AJ,
In another thread you wrote: "It is generally thought that the pH going into the kettle should be around 5.2."
I'm still not getting that low, probably because I'm overly-cautious of using too much acid malt or lactic acid and getting some sourness. But I've only had my meter for a few brews. Still tweeking but skeered to go over 3%.
There is definitely some latitude here. I often have pH appreciably higher than that going into the kettle and never take action. I have noticed that when using sauermalz with soft water the pH tends to creep up during decoction mashing when relying on salts for pH control it tends to decrease somewhat with each decoction. This is probably because of the formation of calcium carbonate, phosphate and protein complexes.
I noticed a comment in Kaiser's How pH Affects Brewing article: "A mash pH above 5.4 should be used for decoction mashes and/or enzymatic weak mashes (i.e. large amounts of Munich malt or adjuncts)"
It so happens I'm making Vienna lager this weekend using ~88% Vienna malt but only a thin mash-out decoction; infusions otherwise. Is 5.2 too low? I make a fair amount of malty German lagers so I want to get it right.
Kunze says that mash pH should be 5.4 - 5.6 and wort pH 5.1-5.2 but as noted I think there is some latitude here. I think you would probably be OK with a mash pH of 5.2 but 5.5 has worked well for me and does not require (with DI water) more than 3% sauermalz. With that amount of sauermalz I have observed dough-in pH as low as 5.1 but after a few minutes it climbs eventually settling at 5.4 - 5.5 and this is what I usually begin the boil at. Kunze would advise adding sauergut to get the pH down to 5.1 - 5.2 and that might well improve the beer and I'd encourage experimentation with this but I'm sure you will find such an improvement in your lagers just by controlling to around 5.4 that you will wonder if "better is the enemy of good enough".
Also, is it correct that certain base malts just yield high-ish mash pHs? I'm going through a sack of Canadian Superior Pale Ale malt and the malt analysis sheet shows values for the entire season (my lot is end-of-season). It appears the mash pH steadily climbed during the season and if I'm reading it correctly my lot had a mash pH (congress?) of 5.9. Struggling to get a mash pH below 5.5 with it (but it's still early).
Yes, malts vary a great deal in the acidity they contribute. Most seem to have a DI (Congress) mash pH of 5.75 - 5.8 but 5.9 certainly is possible.