Stability of colloidal silica is important for the health and performance of an investment casting slurry. pH is one indicator as to the stability. Conductivity is another important indicator and information on that topic can be found here.
Accurate measurement of pH is critical. I have received numerous phone calls from customers stating that the pH of the slurry is 11.0 or even higher. If the pH of a silica-based sample is above about 10.5, the silica particles can begin to dissolve. Since the slurry additives typically lower the pH, a reading of 11.0 could indicate issues with your meter.
The trending of the pH over time is as important, if not more important, than an absolute value. For example, a pH of 9.2 could be a reason for concern. However, if the pH has been slowly drifting down to 9.2 and then settling there, the slurry stability may be fine. Checking conductivity is another way to confirm the health of the slurry.
Care of a pH meter will vary by manufacturer. Take some time to review the manual and ensure that the proper calibration and maintenance procedures are followed.
If you are still unsure of your readings, feel free to contact me for more information at Julie@keyprocessinnovations.com.