The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #86.
There is not a whole lot of current information on the response time of pH electrodes. Tests done in the 1960s for a clean, healthy pH glass electrode showed that the time constant was smaller for a positive pH change, a higher velocity, and a higher degree of buffering. For a buffered solution at 4 fps fluid velocity, the time constants were 0.75 and 1.5 sec for a positive and negative pH change, respectively. At 0.5 fps the time constants became 3 and 12 seconds for the same electrode and solution.
A case history published in 1990 showed that the time to reach 98 percent of the final response (the “response time”) deteriorated from 10 seconds to 7 minutes for a 1 mm slime coating. The time to reach 98 percent of the final response is four time constants plus the deadtime. The use of a 98 percent response time can be misleading and difficult because pH electrodes typically have a much slower approach to a final value above 90 percent than a first order classical response. Waiting to 98 percent means that the test time is greatly extended and the inferred time constant may be grossly overestimated. Also, a small amount of noise can lead to inconsistent results.
As far as the controller is concerned, what happens at the beginning of the response is the most important. I prefer to use an 86 percent response time for a faster test and a more accurate time constant that is less sensitive to noise. The 86 percent response time is used for control valve response testing in the ISA-75.25.01 standard for some of the same reasons.
Test results for a glass electrode prematurely aged by exposure to high temperature showed an increase in the 86 percent response time from 10 seconds to 50 minutes. Dehydration, abrasion, and chemical attack can also cause a large increase in response time. An increase in the sensor response time (lag time) slows down the response of the loop to upsets. In the study cited, a 1 mm of slime coating increased the amplitude of oscillations by a factor of 10 and the period doubled.
Aging of a glass electrode also shows up as a decrease in efficiency; that is, span. However, this effect is not as detrimental to loop performance. The loss in span causes a decrease in process gain but there are much larger influences on the process gain, such as the shape of the titration curve. For a setpoint at 7 pH (the zero millivolt point) there is no effect on operating point. The actual pH is 7 when the measurement is at setpoint. As the pH setpoint gets further away from 7, the effect becomes larger. However, standardization by the analysis of a grab sample will compensate for the difference between the actual pH and the setpoint, reducing the effect efficiency back to a process gain. Losses in efficiency are less problematic than offsets from the reference electrode (Tip #87).
Concept: A thin coating on the glass and/or aging of the glass will show up as a huge increase in the sensor time constant. (The 86 percent response time is the electrode deadtime plus two time constants.) A large sensor lag causes deterioration of loop dynamics. A measurement of the response time by making a setpoint change or by putting the electrode in different buffer solutions provides a sensitive indicator of the health of a glass electrode.
Details: When buffers are used to calibrate an electrode, estimate the time to 86 percent of the final response. The electrode deadtime is usually negligible. Use the data historian and trend chart in the DCS to estimate the response. If you have a wireless gateway, use a wireless pH transmitter instead of a lab pH meter to get the calibration data into the historian. Errors of less than 10 seconds in the estimate are not important because glass electrode measurement problems show up as large changes in the measurement time constant. Use setpoint changes to measure the response time of a glass electrode.
If you have multiple electrodes, the increase in electrode time constant will show up as a nearly constant time shift between the response curves. A decrease in electrode efficiency shows up as an increasing time shift. Remove and clean or rejuvenate, if necessary, the slowest responding electrode(s). Use a dilute 5 percent hydrochloric acid (HCl) solution to remove alkaline deposits and strip away the outer, aged layer of glass to rejuvenate an electrode. Use a dilute 1 percent sodium hydroxide (NaOH) solution to remove acidic deposits. Use a detergent solution to remove organic deposits (oil and grease). The household rules of cleaning solutions to remove stains may be applicable to cleaning electrodes.
More tenacious deposits may require a solvent. Be careful to avoid solvent attack on the sensor’s o-rings and seals. Limit exposure time to prevent contamination of the reference junction and chemical attack on the glass by the cleaning solution. To minimize coating while in service, ensure that the fluid velocity past the electrode is greater than 5 fps and that the protective shroud provides exposure of the glass surface to the flow stream unless the fluid is abrasive. Use high temperature glass to prevent premature aging from exposure to temperatures above 40 °C.
Watch-outs: The equilibration of reference potentials may make finding the final response difficult, particularly as the measurement electrode response gets slower. The extremely long equilibration time of some solid-state references will make measurement of the 98 percent response time thoroughly inconsistent. The measurement may appear to drift or never reach the buffer solution pH despite repeated calibration adjustments for both offset and span. The percent of final response is often not given in response time statements. Sometimes people mistakenly use response time when they mean time constant. Because the number of time constants is dramatically different between 63 percent and 98 percent (one versus four time constants), the source should be questioned as to the percent of final response.
Exceptions: For loops subjected to frequent changes in setpoint that are not corrected by an upper level loop, a loss in efficiency (span) may become more important than an increase in sensor lag. Electrodes that have an extreme loss in efficiency are in danger of becoming dead electrodes and must be replaced even if they are fast-responding.
Insight: The sensor response time is a sensitive indicator of the effects of coatings and aging on glass pH electrodes.
Rule of Thumb: Use the 86 percent response time to determine when to clean, rejuvenate, and/or replace the glass electrode.
About the Author
Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.
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