface to Mill’s book he notes there is “usually no accuracy specification [actual or reference] on the overall system that includes the sensor.”

Providing reference electrode uncertainty to end users is the fundamental challenge remaining in pH measurement, says Mills.

Further challenges

Others, reflecting the variety of issues facing pH measurement, see the remaining challenges differently.
End user Pastushak considers the primary issue, generally, to be the technical capabilities of pH measurement. “They are lacking compared to industry wide process control.” he says. The second issue is accuracy and response, followed by ease of installation, he says.

Supplying longer lived sensors is the biggest challenge left, Connelly counters. “You use the best device available. If you have to replace once a day, once a shift, once a month, that’s what you do.” But, eventually, pH sensors must be removed, cleaned, calibrated and/or replaced. “Astute suppliers will realize that customers want to spend as little time as possible doing these things,” he adds.

Offering sensors that can resist clean-in-place (CIP) exposure is the biggest challenge Joseph sees. “The glass electrode is severely affected by caustic CIP and the main alternative, the ISFET, is even more so.” What’s been done to solve that? “Nothing has come close so far,” he notes.

Making sensors truly “smart” is the most pressing issue with pH measurements, says Tom Griffiths, product marketing manager for the analytical instruments group within Honeywell’s Industrial Measurement & Control Division, Fort Washington, Pa.

Smart equals diagnostics, which equals instrument health, believes Connelly. Use of these smart or intelligent pH sensors isn’t common, but is growing, he notes. “As end users become more accustomed to diagnostics, they can begin to use them as a valuable tool."

The time’s coming, though, when users won’t want to determine when a sensor needs replacing or cleaning, Griffiths predicts. “They want to be notified by the system either by a diagnostic alarm or even by e-mail notification through communications via Ethernet.” To meet that need, Honeywell has installed in their Durafet III electrodes, which are silicon-based ISFETs, an electrically erasable programmable read-only memory or EEPROM. It collects information about the calibration that, in the future, will be used to help predict the sensor’s health and lifetime.

Diagnostics feature in Rosemount Analytical’s PERpHX. “They [diagnostics] alert the user to a broken glass electrode, a coated sensor and a non-immersed, or dry, sensor,” Joseph explains. “These diagnostics point the user towards refilling the reference gel, replacing the reference junction and replacing the sensor.”

Diagnostics fit directly into what end users are demanding about pH measurement: “Make it easy for me to use your — the vendor’s — equipment and provide me information that makes my job easier,” according to Connelly. After all, he explains, “most handle many different measurements and instruments. They don’t want to spend a lot of time figuring out how to use the instrument.”

Pastushak undoubtedly speaks for many end users when he says more reliable pH data will always be needed, “to ensure the highest project quality in the highest yield.”