Process Capability

It appears that you are part of the quality team at your facility.  Just some additional thoughts:  (1) pH meters have electrodes that typically require more maintenance than say a thermal couple.  Therefore, out-of-control condition detected by the SPC chart may or may not be a process shift.  It could be an instrument shift.  (2) Understand the pH meters calibration procedure and frequencies.  Is it being calibrated frequently enough?  (3) How often are the electrodes replaced?  The probe response time to a change in pH depends upon the age of the electrodes.  (4) Do you have a 2nd or 3rd pH meter that you can use to verify the results.  Have the 2nd and 3rd meters been correlated to the primary pH meter? 
 
All of these issues are important considerations in establishing a stable process.  The last thing you want is a statistically stable process in which the pH is outside the specification limits.  Finally, all of these considerations must be balanced against the specifics of the process.  A world-class process involves SPC-plus.  The plus involves all the specifics needed to produce the best control possible.
 Here is a question that I would like an answer to: What is the impact on measurement accuracy when the pH meter is used continuously for 24 hours…..?

RGP asks some very good questions.   In my food plant we really struggle with pH measurement because the food grade non-glass pH probes are accurate but inprecise and there is the problem of drift.  We chart pH data but it is ignored (I know why chart when you are going to ignore?).   Any body had success with these twin SPC challanges of  a lack or precision and insttrument drift?
 
Andrew