At a luncheon meeting of Automation Xchange in Park City on August 22, many key users from the biopharmaceutical industry were interested in doing a better job of pH measurement. Many bioprocesses are adversely affected by a change as small as 0.2 pH. During the course of a fermenter batch, which can take up to two weeks, the drift of the pH measurement can exceed 0.2 pH. It is suspected that coatings may be cause of many problems. The removal of electrodes is problematic because of concerns about contamination. There are no easy solutions but the following general techniques in the design and maintenance of the pH systems may improve electrode performance.
The following practices are offered to improve the performance of pH electrodes where an accuracy of 0.2 pH or better is needed based on field experience and literature as documented in my ISA book; Advanced pH Measurement and Control – 3rd Edition. It is assumed the user has already selected the best features, such as electrolyte and glass type.
Practices Offered to Improve the Performance of pH Electrodes
1. Use a flowing junction reference and spherical glass bulb measurement electrode for accuracies better than 0.1 pH
2. Use smart digital transmitters with built-in diagnostics
3. Use middle signal selection of three pH measurements
4. Allocate time for equilibration of the reference electrode
5. Use ‘in place” standardization based on a sample with the same temperature and composition as the process. If this is not practical, the middle value of three measurements can be used instead as a reference. The fraction and frequency of the correction should be chosen to avoid chasing previous calibration adjustments.
6. Use a fixed process fluid velocity at the highest practical value to help keep the electrodes clean and responsive
The following file provides some background information on these practices.
The problems in pH control systems for chemical processes and environmental discharge can be more dramatic. Even though the accuracy and control band required may not be any where near as tight as for fermentors (with the exception of chemical reactors and crystallizers), the titration curve can be exceptionally steep and variable and the process fluid extremely harsh. Chemical attack of the glass and poisoning of the reference can be major concerns. Solidified references, special glass formulations, thicker glass, and flat glass electrodes may be important. Still the life expectancy can be so short for some environments (e.g. a few days) that the use of three electrodes is not feasible. In these cases a piston actuated retractable injector assembly may be needed to reduce the time exposure to the process and to provide automated periodic cleaning, rejuvenation, and hydration of the electrodes. A word of caution for low water content streams; variations in the water concentration affect the pH reading even if the gel layer remains hydrated.
Top Ten Signs of a Rough pH Startup
10. Food is burning in the operators’ kitchen
9. The only loop mode configured is manual
8. An operator puts his fist through the screen
7. You trip over a pile of used pH electrodes
6. The technicians ask: “what is a positioner”?
5. The technicians stick electrodes up your nose
4. The environmental engineer is wearing a mask
3. The plant manager leaves the country
2. Lawyers pull the plugs on the consoles
1. The president is on the phone holding for you