There is an excellent discussion going on in the homebrewtalk brewing science forum on mash pH, specifically on changes in pH over time in the mash - in practical terms, when can you measure the pH and be assured its no longer changing.
There's lots of other items in this thread (and the whole science forum, also, it is really excellent), but that jumped out at me and it occurred to me it might be of interest to some CHAOSers too. I just assumed taking a reading after five or ten minutes was good, but apparently its not clear that it is. Brandon's BFF, Martin Brungard, says that in his experience, the mash pH is changing up to 30 minutes into the mash.
https://www.homebrewtalk.com/forum/threads/wanted-mash-ph-measurements.6...
The main topic of the thread is gathering data to evaluate how well the different calculators are at predicting mash pH, btw.
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