Instrumentation Best Practices
- For greatest measurement electrode accuracy, use a semi-spherical glass bulb with the best glass formulation for operating conditions. For high-temperature applications, make sure a high-temperature glass prevents premature aging seen as a dramatic increase in response time (see Figure 1). For cleaning and sterilization in place (CSIP), make sure the electrode structure design minimizes the offset and drift after repeated cleanings and sterilizations (see Figure 2). Use low-sodium ion error glass if excursions occur above 10 pH when caustic is present. Use HF-resistant glass if excursions occur below 8 pH when HF is present. Note that high temperatures greatly accelerate chemical attack, reducing glass life. Also realize that the benefit of HF-resistant glass is marginal. A few ppm of HF at low pH can dramatically reduce the life of the best glass. The best bet is to insure the process stream never drops below 8 pH.
- To minimize plugging and contamination problems, use a solid reference electrode or a readily replaceable reference junction. The entire sleeve of some solid reference electrode designs is the liquid junction helping to ensure electrical continuity minimizing the effect of coatings. Most solid references eliminate the contamination problem. Some liquid reference designs offer a removable reference junction, so the junction and electrolyte can be easily replaced.
- Consider a flowing reference junction as a last resort to quickly equilibrate to a constant minimum junction potential. If you must measure accuracies of 0.02 or better pH in a stream with a propensity for plugging or a high significant ionic strength, realize that a flowing junction can quickly establish a small constant liquid junction potential, and prevent clogging and contamination. This solution is a last resort because the installation requires the pressurization of an external reservoir of electrolyte for the reference to keep a small flow of electrolyte. Excessive electrolyte flow will contaminate process samples and even buffer solutions.
- Use wireless pH transmitters to eliminate electrical interference and spikes. The use of wireless transmitters eliminates the ground path that is the cause of much noise and spikes. Wireless transmitters tend to have the latest improvements in transmitter design, including better diagnostics and signal resolution. Wireless transmitters also offer flexibility for testing electrodes in the lab at process conditions and analyzing the results in the data historian.
- Compensate or be cognizant of changes in solution pH with temperature. The standard pH electrode temperature compensator corrects for changes in the millivolts generated by the glass with temperature, but not for changes in the solution pH, which will change with temperature due to a change in the acid, base and water dissociation constants with temperature. The effect is greatest when the pH is near these constants. The actual effect of temperature on a solution pH should be quantified by tests on representative lab samples. Smart transmitters offer solution pH temperature compensation.